PRESENTED TO THE
CAII-FORNIA ACADEMIY OF SCIENCES.
V'oOL. I, P,/RT II.
TH E NATURAI  SYSTEM OF
AT OTCANIC RTOCKS.
RICHTIHOFEN.
SAN FRANCISCO:
TOWNE AND BACON, PRINTERS.
1868.




Committee of Publictiozn.
J. D. WHITNEY,          W. O. AYRES,
R. E. C. STEARNS.




M E A O AT  I  S
PRESENTED  TO  THE CALIFORNIA  ACADEMY  OF SCIENCES.
VOLUME I.
II. Principles of the Natural System of Volcanic Rocks.
BY F. BARON RICHTHOFEN, DR. PHIL.
[Presented, May 6th, 1867.]
NTRODUCTORY. Among the features peculiar to modern Geology may be noticed
a revival of that speculative tendency which prevailed among the cultivators of this
science at the close of the last century. But while in those early times imagination
exerted a dominant influence in the framing of hypotheses, and discussions between
the adherents of different doctrines were conducted with all the bitterness peculiar to
such struggles, when neither party has a firm basis upon which to found its arguments,
the constant ascendency of the spirit of the inductive method has imparted to those
theories more recently propounded a more logical and scientific form, while, at the
same time, the increasing amount of positive knowledge has given to the different
doctrines a more varied and more definite character, and enlarged the scope of dissenting views.
This renewed tendency to systematize and theorize, which is especially conspicuous in the records of the last twenty years, must be ascribed, partly, to the vast
amount of well-established facts gathered during the previous decades, and which
have since been multiplied and intensified in a constantly increasing ratio, as regards
depth and distinctness of observation as well as the geographical area over which they
extend; partly, and in no less degree, to the rapid progress made by those sciences on
which geology has to draw for the general laws which are alone capable of affording a
philosophical guide to speculation on the basis of facts gained by observing and comparing. The advance of the chemical and physical sciences, especially, has had a
MEM. CAL. ACAD. SCd. VOL. I.  [2E]                   Jan. 1868,       (39)




2                                INTRODUCTORY.
powerful influence, by allowing an immediate application to geological problemns of
such general laws as are established beyond any doubt.  Besides, the improvement of
the practical methods applied in chemical and physical laboratories has given rise to
the execution of numerous experiments, wlhichl were made with a view of imitating the
processes applied in the great natural laboratories.  They suggested frequently the
causes of facts disclosed by the examination of a certain region, or obtained by comparing the observations made in different countries on one certain subject.
It appears to be mainly due to these causes that a number of theories have
been proposed, in rapid succession, relating to the origin of rocks, to the mode and causes
of metamorphism, to the agencies of vulcanism, to the structure and mode of formation
of mountain ranges, to the structure of the entire globe, and other cognate subjects.
It will be admitted, even by those who are most strongly opposed to theorizing, that
geological science has in this way been promoted and enriched in various respects;
since there is scarcely a theory which, even if insufficient to explain what it undertakes, has not some truth in it, or is applicable to some extent in certain cases, or has
at least, even when proving to be erroneous itself, led to discussions on subjects of high
interest, which is indeed a result of no little value. In reviewing the theories proposed on any particular subject, we find them, it is true, often in apparent contradiction with each other; yet almost every one is based upon arguments drawn fromn
observed facts, and there are probably very few which will ultimately be entirely
abandoned. The immense range of varied processes as applied by nature allows the
applicability, in a limited way, of many a theory in certain instances, while in others
it may be refuted on no less valid grounds; and the struggle between the defenders of
different doctrines is often founded only in the difference of their standing-points.
What appears to be true in one instance is frequently not applicable in others; it is
the bold generalizations which render theories so often untenable in that form in which
they are usually first expressed. An instructive example is presented by the different
theories which have been proposed for explaining the mode of formation of mineral
veins. Almost every one of them was based upon a limited range of observations, and
was, from its first application to a few instances, extended to the generality of veins.
Numerous exceptions to it were then found, leading to the rejection of the first, and
the establishment of a different, theory, which, in its turn, shared a similar fate.  Obscure as this subject still is, we are able to state this as certain, with our present state
of knowledge, that every mineral vein is the product, not of one simple but of complex
processes. Nearly every one of the theories proposed will, therefore, have its limited
range of applicability, inasmuch as the agent it suggests may have been especially
active in the formation of the veins of a certain order, while the same agent may have
played a subordinate part in regard to the origin of other veins which were chiefly
due to processes of another kind.
This instance points clearly towards the one principal cause of the divergence
of opinions in regard to some of the most important geological questions.  This cause
is the want of latitude of the basis upon which arguments are founded.  Conclusions
which are obtained by reasoning on1 geological subjects solely on the strength of chemical analysis, are, when generalized, often found to be utterly in discordance with the
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INTRODUCTORY.                                   3
facts revealed by geological observation; and how unsatisfactory general theories may
be when based upon the latter alone, is sufficiently exemplified by the fantastical
attempts made in all ages of geological science to interpret the geological structure of
the world from that of a limited region.  The basis for argumentation can therefore
never be broad enough, and its enlargement should be, as it indeed is, one of the
chief objects of geological science. But it is not sufficient to content ourselves with
an accumulation of primary observations, which are in fact being infinitely increased
by the conjoint labors of geologists in all countries: it should be a higher object of the
student of geology, to compare the established results of observation, and to investigate their mutual relations. The study of the structure of one mountain range, or of
several ranges comprised within a limited district, may lead to the establishment of anll
elaborate theory of the mode of their formation, which may apparently answer perfectly well in that one case, but may be found inadmissible when generalized, even in
those cases where, by imperfect observation, one would expect to detect a great similarity to the structure first observed.  But in determining those features which are
conmmon to a number of mountain ranges, or to certain orders of them  which we may
discern among their generality, we may aspire to form  conclusions which are more
generally applicable.  It is particularly the auxiliary branches of geology to which
these remarks apply.  The value of observations made in limited regions, or fromn  a
limited point of viewv, on subjects such as the outlines of the morphological features of the continents, the occurrence of mineral springs, the structure of minieral
veins, the age of those among them which carry a certain metal, the generality of volcanic phenomena, the mode of action of earthquakes, the nature of certain kinds of
rocks, and their part in the structure of the surface of the globe-cannot be fully realized unless the comparative method is applied in as wide a scope as we may be able to
do, and the mutual relations among the different modes of tanifestation of force, or
among the properties of the kinds of matter upon which it acts, or the bearing of all
these relations to each other and to tile evolution of the globe, are investigted from
as many points of view as we may detect, and ill as many combinations as possible.
We may then be able to gain a foundation for argumnentation on more involved problems, consisting not of imperfect premises, nor of a confused aiccumulation of facts,
but of established truths of a higher order.
The mode of origin of the non-foliated crystalline rocks, made up of silicates,*
is among those subjects which have at all times, but at no time more than of late,
commanded a great deal of attention, and given rise to the establishment of numerous
theories, each of which was applied in a general way, if not by its author, then by his
followers. It is well known how conflicting they apparenltly are, and what weighty argumnents have been brought in favor of as well as against each of them.  The only
method, promising success, of weighing the merits of these different theories, or of
modiltfing therm  in accordalncec with the gelleral advance of science, appears to be, to
X I have for these applied the name " eruptive rocks" in the following pages, considering that, wherever we have
occasion to observe them, they are not at their original seat, but ejected from it towards the surface. The reasons supporting
this position will he more fully mentioned in the chapter on the origin of volcanic rocks.
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4                               INTRODUCTORY.
ascend from the examination of the nataire of these rocks to that of their mutual
relations, to investigate these from as many points of view as we can discover ill regard to physical and chemical properties, mode of occurrence and age, as well as in
regard to geographical distribution: that is, to try to establish the natural system of
eruptive rocks.  The results so obtained may theio, in accordance with what we just
remarked in a general way, be applicable to reasoning on remoter questions, of
which we can only attempt to find the most probable solution.  They regard chiefly
the causes of those relations, the mode of origin of the eruptive rocks, and the
processes connected with their ejection.  The intricate nature of the subject, and
the fact that the present changes on and below the face of the globe, as well as
the events of the past, are often but dimly and imperfectly perceptible to our observation, demand that we should concentrate our endeavors in exploring first the
laws of that which is definite and constant within the infinite range of phenomena, and
await further experience to arrive at an explanation of those isolated facts which form
apparent exceptions to the order of things.
It is with these views that the following pages were written. They extend
chiefly over the comparatively limited, and yet very extensive class of " volcanic rocks,"
and are offered as a mere elementary attempt, which is necessarily very imperfect.
The application to exact reasoning of the numerous observations which have been made
on the subject of volcanic rocks in different countries, is nearly prevented by the extraordinary discrepancy existing in regard to the mode in which the names of rocks
are used by different authors. The first condition of a uniform and harmonious mode
of observation on volcanic rocks, and the phenomena connected with them, is the application of a uniform system of nomenclature.
In concluding these preliminary remarks, I dare express the hope that some indulgence may be had with the imperfections of this essay, if it is taken into consideration that it was written on the Pacific coast, where chenmical laboratories are unknown,
libraries scarce, and little opportunity is afforded of becoming acquainted with the
current geological literature. I fulfill a deep-felt duty if I tender at this place my
sincere thanks to Professor J. D. Whitney, not only for allowing me the use of his
library and revising the manuscript of this essay, but also for the interest which he has
constantly taken in my pursuits, and for what I owe to his personal intercourse, especially
in a country where scientific communication is so extremely limited. The influence
of this intercourse will, long after this, be kept in grateful memory by all those who are
taking personally a part in the development of the California Academy, the members
of which kindly allowed this paper to be published in their Memoirs.
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TIHE NATURAL SYSTEM OF VOLCANIC ROCKS.
In reviewing the various attempts which have been made towards a classification of eruptive rocks-that is, those crystalline rocks made up of silicates, which,
without showing themselves any traces either of stratified deposition or foliation, enter
into the structure of the surface of the globe in such a way as to be unconformable
with the stratification of the neighboring sedimentary rocks, and as a rule to abut
against them without any gradual passage-we are struck by the observation that, if
they are based on any principles at all, these are usually artificial, while none but unsatisfactory results have been obtained when the application of natural principles has
been tried. This want of success is the more striking if we consider that it is peculiar
to petrology, and that the efforts made in the same direction with other branches of
descriptive natural sciences have been attended by extraordinary results.  In zoilogy
and botany, the natural system has long since been considered as the ultimate object
of scientific research; and since the time when its first outlines were discovered, the
progress of these sciences has been admirable. Since then only have the developments
of their different branches cooperated harmoniously towards one common end: the
profoundest investigations into the anatomy of animals and plants, the study of their
geographical distribution in modern time, and of their gradual development in past
ages, have in their final results but been subservient to the establishment of a foundation of the natural system, and the ingenious deductions made by Mr. Darwin on the
origin of species are but its philosophical interpretation. As regards mineralogy, classification was for a long time a simple enumeration of minerals, governed by certain artificial principles. A new era was inaugurated for this science by the progress
of chemistry, and its application to mineralogy, by Berzelius. It led to a more correct
estimate of those principles which had been formerly applied, and to the discovery of
the existence of an intimate connection between crystallographical form and chemical
composition. The combination of these two principles gave rise to the natural system
of minerals, which since their adoption has been constantly gaining in completeness.
These are results which surpass in a surprising degree those obtained in regard
to the natural classification of eruptive rocks. Even the most recent and elaborate systematical arrangements, as those proposed by C. F. Naumnann, F. Senfft, B. v. Cotta,
and J. Roth, though marking a conspicuous progress, are based on almost purely artificial principles. In no other branch of the descriptive natural sciences, it is true, do
difficulties arise so great as those which present themselves in petrology. Prominent
MEM. CAL. ACAD. SCd. VOL. I.                  F                 Jan. 1868.  (43)




6                               RICIITHOFEN-  THE  NATUR            SYSTEM
among these is the entire absence of what we could call "' genus," or " species," not to
mention " individual." If we should succeed in discovering some natural group to
which we might apply the  term   "family,"  (though  even  this can never be used  in
petrology  in  as definite a sense  as it is in  the  organic  kingdoms) we should  find
it made up of an infinite number of varieties; and if we should be able to
establish several groups, the main types of which are conspicuously distinct in
nature, we should find them  linked  together by gradual passage in chemical and mineral composition. It appears indeed utterly impossible to draw distinct boundaries
between cognate groups.   Certain names, such as those of granite, syenite, quartzose
porphyry, trachyte, basalt and others, have been applied to designate distinct types of
crystalline rocks, which can easily be recognized wherever met with. But, practically,
they have to be used for larger groups of rocks, in which those distinct types appear
like luminous centers, surrounded by clouds of varieties blending with each other in
such a way as often to render. it arbitrary whether to bring a certain rock within one
or the other denomination. But there are other groups of rocks, of larger dimensions
than the former, the nomenclature of which is far more indistinct, and which, in regard
to  classification, may indeed be  said to be  still inl  an  entirely nebulous state.   The
vague and arbitrary mode in which different names are used for them shows plainly
that difficulties in regard to them are greater than with other rocks. As will be seen
in the sequel, this indistinctness of external character, as well as of designation, applies
particularly (with the exception of basalt) to those rocks in the composition of which
silica takes a less prominent part, while those which are richer in silica offer much
more distinct characters. Most conspicuous among the names applied for the former
are those  of "trap,"  " greenstone,"  and  " porphyry."   The latter two may be conveniently used to designate groups of rocks having certain external characters in comnmon, but as generic terms they should all be completely abolished.' They never convey a definite conception, as each of them is used for a great variety of rocks, and
they have only too often been made to serve as a convenient cloak to cover ignorance.
The perception of these difficulties has caused the idea to be almost universally
accepted, that only an artificial system of eruptive rocks canll be established; that is,
that classification  should  be made dependent on  one certain  principle previously as1 This applies chiefly to the term " Trap," (or trapp) which had originally a definite meaning, but has gradually been
extended with wonderful elasticity. It was first introduced by Torbern Bergmann for a very ancient, dark colored, augitic
rock of Uddevalla, in Sweden, which is arranged in superposed layers abutting against the slope of the hill in the shape of a
stairway (trappar in Swedish). This rock would be called A" diabase" in modern nomenclature. The name " trapp" was
then applied to other, and gradually to all, dark colored eruptive rocks, particularly to such as were found occurring in
dykes; afterwards the " greenstones" (which name, too, has shown itself capable of wonderful distension) were included in
that denomination; and finally it has become still more comprehensive, though not quite to the same extent with every author. Its present meaning may best be seen from the following passage by Lyell, (Elements of Geology, 6th Ed., 1866, p.
601 of Am. Ed.): "' This term (lava) belongs more properly to that (melted matter) which has flowed either in the open air
or on the bed of a lake or sea. If the same fluid has not reached the surface, but has been merely injected into fissures
below ground, it is called trap."  Thus, the same name which was originally applied to a distinct rock of ancient origin has
been generalized so as to express now a mode of occurrence, and, what is more remarkable, a mode of occurrence the very
reverse of that exhibited by the original type, which must be supposed to have been flowing on the bed of the sea, and the
position of which in relation to neighboring rocks can never be explained by assuming it to have been " injected into fissures
below ground." Would it not be better to drop sqch a name altogether?
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OF VOLCANIC ROCKS.                               7
sumed as the point of issue. Taking crystalline texture, lack of stratification as well
as of foliation, and the fact of thleir being made up of silicates to be the ch-laracteristic
features of eruptive rocks in general. the most obvious external differences among
them are caused by the variations of texture and color.  During the early stages of
petrographical science, rocks were therefore classified on these principles. The terms
trap, porphyry, pearlstone, obsidian, lava, amygdaloid, wacke, as well as greenstone, black porphyry, green porphyry, and others, are the remnants, in our present
nomenclature, of that epoch when the more minute differences of rocks arising from
their mineral composition were but imperfectly investigated.  This principle was
necessarily next in order for serving as point of issue for classification, since, as far as
regards external characters, it is only second to the former in value.  The emergence of petrology from  a chaotic state, by the scientific application of this principle, dates from the investigation of Gustav Rose on the feldspathic minerals entering
into the composition of rocks, and it has since been more generally applied for
establishing subdivisions than any other.  The presence or absence of quartz, the
predominance among the feldspathic minerals of orthoclase, oligoclase, or labrador,
the presence of augite or hornblende. are the usual points of issue, even in the most
recent attempts at classification.  The high value of mineralogy as a basis of classification cannot be denied.  But its exclusive application has caused the combination into certain groups, of such rocks as front a geological point of view are
widely separated, while it has given rise to distinctions in cases where the results of
geological observation would demand close connection, as we shall have occasion to
illustrate in the following pages, with reference to those volcanic rocks which are composed of hornblende and oligoclase.  Gradually, those differences based on chemical
composition, not capable of being detected by the eye, and the knowledge of which
could only be obtained after chemistry had made the necessary advances, have become
an object of scientific research.  But this principle has not yet been used to any great
extent for classification.  It can easily be demonstrated that, when exclusively applied, it leads to a systematical arrangement of rocks which is in even greater contradiction with the natural mode of occurrence than when the same is based upon
mineral composition alone, notwithstanding the fact that its great value has been conclusively demonstrated, especially by the important results which Bunsen obtained
from the chemical analysis of rocks, and which mark an era in petrology.  To combine granite, quartzose porphyry, and rhyolite into one class, because they resemble
each other in their chemical composition, and to place them at the head of the list
because containing the highest amount of silica observed among eruptive rocks, would
be to take no regard whatever of geological facts. Rhyolite is, mineralogically and
geologically, far nearer related to trachyte than to either granite or quartzose porphyry; and these two are quite distinct from each other, while granite is closely allied,
by gradual passage, to syenite, and quartzose porphyry to porphyrite.
It is by slow degrees only that we call hope to reach a more scientific, that is,
a more natural system in this, the most intricate branch of descriptive natural sciences.
The natural differs from  the artificia:l system in this, that it starts from the alpplication
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8                      RICHTHOFEN - THE NATURAL SYSTEM
not of one only but of various principles, compares and weighs the results obtained by
each of them, and accepts them as final only when perfectly harmonizingi among each
other.  It is then that it tries to determine what principles are most available for
establishing the higher orders, and which for the subdivisions. The singular complication which is peculiar to the classification of rocks, is, besides the reasons already
mentioned, due in a great measure to the fact that geology combines the double functions of a historical and an inductive science, while in petrology we have besides the
requirements of a descriptive natural science.  The natural system of rocks should
therefore be based, not only upon the entire range of their petrographical characters,
such as mineral composition, chemical composition, texture, and specific gravity, but
also upon their mode of origin and geological occurrence. Classification of objects
and classification of relations are, with them, closely connected, and should be made
to assist each other.
The question may be raised, whether a natural system of rocks based upon
such principles can be established at all, and if it can, whether it would be of any use
for the advancement of science. To the first question, the answer must be in the
negative, as far as sedimentary rocks are concerned. They have been formed by a
complexity of circumstances, and just so complex and infinite in variety are they, in
respect to chemical and mineral composition and all external characters. To analyze
in detail their mode of origin, and the sources from which their material has been
derived, transcends the faculty of human intellect, and it would be a hopeless task to
attempt to discover ally laws regulating the boundless differences of their composition.
They are thus debarred from natural classification, though its principles may be applied
imperfectly to the establishment of some general groups. We arrive at similar conclusions in regard to those rocks, the sedimentary origin and subsequent metamorphism
of which can be proved. Accidental and local circumstances have played as conspicuous a part in their first deposition as was the case in regard to those sedimentary
rocks to which the term metamorphic has not been usually applied. But as metarmorphic processes of a certain nature have ordinarily affected extensive tracts of
these rocks, and similarly pervaded great thicknesses of them, the local differences of
their action having been apparently more in degree than in mode, they have occasioned a certain similarity of effect which partly conceals the original differences in
the composition of the rocks affected; and it appears that the differences in the kind
and intensity of metamorphic action, though recognizable only in their final results,
will, when better known, afford a convenient principle for a classification which may
have some similarity with, but not the full requirements of, the natural system. It is
different with those rocks which on the surface of the globe appear as intrusive or
eruptive masses. Notwithstanding their infinite variety in character and composition,
they are connected by definite relations which bring their elementary composition
even within range of mathematical calculation. Their recurrence in the most widely
separated countries, with similar external character, identical chemical composition,
and in analogous relative order of succession, is another distinguishing feature of
eruptive rocks.  For these reasons, as well as in virtue of other peculiar characters
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OF VOLCANIC ROCKS.                              9
which will be more fully mentioned in other pages, they appear to owe their present
positions to the action of general planetary processes, and to reveal by their own
nature that of the mineral matter participating ii the original composition of the
globe, and by their order of succession, the mode in which the same is arranged
beneath the theater of those changes which since a remote period have been taking
place on its surface. Only presumptive evidence can be adduced in favor of this
common and yet much disputed theory. The probability of its approximating the
truth could hardly be better established by any other evidence than by the proof that
all eruptive rocks of the globe, taking their historical part into account, are capable
of being brought into a natural system; or, to express it more correctly, that they
form among themselves a natural system, the laws of which we may be capable of discovering. Considering this in its widest bearing, as embracing all the definite correlations of eruptive rocks, and being indeed their philosophical expression, we may
expect that it will make us acquainted with the history of one great feature in the
development of the globe. Bearing in their own character and system the imprint of
their origin, the eruptive rocks will, by their nature itself, allow well-founded conjectures as to the interior structure and composition of the earth.  This, then,
together with a more perfect understanding of everything connected with the agencies
working below the surface of the globe, would be the philosophical use of the natural
system of eruptive rocks.
Only initiatory steps can be taken at the present time towards the establishment of this system. I have confined myself in this essay to an attempt at classifying,
in a way as natural as experience will allow, the " volcanic rocks," that is, the eruptive
rocks of Tertiary and Post-Tertiary ages. The term " volcanic rocks," has been chosen,
because the rocky matter ejected by active volcanoes belongs altogether to this class,
and because almost every kind of rock, generated by eruptive activity during the
period indicated, has partially forced its way through volcanic vents. We must keep
the two-fold mode of occurrence of volcanic rocks clearly separated in our minds. We
see them at the present day flowing from craters in the shape of lava, or being thrown
out as scoria and rapilli; and there is abundant evidence that their mode of origin
was very frequently the salne in past ages.  But in other places, it is perfectly clear that
volumes of matter of the same kind have been forced to the surface through extensive
fissures and accumulated above them in elongated ranges, when the origin of the outbreaks cannot be ascribed to volcanic activity.  These eruptions are evidently similar
in nature to those by which the greater part of the granite, syenite, or quartzose porphyry ascended to the surface in ancient times. We shall distinguish the two modes
of eruptions, for the use in this present paper, as " volcanic eruptions " and " massive
eruptions," and shall dwell in the sequel more fully on the difference between both
manifestations and their probable causes. Then, too, the reasons will be mentioned
which justify the uniting of all recent eruptive rocks into one separate class.
No other class of eruptive rocks offers greater difficulties for a systematical
arrangement, as none presents throughout so great a number of varieties, and so many
accidental modifications of texture and mineral composition.  On the other hand, their
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10                     RICHTHOFEN - THE NATURAL SYSTEM
classification is of especial importance, as it furnishes the key for deciphering the
natural system  of the ancient eruptive rocks.  Volcanic rocks issue from  volcanic
vents under our very eyes, and the record of the history of those which have originated in past ages, as preserved in their geological relations, is far more distinct than
it is in regard to their ancient predecessors. No doubt exists in the mind of any observer in respect to the eruptive origin of all basalt; while in regard to granite, very
different views are entertained by distinguished geologists, and supported by weighty
arguments.  In the former case, we have conclusive evidence, while in the second
speculation has a wider scope. The knowledge of volcanic rocks will, for this reason,
facilitate the correct interpretation of the nature of rocks generated in remote ages.
Volcanic rocks are widely spread over the face of the globe. It would be an
object of great interest to lay down their geographical distribution on maps, and to
explore the laws by which this distribution has been governed.  This has been tried
in regard to active volcanoes, and the importance and interest attaching to the results
obtained are such as to have given rise at once to speculations as to the connection
between volcanoes and other phenomena.  The value of those results would be increased, if to the active craters were added the vastly greater number of those extinct
volcanoes, the mode of preservation of which still allows us to recognize their former
nature.  Even then, howvever, the maps would convey but a remote idea of the general distribution of volcanic rocks; the areas comprised by which should be marked
out with proper distinction of their main subdivisions.  Besides an immediate bearing
on more special geological questions, the knowledge of these subjects proinises to be
of high value for that entire department of geological science, by which the latter is
most closely connected in scope with the science of physical geography. Many weighty
problems, such as the causes of the present direction and extent of mountain ranges,
of the outlines of continents, of the position and shape of groups of islands, of the
secular oscillations of the surface of the globe, and many other questions, appear to be
intimately connected with the geographical distribution of volcanic rocks and their
mutual geological relations.  The deduction from  the latter of definite laws appears
to be the initiatory step towards understanding the laws of eruptive activity of
remote times, and, thereby, towards establishing a chapter in the history of the globe,
which is among the obscurest and least understood.
The following classification, in which existing names are retained, as nearly as
could be done with convenience, is chiefly founded on observations made in the Carpathians and in the States of California and Nevada. In respect to the variety and the
distinctness of the mutual relations of the volcanic rocks, these two countries are hardly
surpassed by any in which this subject hleas, up to this time, been scientifically investigated. Until recently, all volcanic rocks, at least those of more frequent occurrence,
were comprehended in the terms: trachyte, phonolite, trachydolerite, dolerite, basalt;
while, besides, separate narnes were used to distinguish modifications of texture, such
as pumice-stone, obsidian, pearlite; or varieties somewhat more distinct in point of mineral composition, such as leucitophyre. The classification as given by Al. von Humboldt,
ill the fourth volume of the " Cosmos," may be considered as having represented the
C7Th)




OF VOLCANIC ROCKS.                                 11
most advanced stage of the science a few years ago, because it was guided by a definite
principle; though when compared with contemporaneous works on petrology, it gives,
oni the other hand, a remarkable illustration of the great difference in the  vay in which
the same names have been applied.  Since then, the name " rhyolite " (Richthofen,
Studien aus den ungarisch-siebenbiUrgischen Trachytgebirgen, in Jahrbuch der K. K.
geologischen Reichsanstalt in Wie, Vol. XI [1860], pp. 153-277) has been introduced
for a very distinct class of volcanic rocks. Adding this to the previous list, there
results a number of names which, in geological treatises, are either grouped completely at random,2 or in an arbitrary order, or arranged by artificial principles, when
the whole classification ordinarily comprises volcanic and ancient eruptive rocks promiscuously. In order to establish a more natural system, we have, not to make groups,
but to find them. Dropping all of those d priori principles which may be conceived
having an artificial basis, we must endeavor to discover xvhether any great divisions
are established by nature herself, and if so, of what character they are.  We may
then apply, as second in the order of their importance, those results which are obtained
in the laboratory or geological cabinet, for defining and subdividing those groups.
Most of the natural divisions which may be derived from geological observation, coincide essentially with those based on artificial principles, but are more naturally
limited as regards each other. Each of them has its own more or less independent
part in the architecture of mountain ranges, and a distinct geological age in reference
to the other groups. Each of themn comprehends a series of rocks, which, besides, are
closely connected by the relations of their petrographical characters, chemical composition, texture, specific gravity, and other properties. The test of the natural foundation and general validity of these grloups will be their recurrence, with mutual relations unchanged, in different parts of the globe, of which test we are never to lose
sight.
The following is the classification, the approach of which to a natural system
of volcanic rocks, I will endeavor to set forth in the course of this paper:
ORDER FIRST: Rhyolite.
Family  1. Nevadite, or granitic rhyolite.
" 2. Liparite, or porphyritic rhyolite.
"    3. Rhyolite proper, or lithoidic and hyaline rhyolite.
ORDER SECOND: TSrachyte.
Family 1. Sanidin-trachyte.
" 2. Oligoclase-trachyte.
2 That this is even done in books of the highest standard, may be seen by reference to one so prominent as Lyell's
Elements of Geology. The following is the order of names of which, under the head of "volcanic rocks," definitions are
given (p. 592, pp. of 6th Am. ed., 1866): Basalt, augite rock, trachyte, trachytic porphyry (in connection with which the
name' andesite " is mentioned), clinkstone, greenstone, porphyry, amygdaloid, lava, scorize or pumice, volcanic tuff or traptuff, agglomerate, laterite.
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12                      RICHTHOFEN — THE NATURAL SYSTEM
ORDER THIRD: Propylite.
Family 1. Quartzose propylite.
2. Hornblendic propylite.
"   3. Augitic propylite.
ORDER FOURTH: Andesite.
Family 1. Hornblendic andesite.
"   2. Augitic andesite.
ORDER FIFTH: -Basalt.
Family 1. Dolerite.
"   2. Basalt.
"   3. Leucitophyre.
ORDER FIRST- RHYOLITE.
The name "rhyolite" was proposed, early in 1860,3 for certain rocks frequently occurring on the southern slope of the Carpathians, and distinguished, in mineral character, from trachyte, which they otherwise resemble, by the presence of quartz
as an essential ingredient, and an almost infinite variety of texture.  Beudant4 had,
long before, described certain varieties of these rocks as porphyre trachytique, pumicestone, pearlite, etc. In 1861, the name " liparite" was proposed by J. Roth 5 for rocks
of similar nature occurring on the Lipari Islands.  The term  "rhyolite," however,
being of prior date, has since been almost generally adopted, among others by F. v.
Hochstetter, for rocks from New Zealand, by C. Peters, G. Stache, and others for those
of Hungary and Transylvania, by Ferd. Zirkel for those of Iceland, by B. v. Cotta as
a general term  in his " Gesteinslehre."  The word "rhyolite" is designed to express
one of the prominent features of these rocks. It is this: that their chief varieties
have the appearance, as it were, of natural glasses, and bear evidence, more than any
other rocks do, to the unpracticed eye, of having been flowing in a viscous state.
Mode of Geological Occurrence.-Rhyolite has had its distinct epoch of eruption in
relation to other volcanic rocks.  Wherever it occurs it may be easily proved to have
been of more recent origin than either propylite, andesite, or trachyte, but to have
preceded basalt in age. As to its geographical distribution, it is confined to the immediate neighborhood of one or all of those antecedent rocks, and occurs ordinarily
within their very limits. Until lately, not much attention had been paid to it. But
since the establishment of the name, rhyolite has been found to be widely distributed,
though always occupying a subordinate position. In Hungary it usually skirts the
lower part of the flanks of andesitic ranges, forming hillocks and ridges of little
elevation, filling depressions, and issuing in currents from  fractures, and, in general,
giving evidence of its entire dependency on the places of previous eruptions.
The greater portion of the rhyolite has, in that country, been evidently ejected
by volcanic activity. It appears' that the same may be said in regard to the
3 Loc. Cit.   4 Voyage en Hongrie. Paris, 1820.    5 J. Roth, die Gesteins-analysen. Berlin, 186f.
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OF VOLCANIC ROCKS.                                    13
rhyolitic rocks of New  Zealand, St. Paul, and  Iceland.   But it has not been
so generally the case on the western coast of America.  The volcano of Lassen's
Peak, and the environs of Mount Helena, in California, present grand instances
of a volcanic  origin of rhyolite.   But in the  adjoining  State of Nevada it appears to have been extensively brought to the surface by massive eruptions. It is of
unusually frequent occurrence along the eastern slope of the Sierra Nevada, and far^ther east in the Great Basin. Ranges of hills are there completely built up of rhyolitic rocks, not always in as close proximity to such as are antecedent to them in age,
as is the case in the Carpathians.
It is one of the characteristic features of rhyolite, that it presents, more than
any other rock does, signs of having been in a state of what Daubree has called
" aqueous fusion," or the fusion of its mass by solution, under great pressure, in superheated water. Another peculiarity is the circumstance that the eruptions of rhyolite,
whether massive or volcanic, bear evidence of having been generally accompanied by
extremely violent solfataric action, which probably surpassed, on an average, that connected with the ejection of other volcanic rocks. This action appears to have been
one of the chief agents in the formation of the rich silver-bearing veins of Hungary,
as well as of some in Mexico, and to have also been peculiarly characterized by the
occurrence of an unusually large amount of fluorine and chlorine among the escaping
gases.
Mineral Composition. -Rhyolite may boe concisely defined as trachyte with an
addition of silica, not chemically combined, and which is either segregated into crystals
of quartz, or dissolved in the rock, and then no longer recognizable to the eye.  It is,
owing to the high proportion of silica entering into its composition, the representative
of granite among volcanic rocks.   Chemically, it is its complete counterpart, as far as
ascertained by analysis, and even in outward appearance certain varieties of rhyolite
offer at first sight a striking similarity with granite, though closer observation will at
once reveal well marked differences between both.  Rhyolite outrivals any other rock
in respect to the truly astonishing number of its varieties, which are chiefly occasioned
by modifications of texture. It consists in general of a paste, with or without minerals
enclosed.
The paste, chiefly, is liable to variation.  Its colors are: white, gray, yellow,
green, red, brown, which occur in all mannler of shades; light ones prevail, while
perfect black has not been met with. The texture is as varied as the color. First
are to be noticed a number of hyaline varieties, which are represented by obsidian,
pumice-stone, and pearlite, and the frequent occurrence of which is a peculiar feature
of rhyolite. Though associated with volcanic rocks of every composition, these natural glasses, as they may be called, decrease in relative quantity and variety with the
decrease of silica.6 Obsidian when having the composition of rhyolite, offers little
6 There is no better example of tile artificial principles on which the classification of rocks has usually been
based than the fact, that accidental modifications of texture, which appear to result chiefly from the difference of the conditions attending either the fusion or the cooling of the mass, have been considered as of equal value with other differences of
the greatest importance; and pumice-stone, trachyte, basalt and pearlite have been considered as coordinate subdivisions,
even long after classification had been made dependent chiefly on mineralogical principles.
G                                                                       (51)




14                        RICHTHOFEN-NATURAL SYSTEM
difference in character from that which is a modification of other volcanic rocks. In
regard to pumice-stone, however, Abich has proved that when formed of the material
of trachyte or andesite it has rounded pores, and ordinarily a green tint; while those
varieties which have the composition of rhyolite, excel by the elongated and irregular
shape of their cavities, which are enclosed in a fine tissue of fibres of silken appearance
and white color. Between both kinds of pumice-stone there are gradations apparently
dependent as to their character on the amount of silica entering into their composition.
Pearlitic texture is peculiar to rhyolitic rocks. From these more or less perfectly glassy
varieties there are gradations to the texture of enamel and porcelain, and to a certain
cryptocrystalline texture very frequent among volcanic rocks in general, and for which
we may apply the obsolete term " lithoid." This passes into the microcrystalline, and
always more or less vesicular, texture of trachyte.
The paste constitutes occasionally alone the substance of the rock. But these
instances are rare. More frequently it contains enclosed mineral substances differing
from it in nature, and in a few instances these accumulate to such a degree as almost
to exclude the paste. Quartz is of the most general occurrence among those which
are crystallized. Sanidin is its almost unfailing companion. Oligoclase, usually of a
vitreous variety, and black mica, are, too, among the usual ingredients, while hornblende is generally less conspicuous. Besides these minerals, there are two substances
entering accidentally into the composition of rhyolite, which are, however, among the
characteristic features of the rocks of this order. One of them consists in small globular grains, from the size of a pin-head to that of a rifle bullet, called " sphmrolites "'
by Beudant. They have a radial structure, and contain ordinarily a small crystal of
feldspar in the center. Certain hyaline, and, in a greater measure, lithoid varieties
of rhyolite contain them in large quantity.  They occur also, though less frequently,
in other natural glasses not of rhyolitic composition, and may be produced artificially,
by allowing molten glass to cool very slowly to what is known by the term " Raumur's porcelain." The second formation frequently met with in rhyolitic rocks are
the "lithophyse," consisting in larger and smaller cavities filled by a substance
strangely inflated by some gaseous evolution which apparently originated in the matter itself (Richthofen, 1. c.). They constitute sometimes nearly the entire mass of
the rock.
The endless varieties of rhyolite appear to be due to the susceptibility of the fluid
mass to be influenced by accidental circumstances to which it may have been exposed,
partly before being ejected, and partly during the process of solidifying. A considerable influence, which, however, has not yet been investigated, is probably exercised
by the difference in the amount of water which entered into the composition of the
molten mass, and partly expanded to steam in the instant of ejection. The vesicular
inflation proper to trachytic texture, the spongy inflation of pumice-stone, and the
concentric separation of infinitely fine laminge, as is often shown in perfect pearlitic
texture, are probably three different modes of manifestation of one slightly varied
cause, which may most likely be found in the conversion of water into steam, which
participated in the composition of the molten mass.
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OF VOLCANIC ROCKS.                              15
Difference of Rhyolite from  other Rocks nearly related to it. —Several varieties of
rhyolite bear so close a resemblance to other rocks, that some mention must be made
of their distinguishing features. Rhyolite may be easily distinguished friom granite. The
varieties by which it mostly approaches the same are those which contain crystals of
quartz, feldspar and mica in unusually large proportion and size. But in the case of rhyolite, the paste in which they are imbedded is never wanting. Moreover, the orthoclase
and oligoclase are of the vitreous varieties, and quartz is present either in crystals or in
rounded crystalline grains, while in granite it usually permeates the interstices between
the other component minerals. Much closer is the affinity which certain other varieties of rhyolite bear to quartzose porphyry, especially those which have a paste of
homogeneous appearance containing no crystals but those of quartz inclosed. Geological observation will never fail, in such instances, to establish the nature of the
questionable rock, as it will show its association either with true rhyolite or with
true porphyry. The same test has to be applied occasionally with respect to some
other varieties which contain the silica equally diffused through the paste, and bear a
close resemblance to trachyte. In this instance, however, even geological observations
will sometimes fail to determine the exact position.  There is a gradual passage in
character between every two nearly related rocks, such as rhyolite and trachyte, or
granite and syenite, and it frequently happens that either name may be used with
equal right.
Subdivisions.-In establishing the subdivisions of most orders of eruptive rocks,
mineral composition affords a principle, not only the most convenient for practical
application, but one that answers well the requirements of the natural system, when
made subordinate in value to'those higher principles which determine the limits of
classes and orders.  In the case of rhyolitic rocks, however, it is not as applicable as
in that of other orders. They should, from this point of view, be subdivided into
those which contain quartz and those which are devoid of it, or into such as carry
sanidin and such as contain both sanidin and oligoclase. But, since rhyolite of any
certain chemical composition may contain its surplus of silica, either visibly segregated in
crystals of quartz, or dissolved in the mass of the rock, and as the case may be similar
in regard to the occurrence of either species of feldspar, the application of this principle would lead us to combine into one group quartzose rocks differing considerably
among themselves as to the proportion of silica they contain, while another group
might comprehend rocks of virtually the same nature as those of the first, and differing
from them only accidentally in external character. More natural subdivisions of rhyolitic rocks are obtained by taking as a basis of classification their difference in texture,
which either approaches, to a certain degree, that of granite or is porphyritic or hyaline.'It is a singular fact, and one difficult of explanation, that rhyolite, at every place
where it has been hitherto observed, presents, either solely or chiefly, one of those
three modes of texture. Lassen's Peak, for instance, presents the granitic variety
almost exclusively. Sonoma, in California, and the Tokay Mountains in Hungary,
only the hyaline, and other places exclusively the porphyritic varieties. This circumstance, which is peculiar to none but rhyolite among eruptive rocks, indicates the




16                        RICHTTHOFEN —NATURAL SYSTEM
dependency of the mnode of texture upon deep-seated influences which acted at the very
source of the eruptive matter, and produced a certain molecular condition of the latter,
varying at each locality. Some light on this subject may be expected from minute
geological observation, accompanied by exact chemical and microscopical researches.
The following are the subdivisions which may be distinguished in regard to the
texture:.Parn. 1st. Nevadite or Granitic Rhyolite.-The name " Nevadite " is derived from
that of the State of Nevada, where these rocks have been first met with in larger
accumulations. The local derivation may answer in this instance, as granitic rhyolite
is little known from  other countries, excepting the neighboring California.  In the
Carpathians, it occurs isolated in Transylvania, but by no means as characteristic as in
Nevada. The name " granitic rhyolite " is designed to indicate the general resemblance of these rocks to granite, which is conspicuous in boulders, or on large exposed
faces, but disappears on closer examination. It is chiefly produced by the similarity
in color, which is of light shades of gray and red in Nevadite, and by some affinity
in mineral composition. Nevadite contains crystals of quartz in large proportion; the
corners are usually rolunded, and the quartz itself cracked, like glass when rapidly
cooled.  Sanidin occurs in crystals of larger size than oligoclase, sometimes of an inch
in diameter. The crystals of both are often cracked throughout their mass, and
rounded at the corners. Black mica and hornblende are ingredients of frequent occurrence. These minerals are, in more or less quantity, enclosed in a paste which is
probably a partially microcrystalline, and partially amorphic aggregation of the same
ingredients, and has a highly vesicular texture, rendering it rough to the touch, even
more so than is the case with trachyte. Geologically, Nevadite appears to have been
produced as frequently by volcanic activity as by massive eruptions. An interesting
occurrence is that at Lassen's Peak, in California, where it was discovered by Prof. W.
H. Brewer and Mr. Clarence King.
Fam. 2d. Liparite, or Porphyritic Rhyolite.-The name "Liparite," which was proposed by J. Roth for this whole class, may be conveniently retained for those varieties
of rhyolite which approach quartzose porphyry in character, as they appear to occur
on the Liparic Islands, either solely or at least in larger proportion than other varieties.
They consist of a paste which has a similar texture to that of quartzose porphyry, and
incloses crystals either of quartz only, or of quartz and sanidin, or of quartz, sanidin,
oligoclase, and black mica, or of one or both kinds of feldspar, without quartz being
present.  The crystals have sharp corners and are seldom cracked; oligoclase is rarely
of the vitreous variety. Typical varieties of the rocks of this family occur largely in
the hills of Bereghszasz in Hungary.
Farn. 3d. Rheyolite proper, or Hyaline Rhyolite.-The extensive range of varieties
afforded by all manner of modifications of hyaline texture are a peculiar feature of
rhyolite, distinguishing it from any other eruptive rock.  In outward appearance they
remind one of artificial glasses cooled under the most varied conditions. Obsidian,
purnzice-stone, and pearlte, constitute but a small portion of the varieties occurring;
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OF VOLCANIC ROCKS.                              17
others resemble enamel and porcelain, or present appearances which are difficult to
describe. The occurrence of sphmerolites and lithophysoa add to the variety of their
aspect. Another feature peculiar to the rocks of this family consists in a foliated
structure, the folia being often thinner than paper, and presenting an endless variety
of color and modifications of texture. Pearlite alone does not participate in the peculiar form of foliated structure.
All these varieties again contain, enclosed, all the different minerals before mentioned, or only a few of them, or they enclose no foreign substances at all.. The rocks
of this family are chiefly of purely volcanic origin, but in some instances currents of
them appear to have been ejected through crevices in the older volcanic rocks.
ORDER SECOND-TRACHYTE.
The name "trachyte " was first used by Hauy, in his academic lectures, to
designate the well-known volcanic rocks composing the Drachenfels on the Rhine.
But it did not come into general use, until Beudant, the pupil of the former, introduced the name into geological literature, by his justly celebrated work " Travels
through Hungary," a book which abounds in sagacious observations on the subject of
volcanic rocks. Beudant extended, however, the application of the name over a much
wider range than his teacher had done. Several years later, in 1835, L. v. Buch
introduced into literature the name "andesite," designating by it certain darkcolored rocks, which were then known, especially through the collections of Al. v.
Humboldt and Boussingault, to enter largely into the composition of the volcanic portions of the South American Andes. These rocks, which form obviously a part of
those which Beudant had comprised under the name " trachyte," were supposed to
be particularly distinguished by containing a peculiar species of feldspar which Abich
(in 1840) called " andesine." When, however, a few years later, this mineral was no
longer considered to be a separate mineralogical species, and its name was dropped,
that of " andesite " became obsolete with it, notwithstanding its prior origin. Thenceforth, the ranger of the varieties of volcanic rocks comprised in the term  " trachyte "
has been even more enlarged than Beudant had proposed in his dissertation. Eruptive
rocks, widely differing in nature-in fact nearly all those of tertiary and post-tertiary
age, with the exception of basalt-have been united in it.  But by no author, probably, was the application of the name as much extended as by Al. von Humboldt.7
On the other side, however, he was the first, by the establishing of numerous subdivisions, to draw attention upon the necessity of using separate names for more limited ranges of varieties. Recently, B. von Cotta, J. Roth, and others, have tried to
demonstrate, how little reason there had been for dropping the term  "andesite."
They re-introduced it into petrology; but, in drawing the limits between andesite and
cognate groups by principles of artificial classification, they used the name in a sense
differing to some extent from that in which it has been applied in the following pages.
7 Cosmos, vol. iv. The first four orders of trachyte were proposed by G. Rose, the other two were added by
JHumbolclt.
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18                       RICHTHOFEN-NATURAL SYSTEM
There is plainly indicated, from a geological point of view, the existence of two large
groups within the limits of Beudant's " trachyte." The two existing names, " trachyte "
and " andesite," may conveniently be used for their designation, since those rocks,
for which either of the two names was first introduced, are indeed the types of the
two natural groups.
Mode of Geological Occurrence. —The trachytic rocks have had their independent epoch of eruption in every volcanic country. They preceded rhyolite and basalt
in age, and were posterior to the ejection of propylite and andesite. The trachytic
epoch was usually of long duration. In many localities, its later part blended with
the earlier of the rhyolitic epoch, which is manifest by the alternate emission of trachytic and rhyolitic matter, during that time which was intermediate between the epochs
of the ejection of the principal bulk of either of them. As regards geographical distribution, trachyte is as much dependent upon, and as closely allied to, the preexisting
masses of propylite and andesite, as is the case with rhyolite. It towers up in peaks,
cones, and ridges, which are distinguished by their rugged outlines, and rest, in the majority of cases, upon the summits or the flanks of the ranges composed of those older
volcanic rocks. But there are also numerous instances when these may be seen to be
accompanied, at some distance, by trachytic outbursts, when a cursory examination might make the latter appear to occupy an independent position. Trachyte does
probably not compose, by itself, any extensive mountain ranges, and it remains, in
general, greatly inferior in bulk to andesite. In Europe, its outbreaks were scattered
and isolated, and, though they have been quite numerous, the aggregate quantity of
trachytic rocks is not considerable. They occur in Hungary and Transylvania, on the
Lower Rhine, in Central France, in the Grecian Archipelago, and in other parts of
that continent, as well as in the adjacent portions of Asia. Specimens of trachyte,
owing to their beauty and varied aspect, are usually much more numerous in geological
cabinets than those of andesite-a fact which has frequently occasioned some misconception regarding the relative proportion and importance of trachyte and andesite
among volcanic rocks.
In the structure of the North American Andes, trachyte takes a more irmportant part than in Europe. A continuous range of it, at least ten miles in extent, and
forming rugged crests, encircl's the Washoe Mountains to the east, in the shape of a
crescent. Trachyte rests there on propylite, and its ejection has probably had an
intimate connection with the formation of the Conlstock Lode. Other accumulations
of similar extent may be noticed at Esmeralda, on the eastern slope of the Sierra
Nevada, around Red Rock Canon, south of Walker's Pass, in the surroundings of Lake
Tahoe and Sierra Valley, and at other places east and west of the Sierra Nevada. In
the northern provinces of Mexico, trachyte is known to occur quite extensively. At
all the places mentioned, the greater part of its bulk bears evidence of having been
brought into its present position bymassive eruptions, while traces of extinct trachytic
volcanoes are scarcely wanting at any of them. Trachyte is still being ejected by a
number of active volcanoes. Among them may chiefly be mentioned those of Central
America, the greater part of the modern lava of which has been proved to have the
clhemical and mineral composition of trachyte.
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OF VOLCANIC  ROCKS.                                               19
Mineral Coonposition.-Trachyte is only inferior to rhyolite  in the number of its
varieties. Yet its chemical composition is as simple as that of the latter, and ranges
within as definite limits.   The rock appears to contain, on an average, from  60  to  65
per cent. of silica, and is in this respect, as in others, next allied to rhyolite. The
difference between the rocks belonging to  both  orders, having  its fundamental cause
in the chemical composition, manifests itself externally in certain differences of charac-ter, among which may be mentioned: the absence of quartz among the essential
ingredients of trachytic rocks, which probably contain no free silica at all; the usual
predominance in them  of oligoclase over sanidin; the larger proportion in which hornblende participates in their mineral composition, and  the fact of their specific gravity
exceeding that of rhyolite.   Like almost all volcanic rocks, trachyte consists of a paste
in which are imbedded various crystallized minerals. This paste is of various colors,
and  has usually a more  or less vesicular texture, which, by its property of imparting
to the rock a certain roughness'of touch, gave origin to the name. Chief in order
among the enclosed minerals are sanidin, oligoclase, mica, and hornblende.  They vary
considerably as regards their relative proportion, and therefrom arises, a number of
varieties which  have  been partly distinguished  by separate  names.   More numerous
varieties, however, are occasioned by the differences of texture. Pumice-stone is of
frequent occurrence among the latter, but it never exhibits that perfect long-fibrous
and silken nature peculiar to it when being a variety of rhyolite. Obsidian, in different grades of perfection, is no unusual modification  of the  trachytic masses, and  it is
often filled with sphaerolites, while no pearlite has yet been found having the chemical
composition of trachyte. Foliated structure may frequently be met with; but the
folia are not of that exquisite  fineness which  is  peculiar to those of rhyolite.   The
modes of texture which alternate in the folia, are chiefly obsidian, pumice-stone, and
microcrystalline varieties.
Su6divisions.-It appears that two  natural groups  of trachytic  rocks may be
distinguished, which differ at the same time, from a mineralogical and chemical point
of view, and have therefore been arrived at similarly by the application of artificial
principles.8 We distinguish with B. v. Cotta:
8 I may here remark that I have endeavored to retain existing names, as much as possible, for designating the orders
and families distinguished in this present classification. What I have tried to establish as natural groups, may therefore, on
account of the similarity of nomenclature, coincide apparently, in many cases, with the groups established by artificial
principles, and named in accordance with them. The former do indeed coincide in a few instances with the latter; but in
the majority of cases, the limits of the application of the names differ widely when established from the two points of view
mentioned. It would require too much space to go into detail on this point in regard to every name. I will, therefore,
confine myself to an illustration regarding the order of trachytic rocks. B. v. Cotta, (with several other authors) unites
under the name of oligoclase-trachyte all volcanic rocks consisting chiefly of hornblende and oligoclase, with the exclusion
of certain dark-colored rocks, to which he applies the name andesite. The former name, if used in the meaning of that
author, comprises, therefore, our order of propylite, together with all those rocks for which the term oligoclase-trachyte has
been here applied. It is evident, from the different geological positions occupied by propylite and trachyte, as well as from
the distinct petrographical character which either of them exhibits when occurring in large accumulations, how much their
union would be opposed to a correct representation of natural relations. It is obvious, besides, that the union is unpractical. It would actually prevent the possibility of a clear and simple geological description of those countries in which
propylite and trachyte occur together with other volcanic rocks. The distinction between both is so great that it has
scarcely ever failed to be noticed by unbiased observers in those localities where both rocks occur. To unite them because
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20                                RICHITHOFEN-NATURAL  SYSTEM
Fam. 1st.  Sanidin-trachyte.-The color of the paste  varies, but it usually pre.
sents light shades of gray, reddish, and reddish-brown; its texture exhibits all the
varieties mentioned. There are imbedded in it: crystals of sanidin, or of both sanidin and oligoclase, besides mica and hornblende; the latter, however, is frequently
wanting. To this family belong the rocks which compose the trachytic ranges of
Washoe and Esmeralda, that of the Drachenfels, and many others.
Fam. 2d.   Oligoclase-trachyte.-The paste is of the same color as in the rocks of
the first family, though darker shades prevail, and presents a similar variety of texture. Imbedded are, chiefly, crystals of oligoclase and hornblende, the former being
frequently of the vitreous variety; the latter having usually the shape of broad
needles, with  a  black  color and  bright cleavage-planes.    Besides those minerals,
black mica is of frequent occurrence.   The rocks of this family are ordinarily associated with those of the first subdivision, but in some localities are not accompanied  by
them. At Lassen's Peak there is but one limited space where rocks of both families
intermingle; it is near the place where the lava has been ejected.  Apart fronm  it, the
grand currents of lava, extending to from ten to twelve miles distance from the place
of ejection, consist merely of oligoclase-trachyte. To this family belong those varieties of trachyte which were called " domnite " by L. v. Buch.
The fact that the  occurrence of the rocks of either one of these two families
will frequently exclude those of the other, and that, even in those localities where
they are associated together, they will occupy separate places in regard to geological
superposition, appears to indicate that the distinction of these two subdivisions forms
an approach to the requirements of the natural system.
ORDER THIRD-PROPYLITE.
The rocks of this order have hitherto  occupied a very  undecided  position  in
the different classifications of rocks proposed, and just as various has been their nomenclature when they had to be mentioned in geological descriptions. The fact that they
bear close resemblance in mineral character to ancient diorite, while, geologically, they
are intimately allied to volcanic rocks, has been the principal cause of this uncertainty
of their position.  In  Hungary  and Transylvania, they occur quite  extensively, and,
being of practical importance as the bearers of rich metallic veins, have had to be
noticed frequently in treatises on the mines of those countries. Beudant applied for
them the name " porphyric greenstone," and classified them, along with syenite, among
they are both chiefly composed of oligoclase and hornblende, would render it indeed, practically, a very difficult and complicated task to compare the geological relations of different volcanic countries on the strength of written descriptions. If we
now follow the classification of J. Roth, we find the application of the name trachyte limited to those volcanic rocks which
contain sanidin, but are devoid of quartz. All those numerous varieties which do not contain sanidin, but are intimately
allied to sanidin-trachyte, by their mode of geological occurence as well as by their physical characters, are excluded from
the denomination, and, together with all volcanic rocks composed chiefly of hornblende and oligoclase, are united into one
subdivision of andesite. The reasons which justify the separation of the compounds of oligoclase and hornblende into three
different groups, (oligoclase-trachyte, hornblendic propylite, and hornblendic andesite) will be detailed in the following pages.
It may then be understood why the adoption of the different systems of the current nomenclature would render the concise
geological description of volcanic countries extremely difficult, and would conceal the harmnony really existing in the relations which they present in different countries.
(58)




OF VOLCANIC ROCKS.                             21
the "''transition rocks."  Since then, the names greenstone, greenstone-porphyry,
diorite, dioritic porphyry, and others, have frequently been applied for the same rocks
of the Carpathians. Similar names have been used for them when they were mentioned
as occurring in other countries, as for instance Mexico, where still oftener they have
been simply styled " porphyry."  In 1860, having had sufficient evidence of the Tertiary age of these rocks and their close connection with the volcanic rocks of that
period, I separated them, in a treatise on some volcanic countries in Hungary, already
referred to, by the name of " greenstone-trachyte," from the remainder of those rocks
which then were usually comprehended by the name trachyte. That designation has
since that time been fiequently applied in geological descriptions. The attempts made
to classify the rocks included in it have, however, been rather unsuccessful.  J. Roth
combines them into one group with amnphibol-andesite, from which they are quite distinct as regards their petrographical as well as their geological propertiesd; while others
considered them as belonging to the dioritic rocks. The grounds upon which they
have been united with the latter are purely artificial, since diorite is, from a geological
point of view, widely separated from " greenstone-trachyte."  Breithaupt established
a new name, " Timacite," for a variety of our propylite, which is of very limited occurrence, and in which he discovered a new variety of hornblende, called by him " gamsigradite."  Most valuable contributions for the knowledge of our "propylite," were
recently given by Dr. Guido Stache, from observations made in Transylvania. (Hauer
and Stache, Geologie Siebenbiirgens, Vienna, 1863.) Hie discovered the occurrence,
in that country, of quartz-bearing varieties in greater extent than they have been
found hitherto at any other place.  Stache retains the name " greenstone-trachyte,"
for those varieties which contain no quartz, and proposes the name " Dacite " (fromn
the Roman province of Dacia to which Transylvania belonged) for those of which
quartz is a common ingredient.
These statements will show the discrepancy of the views which have been
entertained in regard to the systematic place and the nomenclature of the rocks
under consideration. It is owing, partly to their twofold affinity with other rocks,
(mineralogically to diorite, and geologically to volcanic rocks) and partly to the fact
that they had, until lately, not been made the object of study. All observations made
during the last few years concur in this, that those rocks, wherever they have been
encountered, form a distinct link in the range of Tertiary and Post-tertiary eruptive
rocks, being everywhere the first of them in age, while they are, in regard to their
mineral character, no less distinct from any other eruptive rocks originated in those
periods. They constitute, indeed, a more natural and more distinct group than any
of the other volcanic rocks, and it has become desirable to unite themn under a comnmon designation. As no prominent property distinguishes them from diorite, and the
derivation of the name from one certain locality did not appear proper for the designation of rocks of wide distribution, geological relations alone could be used as a basis for
the nomenclature.  The roclks under consideration, as we shall hereafter more fully
develop, give evidence, in all localities where they have been met with; of having reopened the eruptive activity after ages of comparative repose. It is since then only,
1(9)




22                        RICHTHOFEN —NATURAL SYSTEM
that this activity has continued with extreme violence over all parts of the globe,
through the remaining part of the Tertiary and the Post-tertiary periods, growing,
however, more and more faint during the latter. Propylite was, in fact, the precursor
of all other volcanic rocks, and its appearance on the surface inaugurated a grand
revolutionary activity on the globe.  It is this position, at the entrance as it were to a
new era in the history of the earth, which has given rise to the name " propylite."
Mode of Geological Occurrence.-The position of propylite at the bottom of all
volcanic rocks is its most important geological feature.  Its age has in no instance been
ascertained with exactness. The nearest approach to its determination was made in
Northern Transylvania, where I found, in several localities, nuinmulitic strata intersected by dykes and large intrusive masses of propylite, and covered by accumulations
of it.  As the greater part of the volcanic rocks in that country are of Miocene age,
the ejection of propylite must have taken place either in the latter part of the Eocene
or in the earlier part of the Miocene epoch.  In view of the facts, that volcanic rocks
have nowhere been observed to be anterior to the Eocene (probably even not prior to
the Miocene) epoch, and that propylite is always allied to them as the first link in the
order of succession, it may be inferred that propylite is, in general, of Tertiary age,
until proofs to the contrary may be found.
The forms of propylitic mountains can be observed only in rare instances, since
they are usually covered by other volcanic rocks, especially by andesite. This circumstance may also explain the fact of the comparatively rare occurrence of propylite as a
surface-rock.  In the environs of Bisztritz, in Northern Transylvania, it forms several
high, isolated cones with steep slopes, resting on Eocene strata; their fine, domeshaped appearance is scarcely surpassed in beauty by that of any other kind of rock.
No estimate can be made in regard to the relative bulk of propylite which has been
ejected, on account of its being overlaid by andesite and trachyte.  It occurs, as far
as exposed to view, in quite considerable accumulations, at Nagybanya and Kapnik,
in Hungary, in Washoe, and at Silver Mountain, and covers large areas in Mexico,
where it overlies the Cretaceous. It appears to have been profusely ejected through
fissures, and its emission not to have been accompanied by volcanic action proper; no
distinct traces, at least, have been found of propylitic volcanoes. Massive eruptions
are known to have occurred, besides the places mentioned, in several other parts of
the southern slope of the Carpathians, on the highlands of Armenia, and in Mexico
while the occurrence of propylite in Bolivia, the Altai MIountains, Nrorthern China,
and some other countries, may be inferred from the description of their geology.
Propylite has been repeatedly considered to be a sedimentary rock, mnetamorphosed int situ.  The cause of this opinion was probably the fact that there are rocks
which undoubtedly have had that mode of origin, and share with propylite the resemblance in mineral character to diorite.  The eruptive origin of propylite is, however,
evident, as it intersects stratified rocks in the shape of dykes.  In Washoe and Silver
Mountain, moreover, there are extensive accumulations of propylitic breccia and
stratified tufa. The latter consists of alternate layers of coarse conglomerate, fine(60)




OF VOLCANIC ROCKS.                               23
grained propylitic detritus and massive propylite, and the entire system is intersected
by dykes of the latter.
A particular interest, which hlas a practical bearing of some importance,
attaches to propylite, inasmuch as, notwithstanding its limited occurrence, it yields
probably a larger amount of silver than any other rock. Several of the principal
silver-bearing veins, as the Comstock vein in. Washoe, the celebrated veins in Hungary
and Tranlsylvania, as well as some of those in Mexico, and probably too in Bolivia, are
enclosed in propylite.
Mineral Composition.-Iln propylite are united the petrographical properties of
ancient dioritic rocks with those of andesite and oligoclase-trachyte.  Varieties are
numerous, but produced more frequently by the difference of the component minerals in
size and relative proportion than by the texture, which remains porphyritic in every instance.  The most common varieties consist of a fine-grained, microcrystalline paste of
dark green or greenish brown, more rarely of reddish and dark gray colors, in which
are imbedded crystals of oligoclase and hornblende; the former is of whitish or light
green color, the latter ordinarily dark green and fibrous, seldom  black with bright
cleavage-planes (as is the case with gamsigradite).   The paste appears to be a finegrained aggregation of the same two minerals (the feldspathic ingredient prevailing),
with the admixture of titanic iron, and to owe its ordinarily green color to the profuse
disseminatioin of small particles of green, fibrous hornblende. Sporadic crystals of
augite are occasionally met with in some of the common varieties, while others contain
rounded grains of quartz sparsely enclosed. Recent observations have led to the discovery of two series of varieties deviating from  the usual composition as explained.
One of them is produced by the increasing proportion of the grains of quartz; the
other by the more profuse occurrence of augite.  The former was found by Stache, in
Western Transylvania; the latter observed by me at Silver Mountain.  Both appear
to be of a limited geographical distribution, but are of great interest, as they extend
the limits of the order of propylitic rocks, without rendering either their petrographical character, or their geological relations less distinct.  In regard to the latter,
it may be stated that the entire range of the quartzose to augitic varieties was anterior
in age to andesite.
Difference of Propylite from other Volcanic Rocks nearly related to it. —The rocks of
two other orders have an affinity to propylite, namely oligoclase-trachyte and andesite. The principal ingredients of either are hornblende and oligoclase, and either of
them contains crystals of both these minerals imbedded in a paste.  Yet there is a
notable difference in character between these two, as well as between either of them
and propylite. It is so conspicuous to the eye that the respective rocks have ordinarily been distinguished in geological descriptions, even in those of older time; and
even the unprofessional eye would be able to distinguish the three groups in a collection of specimens belonging to tlhemn.  Yet, it escapes description.  It may, at this
present time, safely be founded on what the botanist would call "habitus," a certain
general character which it is as easy to recognize by the eye as it is difficult to describe
it in words, and impossible to define its causes.  It is probable that observations, such




24                        RICHITHOFEN-NATURAL SYSTEM
as Sorby has made in reference to those minute differences of texture, which can only
be detected with the aid of the microscope, and H. Rose in regard to the modifications
of silica and their causes, aided by exact chemical analysis and experiments made
with the view of enquiring into the differences of origin of such eruptive rocks as
differ from each other in texture, will, if further prosecuted, reveal the true nature
and cause of the properties which distinguish the rocks of these three different
orders.
A few of the more palpable differences may here be noticed. Propylite is
essentially of greenish color, and some of its varieties resemble diorite in composition
and texture; andesite is of blackish color and approaches basalt in aspect; trachyte is
of various colors and shades, among which green and black are rarest of occurrence,
and in regard to its external characters resembles rhyolite more than any other rock.
Oligoclase, in trachyte, is frequently of the vitreous variety, scarcely ever so in propylite and andesite. Hornblende is an essential ingredient in these two, not so in the
former; it is of fibrous texture and green color in most varieties of propylite, black in
andesite and trachyte. Mica is seldom wanting in the latter, while it is not of common occurrence in propyiite and andesite.  Titanic iron enters largely into the composition of these, and is contained in smaller proportion in trachytic rocks. The latter
excel by having the greatest variety of texture, while propylite has among the
three the most perfect porphyritic texture, which has given rise to its frequent
popular designation "porphyry;" this name has never been applied to andesite or
trachyte.
The enumeration of all these trifling differences is, however, insufficient to
express the marked distinction which exists in the external characters of propylite,
trachyte and andesite. As a similar, and even more conspicuous, distinction manifests
itself in their geological relations, we have to consider the existe:ce of those three
natural orders as a fact founded on observations, although we may be utterly unable
to explain, and even to express it in words.
Subdivisions. —The remarks made in regard to the mineral composition of propylite have shown that the range of its varieties may conveniently, and in harmony
with geological occurrence, be subdivided into three parts:
Farn. 1st.  Quarlzose Propylite or Dacite.-This embraces rocks which, though
having in general a similar composition to those of the following family, contain besides, rounded grains of quartz, sometimes in considerable proportion. They occur in
the western part of Transylvania, where their outbreaks succeeded those of rocks of
the second family, and preceded those of andesite.9 Similar rocks have been observed
in Sinaloa (Mexico).
Farn. 2d. Hornblendic Propylite. —Rocks composed chiefly of hornblende and
oligoclase, as described above. This family embraces vastly the majority of all propylitic rocks observed, among others those of Washoe.  Breithaupt's "timacite "
is one of its varieties.
9 Fully described by G. Stache, loc. cit.
(62)




OF VOLCANIC ROCKS.                             25
Pam. 3d. Augitic Propylite.-Rocks distinguished by the accession of augite
among the ingredients of the rocks of the foregoing family. It is present in greater
or less quantity, sometimes predominating over the hornblende. To this family belongs the propylite of Silver Mountain, which contains augite in larger proportion
than any other known variety.
ORDER FOURTH-ANDESITE.
The history of the name " andesite " has been noticed conjointly with that of
"trachyte."  At the time when the former name was first proposed for certain rocks
of the Andes, a few specimens of which had been brought to Europe, it was intended
as a designation of an accidental variety, proper to that mountain range. But those
same specimens have proved since to be the type of one of the most important groups
of volcanic rocks, which is distinct from others in character, and has a wide distribution.
Mode of Ceological Occurrence.-Andesite vies with basalt in regard to the
quantity of matter ejected to the surface, and probably excels the same in this respect.
In most of those parts of the Andes, in regard to the geology of which ve possess
reliable information, it forms the chief bulk among volcanic rocks. The same is the
case on the southern slopes of the Carpathians, at Nangasaki in Japan, and on the
islands of Luzon and Java. Andesite succeeded the ejection'of propylite, and prepared the way to that of trachyte.  Preeminently the greater part of it has, to all
appearance, been ejected through extensive fissures; that is, it has been produced by
what we styled massive eruptions; though andesitic volcanoes, too, are not of rare
occurrence, and, including those which are extinct, appear to have been particularly
grand in their activity. It may be supposed that many of the former craters have been
destroyed. Andesitic mountains are characterized by monotony in scenery. They
form continuous ranges, which are often of considerable elevation and extent, but exhibit gentle outlines in their summits as well as in their slopes. Breccias only, which
accompany the solid rock ordinarily in vast quantities, cause local interruptions of the
monotony by their more rugged forms. They appear in castle-shaped rocks on the
crests of andesitic mountains, and formn high walls, naked and steep, along their slopes.
Being more liable to destruction by the erosive action of water than solid andesite,
they frequently compose the sides of steep ravines and canions.
Mineral CGomposition.-Andesite is always of dark color, mostly blackish,
though frequently reddish-brown on the weathered surface. Its mineral composition
varies, though it is confined within more narrow limits than that of propylite. We
may distinguish, in regard to it, two
Subdivisions. which are connected by gradual passage in composition and, in
a greater measure, by geological relations.
Earn. 1. Hornblendic Andesite.-Paste of bluish-black to dark-gray color, and
of microcrystalline texture, which passes by gradual steps into that of obsidian. In
the former case it is frequently vesicular like trachyte. There are imbedded in it
(63)




26                         RICHTHOFEN-NATURAL SYSTEM
smiall but distinct tabular crystals of oligoclase, fine grains of titanic iron, smnall,
elongated colunns of hornblende, sometimes mnica, and in most varieties a few isolated
crystals or rounded grains of augite. Frequently these different crystals are so small
as to be no longer recognizable, except by the aid of the microscope. The rock is
then similar in character to certain varieties of melaphyr, but may be distinguished
by its vesicular texture, which is scarcely ever wanting in the hornblendic varieties.
This family comprises vastly the greater portion of all andesitic rocks.
PEam. 2.  Azugitic Andesiie. —Two groups of varieties may be distinguished
among the rocks of this family.  Those of the first have a paste of an oil-brown color
passing into black, a compact appearance, and ordinarily a microcrystalline texture which passes into that of porcelain.  Vesicular inflation does ordinarily not
occur.  Crystals of a monoclinohedric feldspar (probably labrador) are almost invariably enclosed, and frequently accompanied by hornblende and augite. For the rocks
of this group the name " traclzydolerite " has particularly.been applied.  The other
group comprises certain varieties for which the name " anamesite " has not rarely been
used, and which are of dark-gray colors.  Their texture is in most cases vesicular.
The enclosed minerals are the same as with the other group; but, while for this
the feldspathic ingredient is more characteristic, and frequently alone present in large
crystals, the rocks of the second group are distinguished by the predominance of well
formed crystals of augite and hornblendle, while those of feldspar are often so small
that they cannot be distinguished by the eye. The rocks of both these groups contain
titanic iron in larger quantity than those of the first family, and are of greater specific
gravity.  Olivine enters occasionally into their composition, in very subordinate quantity.  Geologically, the rocks of this family are closely allied to hornblendic andesite.
They have succeeded it in age, and are limited in their geographical distribution to those
places where hornblendic varieties had been ejected before, frequently intersecting and
overlying them.  Notwithstanding the resemblance which they bear to certain varieties of basaltic rocks, they appear to be never associated with them geologically.
ORDER FIFTH —BASALT.
No one of the names applied to volcanic rocks is of equal antiquity with that of
"basalt," and with none there has ever been less change of opinion, and uncertainty in
respect to its application.  It may be inferred fromi this fact that the rocks comprised
by that name are very distinct in character and little liable to variation, which is indeed
true for the typical rocks of the order. But there have to be associated in the same
order with them certain other rocks, which, though nearly related to basalt in regard to
their chemical and mineral composition, differ from it in the more conspicuous external
characters.
7fode of Geological Occurrence.-Basaltic rocks are more independent than those
of any of the foregoing orders (perhaps with the exception of propylite), in respect
to their epoch of ejection as well as to their geographical distribution. In the order
of time they succeeded next to rhyolite, but locally, both are usually separated.
Basalt occurs always in the neighborhood of more ancient volcanic rocks, but the cases
(64)




OF VOLCANIC ROCKS.                               27
are rare when it intersects them and expands over them. Almost invariably it is found
accoinpanying their ranges at some distance, forming itself extensive ranges; more
frequently, however, it occurs in groups or lines of isolated outliers.  It may sometimes appear to have no connection with the distribution of older volcanic rocks.  But
closer investigation will always show it to be within or in the neighborhood of the
limits of former eruptive activity.  The belts along which this had taken place may
be locally interrupted for quite a distance, and the gap be filled by some isolated outbreaks of basalt, or the latter nmay extend the limits of those belts, in length as well
as in width.  It is a strange phenomenon that these isolated outbreaks of basalt occur
particularly in connection with granite. It intersects this rock very frequently in small
dykes, and expands over it in thin sheets.  This connection of granite and basalt is
very conspicuous on the eastern slope of the Sierra Nevada, and along the belt of
basaltic eruptions which traverses the middle part of Germany.  Such places are often
the seat of basaltic volcanoes, which, though now mostly extinct, are also otherwise of
very frequent occurrence.
Mineral Conmposition.-Basalt is the representative of what Bunsen has called
the " normal pyroxenic type," and is thereby, as well as by its specific gravity, mineral
composition, and little variety of texture, the reverse of rhyolite, which represents the
" normal trachytic type."  All the essential ingredients of basalt, and most of the
minerals which enter accidentally into its composition, are different from those which
are characteristic of rhyolite. Augite, labrador and titanic iron, imbedded in a paste
consisting essentially of the same minerals, constitute generally the rocks of this order;
or labrador may be replaced by leucite, nepheline or a zeolitic substance; or other minerals may enter into the composition, such as olivine, basaltic hornblende, hauyne,
apatite and black mica; and in many instances basaltic rocks consist merely of a
fine-grained aggregation of the different minerals mentioned.  Ordinarily, the paste
has a microcrystalline texture, and may either contain crystallized minerals imbedded,
or may be devoid of them. This mnode of texture passes by gradual steps into that of
obsidian, which is the only hyaline variety occurring. Among the peculiarities of these
rocks, and which may also in some measure be observed with augitic andesite, is the
occurrence of rounded cavities filling the rock and giving it a cellular, sometimes
spongy appearance.  It is different in nature from  pumice-stone, which is a modification of hyaline texture, while those cavities occur in microcrystalline rocks.
Subdivisions.-Famn. lIst.  D)olerile (including nepheline-dolerite and tile greater
part of anamesite).  A crystalline aggregation of augite and labrador with titanic iron.
Labrador is usually replaced by nepheline, either in part or totally.  Accessory minerals are: olivine, hornblende, apatite, black mica.
Eaam. 2d. Basalt.-Paste of dark-gray or black color, and of the varieties of texture mentioned; it constitutes either the mass of the rock by itself, or encloses olivine,
augite and labrador, in crystals or crystalline grains.  Besides these are frequently
enclosed: titanic iron, black mnica, rubellan, zircon, apatite and other minerals.  This
family comprises nearly the whole bulk of the rocks which constitute the basaltic
order, the two other families being of rare occurrence,
(65)




28                        RICIITHOFEN-NATURAL SYSTEM
Panm. 3d.  Leucitophyre.-Rocks of porphyritic texture, crystals of augite and
leucite being imbedded in the paste.
It vill be noticed that no place has been assigned in this classification to phonolite.  As regards those rocks for which this name was first proposed, our knowledge
is still quite limited. It appears that, notwithstanding their comparatively rare occurrence, they form a distinct natural group closely allied to basalt; but they are so different from true basalt as regards lithological characters, that they should not be
classified with it before further observations will have determined their real position
in reference to the natural families of volcanic rocks.  The name phonolite has, however, been so much extended in its application, that this task is not so easy to accomplish. Some external properties, easy of recognition to superficial observation, such
as a certain tabular structure, lithoid texture of the paste, with small, bright crystals
of feldspar enclosed, and the peculiarity of ringing by a blow of the hammer, which
have been often considered as the characteristic features of phonolite, are just as common with certain varieties of trachyte and rhyolite, and even with some of propylite.
It must be ascribed to this reason, that the name phonolite has been used for the
designation of rocks which bear an accidental resemblance to true phonolite, but are
distinct from it in nature.  It has probably been oftener applied to rocks belonging to
the trachytic order than to such as have the distinguishing features of those varieties
for which the name has been first used.
CORRELATION OF THE FIVE ORDERS OF VOLCANIC ROCKS.
In the foregoing pages I have attempted to lay down the outlines of a classification of the volcanic rocks by natural principles, and to apply a nomenclature which
should be appropriate to these, and embrace, at the same time, the most current of
existing names.  It is the next object of this paper to prove that these rocks are
mutually connected by definite relations, and that their totality, in virtue of this
property, forms actually what may be called a system in nature, and that the form into
which we have tried to bring it, imperfect though it must be, is an approach towards
its expression.  In order, therefore, to fully realize the philosophy of the natural
system, we have to contemplate the relations which, firstly, the rocks of the different
orders offer mutually among themselves, and by which, secondly, they are connected
as an entire class with ancient eruptive rocks; while we will have, thirdly, to examine
into the mode of origin of volcanic, and of eruptive rocks in general, in order to
establish the nature of their fundamental difference from sedimentary and metamorphic rocks. Our task is thus three-fold. The present chapter will be devoted to the
first order of relations. They may be considered from several points of view, the
moreimpnortant of which are: chemical and mineral compdsition, geographical distribution, and all those complex relations which may be comprehended in the term
"mode of geological occurrence."  WVe will confine ourselves to the last point of view.
But even with this restriction, we can only trace general outlines.
(66)




OF VOLCANIC ROCKS.                              29
Lcaws relative to the Age of Massive Eruptions.
The succession of 9nassive eruptions during the Tertiary and Post-tertiary ages
has taken place in the following order:
1st.  Propylite.
2d.  Andesite.
3d.  Trachyte.
4th.  Rhyolite.
5th.  Basalt.
This singular mode of succession, in which no regularity (as to increase or
decrease of silica or specific gravity, or as to a gradual change of maineral composition)
can be discovered at first sight, and which might indeed appear to be devoid of order,
and to bear the character of such a succession as might have been occasioned by the
coiiperation of accidental circumstances in one single country, can nevertheless be
proved to exist in widely separated parts of the globe.  It may justly be objected to
this assertion, that observations in regard to the relative age of different volcanic rocks
are scarce, and hardly sufficient to establish definitely such a law.  But hitherto no
deviation from it has been discovered,0 and it appears to be true for all volcanic regions
on the globe, though with this restriction, that the epochs marked in each country by
the ejection of rocks of any certain orders have not been contemporaneous in different
countries.  The commencement of eruptive activity in the Tertiary epoch has been earlier at one place than at another; it is its further mode of development in regard to the
nature of the matter ejected which has everywhere been regulated by the same definite
relations, thouglh it has been indeperndent, in some measure, at each place, or, to use
a more correct expression, over the area of each belt of eruptive activity.   We have
to mention another restriction.  Abrupt passage may be said to be an almost unknown
conception in geological matters, where the order in time is concerned; nor has it to
be applied to the order of succession of volcanic rocks.   The eruptions of propylite
appear not to have been interrupted by the ejection of any other rocks.  This may
too be said, though less strictly, of the andesitic epoch.  But some, as it were, retarded
eruptions of andesite, which, however, have always beets insignificant, may occasionally be traced during the first part of the trachytic epoch, and similar relations exist
between trachyte and rhyolite.  Basalt, however, appears to have had everywhere its
own epoch of ejection, uninterrupted by the massive eruptions of any other rock of
the orders just mentioned.
We proceed to a short review of some salient facts observed in those countries where the mutual relations of volcanic rocks in regard to their age have been
made an object of study.
In Hungary and Tranrsylvania, propylite, as we have had occasion to mention,
was ejected first of all volcanic rocks, and may be said to have inaugurated all sub10 It must be borne in mind that we are speaking of massive eruptions. Apparent exceptions are known to occur
with rocks ejected from volcanoes. of which mention will be made hereafter.
I                                                        (67)




30                         121IC'ITHOI EN - NATURAL SYSTEM
sequent eriuptions of those belonging to other orders.  At several places it has
been observed to intersect Eocene strata, and to expand above them.  The country of NagyblTnya, Fels6ibtnya and Kapnik, the celebrated silver-bearing -veins of
which are enclosed in propylite, offers especially colnspicuous illustrations; not less,
from  the descriptions given by G. Stache, the " Erzgebirge " of Transylvania, where
the same kind of rock is rich in mineral veins.  In this country, hornblendic propylite
forms an older series, followed by eruptions of highly quartziferous varieties.  In
both countries, but chiefly in that first named, and at several other places along the
southerl slope of the Calrpathians, andesite may be seen intersecting propylite in large
massive dykes, and towering up above it in mountain ranges.  Andesite composes
entirely the Hargitta-range, which extends over one hundred miles in length, and
twenty-five in width; the Vihorlat-Gutin-range, which is of still larger dimensions,
and the Eperies-Kaschau-range; all of which are densely wooded, and of a gloomy,
monotonous aspect.  The only change observable on their summit ranges is a more or
less dark color of the rock, occasioned by the predominance of augite or hornblende in
its composition, (the augitic varieties being invariably of more recent age than the
hornblendic), while their slopes, and particularly their ends, present a much greater
variety in rocks as well as in scenery.  It is here that the more silicious volcanic rocks
are encountered.  Trachyte is of rare occurrence; but it forms several isolated cones,
some of which, on account of their prominent position, werc crowned by castles in the
middle ages.  Rhyolite is much more frequently met with, bursting forth on the
flanks, and skirting the foot of the andesitic ranges, particularly where they verge
towards the Hungarian plains, which iu the rhyolitic epoch were still covered by a
shallow and slowly retiring sea.  It projects against this in promontories, which are
now covered by the most celebrated of the Hungarian vineyards, those of Tokay
among others.  The boundaries of these vineyards towards the adjoining beech forests
mark the dividing line between rhyolite and andesite.  An interesting mode of occurrence of the former may be witnessed in large circular or ainphitheatrical basins which
are surrounded by andesite.  Such places are the theater, especially, of the volcanic
activity connected with the outbreak of rhyolite, and abound in endless hyaline varieties of the same.  Telkibanya is the most interesting among the localities of this
description.  There is no lack of evidence to prove that trachyte and rhyolite are both
of more recent age than andesite, while the assertion that trachyte preceded rhyolite
in age, rests only on a few though conclusive observations.  Basalt occupies a singular
position in the geology of Hungary.  It keeps altogether aloof from the places occupied
by the other four orders of volcanic rocks, and forms extensive, though isolated, hills at
some distance from them, scattered over a wide range of country.  It would be difficult
to determine its relative age, but for the volcanic sediments which were of formation
contemporaneous with the ejection of the different volcanic rocks, and have been spread
over wide areas at the bottom of the then existing sea, with fossils occasionally imbedded.
In several localities, especially in the neighborhood of Kascllau, basaltic sediments may
be seen, covering those composed of r~hyolitic matter, while at Gleichenberg in Styria,
Mr. Franz von Hauer has observed fragments of rhyolite enclosed in basalt.
The observation of these relations in Hungary and Transylvania has first given
(68)




OF VOLCANIC ROCKS.                               31
rise to the establishment of the above mentioned law of the periodical succession of
volcanic rocks. Ad  It has since been corroborated by observations made in other countries far remote from the Calrpathians.  Near Nangasaki, ill Japan, andesite preceded
trachyte in age, and of the formner there are two varieties represented, one augitic
and one horublendic, of vwhich the former is of more recent origin than the latter:
thus, even the more minute relations observed in the Carpathians are repeated in
other countries.  The islands of Java and Luzon are too intensely volcanic and
covered with lava to aid in establishing the laws in respect to massive eruptions
without very close obseraation.  lut another region offering copious evidence is that
of the Sierra Nevada, together with the adjoining parts of the Great Basin. Observations in these countries are still limited as regards our present subject.  The great
part, however, which volcanic rocks take in their composition, as well as in that of the
highlands of Mexico, and of the entire range of the Andes, promises to make the
Western Coast of America tihe most prolific source of observations necessary for the
definite establishment of geological laws of which, at tile present day, we can only
trace the first foundation.
In. Washoe, a country adjoining the Sierra Nevada immnediately to the east, propylite forins extensively the foundation for all other volcanic rocks, which fact proves
clearly its priority in age. It composes the plateau of Virginia City and Gold Hill, and
derives a practical interest from the fact that the Comstock vein is enclosed between
propylite and syenite, though in some parts of it both walls consist of the former rock.
Andesite is insignificant in bulk in that region.  It composes a few small hillocks on
the propylitic plateau, and in some cuts and tunnels andesitic dykes may be seen, which
appear to have been the feeding channels of the surface accumulations.  Trachyte, on
the contrary, is among the prominent rocks of Washoe.  It forms a high and rugged
crest, encircling the plateau of Virginia and Gold Hill to the east, and extending for
miles to the north, while to the south it reappears across the Carson River.  No evidence is afforded, in Wlaslho, for the establishment of the mlutual relations of andesite
and trachyte, while it is conspicuous that the latter was of later origin than propylite.
Besides the evidence offered by intersection and superposition, another fact may be
noticed which is suggestive for the length  of the period that elapsed between the
eruptions of both rocks.  It is this, that propylitic sediments occur, at least, at one
thousand feet more elevation than those consisting of trachytic matter, which fact
appears to indicate that the former have been deposited at a much earlier part of the
period marked by the gradual subsidence of the inland seas of the Great Basin than
has been the case with trachytic sediments.
At Silver Mountain, propylite, of the augitic variety, fills the bottom of a deep
basin encircled to the west by granitic walls several thous.andl fect in height.  Its
massive accumulations are intersected by andesitic and trachytic roclis, which latter
appear to compose the sumullit of the high peak of Silver MnountLin itself, while rhyolite occurs in such a position as to make it probable that it has arrived at the surface
last of all eruptive rocks.  Basalt appears to occur only to a limited extent at Silver
I{' licl.thofen, loe. cit.
i'Wi))




32                         RICHTEOFEN - NATURAL SYSTEM
Mountain.  But this rock is largely distributed in the regions adjoining the Sierra
Nevada to the east, and bears evidence of its recent origin.  It is the only volcanic
rock which covers in places the sand of the deserts, and the belts distinguished by its
eruptions are still marked by the occurrence of hot springs and other post-volcanic phenomena.  -Most of these are contiguous to those places where basalt and
granite are in close contact, as is very conspicuously the case at Steamboat Springs,
near Washoe, and in the Coso Mountains.  No phenomena of similar nature appear
to be connected at the present time with any other volcanic rock east of the Sierra
Nevada.  There are not a few instances where basalt may be seen covering propylite
or andesite; but I mnet with only one case where it comes in contact with rhyolite,
close enough to establish their mutual relations.  This is in Esmeralda, on the eastern
slope of the Sierra Nevada, a country of unusual interest for the study of volcanic
rocks in general.  Propylite encloses the silver-bearing veins of that place.  It is
overlain by trachyte and rhyolite, both of which occur in very great variety.  To
the east of the place, basalt has not only flowed over rhyolite, but contains numerous
fragments of it enclosed, which fact confirms also for this country the more recent
origin of basalt.
Many other examples might be added to this short list, partly of positive observations made in the countries already mentioned, and partly of facts described in
treatises on the geology of other countries, such as Armenia, the Caucasus, Central
France, the Eifel, Bolivia, Mexico.  As these descriptions, however, have not directly
in view the illustration of our subject, great care should be used in drawing from
them conclusions in regard to it.  Let it suffice to remark, that every observation on
record w1hich bears on our subject, appears to confirm  the proposed law, while none
can be found giving evidence against it.  This may justify the assumption that the
periodical succession of volcanic rocks, in the order above mentioned, is a general law,
true for all parts of our planet.
Laws regardinyg Ite Jltual Relations oS MalcGssive Erucptionzs and Volcanic Actitvity.
It has been mentioned in this chapter, that the law of the periodical succession of
volcanic rocks regards those outpourings of large volumes of matter not resulting
from volcanic activity proper, and which we called massive eruptions.  For convenience' sake, we mnake use of the followilng expressions: propylitic epoch; nclndesitic epoch;
trac/ytic epoch; rlhyolitic epoch; basallic epoch   designating thereby those epochs
in which tihe massive eruptions of rocks belonging to each of these orders, have taken
place in every different country.  If we now direct our attention to the other mode
of manifestation of subterraneantl  energy, the volcanic eruptions, a cursory review of
active volcanoes in regard to the nature of the rocks whichl they eject, shows that the
samne law is no longer true for them, as their lavas belong to several diffierent orders
of volcanic rocks.  It is, however, knlown that each volcano ejects lava or scoria
belonging petrographically only to one distinct order, and the examination  of the
rnmaterial accumulated by former activity will show  that with  most volcanoes the
natulre of the rocks ejected hlas nlever materially  varied, whlile with some of the
(70)




OF VOLCANIC ROCKS.                               33
grander vents it has undergone a periodical change.  Volcainoes, whether active or
extinct, maybe classified from this.point of view.  We shall distinguish: andesilic,
trachytic, rlhyolitic and basaltic volcanoes, according to the nature of the mineral matter
which each volcano has ejected in the first epoch of its activity, regardless of ally later
changes.  Two noteworthy relations may be traced between these different orders
of volcanoes and the massive eruptions of the synonymous orders of volcanic rocks.
The first of them is the alliance of both in regard to geographical distribution, the
volcanoes of each order being limited, in this respect, to the immediate neighborhood
of massive accumulations of rocks similar in nature to their first lavas.  From  this
may partly be inferred the second relation, that the massive eruptions of each order
have been succeeded by volcanic activity, which occasioned the ejection of lava corresponding in nature to their own rocks, and continued for long after-time, in many instances to the present day. This dependence of volcanoes upon massive eruptions explains
why the number of active volcanoes is so small when compared with those which
are extinct, and why the present activity even of those which are still in operation, appears to be only a faint remnant of that which the same vents exhibited in former time.
It will further explain why no vestige can be found of a rhyolitic volcano having been
active before the rhyolitic epoch, or of a basaltic volcano having originated before the
basaltic epoch, while geological observation goes to show that during, and immediately
after those epochs, the volcanoes of either order have been most intense, numerous
cand extensive, and their activity has, from that epoch of culmination, gradually relaxed,
in most cases to perfect extinction.
An instructive instance of one of those grander volcanoes which have undergone
a periodical change in regard to the nature of the matter ejected from them, is afforded
by the extinct volcano Lassen's Peak, in Northern California, which Professor J. D.
Whitney and I visited in 1866.  We found it to have been originally an andesitic
volcano, and it has to be ranked as such in our proposed classification.  The enormous
bulk of thle ancientvolcano is totally built up of stratified layers of andesitic tufa
and rapilli, which, in the steep gorge issuing from  its lower crater, are exposed in a
thickness of near]y four thousand feet, notwithstanding the total destructionl which
the upper part of the former cone has undergone, and the fact of its lower parts
extending down far beneath the present surface, and being therefore concealed to view.
Besides these stupendous accumulations of loose mlatter, currents of andesitic lava
appear to have been emitted from the crater, extending at least twenty miles from
the place of ejection. At a later epoch, the activity of the same volcano has been
distinguished by the emission of trachytic lava from the northeastern part of the
wall of the crater; its currents have expanded to elongated and sloping tables.
bounded by abrupt descents. A third epoch is marked by the outbreak of rhyolite
at the same place whence the trachytic rocks had issued.  Rhyolite composes the
present summit of Lassen's Peak, 1on  which it is accumulated in a thickness of more
than fifteen hundred feet, also some other summits of less altitude, and at least one
prominent current of lava of great volume.'2  The noteworthy fact illustrated by these
observations on Lassen's Pealk, and corroborated in numerous other instances, is this:
12 Mr. Clarence King has observed the occurrence of basalt of apparently very recent origin immediately north of
(71)




34                              IRICIITHOFEN -  NATURAL  SYSTEM
that the same  law  of periodical succession which  has  been  established  in regard to
massive  eruptions, is true for volcanic action, particularly  when this happened  to
assume such ulnusual intensity and  dimensions, and has been  of as long  duration as
was the case at that volcano.l3
Suniming up these  considerations on the correlation of the different orders of
volcanic rocks in respect to the age of their emissiorn throtugh volcanic vents, we arrive
at the following conclusion:  The commencement of the  activity of the volcanoes of
each separate order has been nearly coincident with, though in every instance successive to, the main iphase of the corresponding massive eruptions.  Thence it has, by each
separate vent, either continued emitting similar material to that first ejected, until its
extinction, or it continued in the same way to the present day, or it has been subjected
to a periodical change in regard to the nature of its lavas, and this change is anlalogous
to that exhibited by the succession of massive eruptions. In this case, as in the former, the volcano has either become extinct when in a certain phase, or it is still active.
We are thus furnished with a natural cause of the fact, that most active volcanoes are
emitting  basaltic, a smaller number of them  rhyolitic or trachytic rocks, while alnclesitic lava is peculiar only to a few of them, especially to some of the prominent volcanoes of South America (Chimborazo, Cotopaxi, Antisana, Tungurahua, also Popocatepetl, Colima, and Teneriffe) which appear never to have changed  in mnineral character.  It will, too, be self-evident, why generally  no material change in the nature
of their lava should have been observed in regard to those volcanoes Which have originally emitted basalt and constitute our order of' basaltic volcanoes.
A  few  more instances may here be mentioned  in support of our propositions.
The  interest attaching  to  volcanoes has furnished  us with a much greater number
of facts in regard to volcanic rocks when  occurring as lavas, than we possess in regard
to the grander and more frequent instances when similar rocks have been  produced
by the  comparatively neglected  action of massive eruptions.  Among those observatiolls, none will be better evidence than such as prove the abrupt succession, by ejection  from  the same volcano, of two rocks so dissimilar in composition as rhyolite and
basalt.  On the other hand, the nature of volcanic action will explain why we should
meet among lavas, more frequently than  among massive  eruptions, with  the fact of
two successive epochs blending into each other by the alternation of the two kinds of
rock peculiar to them separately, and it cannot be surprising if instances are occasionally observed exhibiting, at least partly, a reversed order of succession.
Lassen's Peak. It is probable that it indicates the existence of a fourth epoch in the activity of that volcano. The ejection
of basalt has been so frequently connected with the opening of vents in the neighborhood of, but not coinciding with, channels
through whichits predecessors had ascended, that its local separation cannot be an argument against its belonging, in our
case, to the system of Lassen's Peak.
13 The fact that tracllytic lavas are frequently followed by such of basaltic character has been known since long
time, and was till now the only law of succession observed. Mrl. Scrope has suggested the hypothesis indorsed by Mr. Darwin, Sir Charles Lyell, and other distinguished geologists, that in the sobterranecan reservoirs of volcanic matter, the heavier
particles will occupy the lower part, and the lighter ones be nearer the earth's crust. It will easily be seen how totally
inadmissible this theory is in the case of Lassen's Peak. It is not less so in those cases where rhyolite was succeeded by
basalt, since the process ot liquation can certainly not be supposed to have produced an abrupt passage unlder ground from
one mass to the other, and it would be much more natural to suppose a gradual transition to take place, there as well as in
the succession of the rocks emitted to the surface, if liquation had really taken p!ace.
(72)




OF VOLCANIC ROCKS.                             35
An instance, which is instructive on account of its simplicity, is furnished by
the island of St. Paul, in the Indian Ocean.  F. von Hochstetter found its foundation
to consist of rhyolitic rocks. These are intersected by basaltic dykes. Rhyolite overlays the first basaltic formation, and is itself superposed, first by dolerite and then
again by basalt. These two rocks of the basaltic order constitute the main body of
the island, and encircle its crater.  Similar relations, though on a much grander scale,
have been observed by the same eminent geologist on New Zealand. More frequently
than this order of succession between rhyolitic and basaltic lava, has been observed
the sequence of basalt to trachyte, with the omission of rhyolite, or immediately to
andesite, when both those rocks are absent. Vesuvius is built up of rocks of the
basaltic order, and still emits lava corresponding in mineral character to its predecessors, while the rocks of its surroundings (Campi Phlegrxi), on the prior origin of
which geologists agree, are trachytic. The industrious explorer of Mount Etna, Sartorius von WValtershausen, has describecl its foundation as being composed of white and
reddish colored trachytic rocks, which contain hornblende as a characteristic ingredient, while among those rocks which build up the summit, as also in all modern lava of
the volcano, no hornblende but, in its place, augite is visible. This mineral and labrador compose the recent lava, which belongs to the basaltic order.  The much more
extensive recurrence of a similar order of succession in the Eifel and in Auvergne,
is too well known from the accurate descriptions of the geology of those regions, to
require to be here more fully mentioned. It contributes especially to confirml our
proposition, that the volcanoes of the different orders, as regards their origin, have
been nearly contemporaneous with the correlated massive eruptions.  The classical
descriptions of the Eifel, by Mr. von Dechen, give conclusive evidence thereof.
Among those volcanoes the lava of which has never undergone a material
change and is, at the same time, similar in nature over the area of larger volcanic
districts, may be mentioned, besides numerous basaltic volcanoes, those of the trachytic
order in Central, and those of the andesitic order in South America, as far as may be
seen from the descriptions given of them.
We might greatly enlarge this enumeration of observations confirming our
propositions  but, as by most authors only a' trachytic " or a'' basaltic " character
of lava have been mentioned in a general way, they would only furnish evidence in
favor of the general tenor of the law, but would fail to give it in regard to any of its
details.
RELATION OF VOLCANIC ROCKS TO ANCIENT ERUPTIVE ROCKS.
All rocks which, bearing evidence of an intrusive or eruptive origin, preceded
in age the Tertiary period, may, by principles similar to those which we applied in
tracing the natural system of volcanic rocks, be divided into two great classes, for
which we may use the terms "' granitic rocks " and "porphyritic rocks," derived from
the mode of texture predominating in either class.  Granitic rocks are, besides,
geologically associated with granite, which is their principal type, while quartzose
porphyry  occupies a simnilar position among porphyritic rocks.  The annexed table
(73)




FIRST CLASS: GRANITIC RocKS.           SECOND CLASS: PORPHYRITIC ROCKS.                         THIRD CLASS: VOLCANIC ROCKS.                           ESSENTIAL INOREDIENTS.
Order First-Granite.                 Order First-FThailic Porphyry.                              Order First-Bleyolite.
Fain. 1st. Granite.                                                                                                                                          Quartz,
Orthoelase
Fain. 2d. Granitite.                        Fam. 1st. Quartzose Porphyry.          (Quartziferous Varieties.)             Fam. 1st. Nevadite.                Oligoclas
Biotite,
Fain. 3d. Syenitie Granite.                                                                                               Fam. 2d. Liparite.                 (Hornble
-( Faina. 2d. Liparite.                (ilorunblende.)
Fai. 2d. Varieties without Quartz.   (Varieties without Quartz.)   I
J       L Fain. 3d. Rhyolite proper.      9
Order Second-Syelnite.                 Order Second-lorphyrite.              Order Second- Trachyte.              Order Ticird-Propylite.         ) Oligoelase Orthoclase           Z
-Hornblende, (Triotite),
Fam. 1st. Sanidin-Trachyte.          Fai. 1st. Quartzose-Propylite.   3 (Quartz.)
Only Fa~mily:  Syenite.                     Only Fazmily: Porphyr~ite. 
L                                   Fain. 2d. Oligoclase-Trachyte.                                            Oligoelase, Jlornblende,
(Titaniferous  Magnetic
9   Iron-ore.)
Order Tihird-Diorite.                   Order Third-Melaphyr.                Order Fourth-A ndesite.           Fain. 2d. Hornblendic Propylite. 9 Oligoclase, Iornblende,
tTitanifevosMgei 
Faam. 1st. Diorite.                         Fam. 1st. Melaphyr.                    Fai. 1st. Iornblendic Andesite. jitnif                                                 M
Fai. 2d. Rocks intermediate between         Fai. 2d. Rocks intermediate between Fai. 2d. Augitic Andesite.              Fai. 3d. Augitic Propylite.        9 Oligoclase, Lbrado r,
Diorite and Diabase.                         Melaphyr  and  Angitic                                                                              Augite, Ilornblende,
Porphyry.                                                                                           Titaniferous   Magnetic
j   Iron-ore
Order Fourth-Diabase.               Order Fourtlh-Augitic Porphyry.                               Order Fifth-Basalt.                             9 Labronore
Augite,
Fai. 1st. Gabbro and Hyperstbenite.                                                                        Fam. 1st. Dolerite.                                 u
Only Family: Augitic Porphyry.                                                                                   Titaniferous Magnetic
Fai. 2d. Diabase.                                                                                          Fai. 2d. Basalt.                                J   Iron-ore.




OF VOLCANIC  ROCKS.                                                 37
will show   the  mutual relation  of these  two  classes  and  their  subdivisions, and  of
either of them  to volcanic rocks.l4
It appears that this  general classification  is based upon as natural principles as
are within  reach of our still limited knowledge of eruptive  rocks, and therefore  may
at least approach  the  natural system.   The  following  are  the systematical principles
chiefly involved:
1st.   Ckernical Conmposition.-Each class contains all possible compounds intermediate between those which Bunsen styled the normal trachytic and normal pyroxenic types, and may, therefore, be represented by a  numerical series of infinite  gradations  within  two  certain  limits, and  progressing  according  to  a definite  arithmetical
law.   If this law  applied to  the  composition  of rocks with  mathematical precision, it
would  be sufficient to  know  the relative  quantity in which  any  one  single ingredient
enters into  the same, in  order  to find  by  calculation the relative  proportion in which
every  other ingredient should  be  present.   It is, however, well known, that analysis
shows ordinarily  a slight deviation  from  the composition  as required  by theory;'and,
considering the various influences to which the rocky masses must have been exposed
before and after their consolidation, we  should  naturallly  presuppose  that such deviations would be the rule, and may indeed be astonished to see how  slight they generally
are.   Silica  has  been found to  be not only  the most convenient, but also, on account
of its predominance  over other ingredients, the  safest element by  which to determine
the place any rock occupies in the series.   In the classification  as proposed in the preceding page, each class of eruptive rocks  commences with  those  varieties of granite,
quartzose porphyry  and rlhyolite, wihich contain the highest amount of silica as found
by analysis, and descends to the  most basic varieties of diabase, augitic porphyry  and
basalt.   In a chemical respect, therefore, the three classes are identical.l5
14 IThis table is only designlel to show the mutual relations of the subdivisious of the three great classes of eruptive rocks in their most general outlines.  I have purposely avoided to detail them  any nmore, since it appears that great
progress in regard to their knowledge will be made in the next years, and considerable changes in the details of classification may have to be looked for. The composition of eruptive rocks is just at the present time being made an object of
more careful study than it ever was before.  Contributions of high value in regard to the chemical and mineral composition of volcanic rocks are being furnished by the members of the Geological Institute of Vienna; while I. Abich
continues with untiring energy his fruitful researches on the same rocks of Armenia, the Caucasus, and the borders of the
Caspian Sea.  The examination of the granitic rocks by G. vorm Rath, A. Streng, Th. Scheerer and many others; the
more general labors of G. Rose, Robert Bunsen, A. Delesse, G. Bishoff, F. Zirkel; the microscopic investigations which
have been commenced with so much success by A. Sorby, and are being extended by his numerous followers-all these
labors pursued zealously by those named and a great many other workers in the same branch of geological science, whom it
would be too lengthy to mention, promise a rapid advance of our knowledge of the properties of eruptive rocks.  In
respect to their mineral composition, the discovery of some new species of feldspar (such as the plagioclase of Rath and the
microtine of G. Tschermak) which appear to be of wide distribution among the components of eruptive rocks, promises
alone to enlarge quite considerably the basis of classification, as it appears to give a clue to the causes of the differences in
outward appearance (frequently referred to in this paper) of rocks which are alike in regard to their chemical composition,
and among the mineral ingredients of which no difference could be recognized heretofore.
15 Objections have been raised against the validity of the law of Bunsen, partly on the ground of the frequent
discrepancy of the figures obtained by chemrical analysis from those which would be required  by the theory, aad partly on
account of the highest and lowest amounts of silica occurring in eruptive rocks having as yet by no means been fully
ascertained. We referred above to the first objection.  As regards the second, it is evidently very trifling in importance.
l'he limits of the series may, and probably will, be somewhat extended, and the figures representing the compounds which
form those limits may have to be slightly changed, yet the series itself will remain essentially the same.  The law of Bunse will have to be revised and corlectel  when increased expericnce shall have establislel  a broader basis for it;  )ut no
clhang e of its pr inciples miay evrtil be(! expeetetd, as anl overwhelmnllgr atmonillt of evidenice hats aIccumulatedl ill support of its
essentiail teo l e  rim




38                         RICHTHOFEN - NATURAL SYSTEM
2d. Mineral comp2osition, in respect to which each class represents a series of
gradations essentially dependent on the chemical composition, and therefore chiefly
coincident with the chemical series, but differing from it inasmuch as it is more articulate.  Certain types, corresponding to certain steps in the chemical scale, and consisting in distinct aggregations of a few minerals, are the centers for which the petrographical names have been applied in the first place; around them other members are
grouped which connect every two types by gradual passages, and are ordinarily composed of an aggregation of all the minerals peculiar to either of them. Expansion of
the series, as it were, in a lateral sense, occasioned by the accession of minerals inferior
in importance, are not unfiequent, but do not affect in a great measure the definite
character of mineralogical gradation, as they are of local occurrence, and probably less
dependent upon any material differences in chemical composition, than upon certain
influences which acted upon the mass of the rock; eithler before its ejection, and
then by the admixture of matter differing from it in nature, or after its solidification,
and then by chemical metamorphism.
3d. Specific gravity, which increases in the reverse proportion of silica.  In this
respect, too, each class represents a series of infinite gradations from. the lowest to the
highest value.
Taking the three foregoing principles exclusively as the basis for classification,
all eruptive rocks would have to be united into one class.  This union, made regardless of any other relations, may be traced as the leading feature of nearly all systemnatical arrangements proposed.  There are, however, other points of view which mIust be
considered if a more perfect classification is aspired.  They lead to the establishment
of further separations, by principles similar to those which we had to apply above in
defining the different orders of the compounds of hornblende and oligoclase among
volcanic rocks. These points of view are the following:
4th. Mode of Textlre.-Eruptive rocks exhibit, in regard to their modes of texture, very peculiar differences, which are little capable of explanation with our present
state of knowledge. They are especially conspicuous with the most silicious rocks.
Free silica, in granite, has probably been solidified cotemporaneously with the other
component- minerals; but its solidification appears often to have been completed last
of all, as the quartz does envelop the aggregation of the other crystals. In quartzose
porphyry and rhyolite, on the other hand, free silica, at least the greater part of it,
has been solidified first, which is obvious from  the fact that its perfect crystals are
imbedded in an ordinarily microcrystalline aggregation of the other ingredients, which
however contains, in most cases, some surplus of free silica that had not entered into
the composition of the crystals.  This fundamental difference from  granite points to
some difference in the mode of origin. Quartzose porphyry and rhyolite differ in regard
to the texture of their paste, which has a compact aspect in the first, while it is more or
less vesicular throughout the mass in most varieties of the latter. This difference, like
the former, appears to indicate, that the molecular condition of the liquid mass, at the
time when it was ejected, was different in either case. The variety of texture diminishes
with the decrease of silica, and the more basic rocks of the three classes bear a much
(7 ()




OF VOLCANIC ROCKS.                                39
closer resemblance to each other than the silicious rocks.  The cause must be, either
that the conditions of the mass before cooling were less varied with basic than with
silicious rocks, or that the differences were alike, but manifested themselves less conspicuously ill the character of the solidified rocks; the latter cause is the most probable
of the two, since even our cotemporaneous basaltic and andesitic lavas offer but a small
number of varieties comparing with those of rhyolite or trachyte, although it is not
likely that there is any more variety among the circurnstances modifying the latter than
among those which are acting upon the former. It is chiefly the similarity in texture
which has occasioned the general application of such names as' trap," " greenstone,"
" aphanite," and others, for basic rocks of all ages, while those containing a large
amount of silica have scarcely ever been similarly confounded. We may, however, note
this difference among the former, that granitic texture — that is, a crystalline aggregation of the component minerals-is peculiar to those basic rocks associated with granite,
while the vesicular or trachytic texture is only proper to volcanic rocks. Occasionally,
though very rarely, rocks of granitic texture are geologically associated with porphyritic rocks, as is the case near Predazzo, in southern Tyrol.
5th. Geological age; granitic rocks being generally the most ancient in origin,
volcanic rocks the most recent, while those of porphyritic structure are intermediate in
age between both.
6th. Other geological conditions resulting fromn the correlation of the different
foregoing principles, the outlines of which I will attempt to trace in the following
pages.
Correlation of Age and Texture.
It is scarcely possible to treat of one of the foregoing principles in its application to eruptive rocks singly, without constantly encountering points of intimate
connection with otlhers, so thoroughly are they intertwined and mutually dependent.
It is the object of petrology to develop these correlations. We contemplate them
here only in their purely geological bearings, in order to try to establish the true relation of volcanic rocks to their ancient predecessors.  We have, while occupied with
these considerations, constantly to keep in view how few are the observations upon
the strength of which we have to base conclusions. The area on the globe whose
special geology has been made the object of investigation, though extending every
year, is still very limited; and even in the best explored countries, little attention
has, in most instances, been paid to the distinction of eruptive rocks. Observations in
regard to them are abundant in some parts of Europe, fortunately in such countries
as are especially capable of giving a clue to their general knowledge.  Distinct
conclusions may be arrived at in regard to portions of that continent, but little scope
is afforded for giving them latitude by comparison with the relations presented in other
countries. We have, therefore, to keep well separate in general petrolog-y, those positive conclusions which are founded upon sufficient observations, and are applicable to
the relations in those countries where the latter were mnade, and the realm of theories,
which are arrived at, partly by the generalization of those conclusions, and partly by
making deductions fromn hypothetical suppositions; because, the premises being founded
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40                        RICHTHOFEN-NATURAL SYSTEM
on local occurrences, and their general validity not being proved, the theories must
necessarily have a great deal of uncertainty, which will only gradually be dispelled by
the advancing knowledge of the geology of the globe.
The correlation of age and texture, as resulting from  observations made in
Europe, will occupy us first.  Granitic rocks are widely distributed on that continent.  Their great eruptive masses are of Azoic and Paleozoic age.  The rocks
have almost throughout granitic  texture, though the distinguishing features of
porphyritic rocks are proper to some subordinate varieties of diorite and diabase.
The eruptive activity exhibited in the granitic era gradually relaxed.  It appears
to have been insignificant, though it was by no means extinct, in the latter part
of the Devonian and the first part of the Carboniferous periods.  But it recomnmenced about the time of the deposition of the coal measures, thence increased in intensity, and was most violent in the Permian age, though it was, during all that
time, much more limited inf extent than it had been during the granitic era.  The middle portions of Germany were then its principal theater, until it changed, in the commencement of the Triassic age, to the southern slope of the Alps and Carpathians.
The rocks produced during this era possess, almost exclusively, porphyritic texture.
The Jurassic, Cretaceous, and the first part of the Tertiary ages, were distinguished
by an almost complete repose of eruptive action, in Europe.  It was only after the
commencemelnt of the Tertiary period when that violent resumption of eruptive activity took place to which we have repeatedly called attention in the foregoing pages,
and which thereafter continued, gradually relaxing, clown to our present era, the manifestations of volcanic action in which are its last faint remnant. -Aesicular inflation,
which is the characteristic feature of trachytic texture, is peculiar to the rocks of this
class; all of them possess it more or less, though there are varieties resembling porphyritic rocks closely in aspect.  A combination of the trachytic and porphyritic modes
of texture is of more common occurrence than either of them singly, and is indeed
the distinguishing feature of volcanic rocks.
These relations of age and texture have been conclusively proved to exist in
Europe, and appeared to justify the conclusion, that the three classes of eruptive rocks
are geologically separated, and represent three successive and distinct phases of the
manifestation of subterranean agencies.  Considering in their generality the facts observed in other countries, they appeared to confirm these views, since granitic rocks
are known to be generally very ancient; volcanic rocks to have been generated during
the Tertiary and Post-tertiary periods; while, in regard to porphyritic rocks, observations
have been scarce, and descriptions lack distinctness, yet no relations have been recorded
in reference to them which would be contradictory to those observed in Europe.  On the
western coast of North America, however, eruptive rocks exhibit some peculiar relations-differing to some extent from those which they offer in Europe.  Positive facts
are known sparsely outside of California, but conditions similar to those observed in
that country appear to be repeated through large portions of the range of the Andes.
Prof. J. D. Whitney's admirable researches on the age of the metamorphic rocks of
the Sierra Nevada have clearly demonstrated that the granitic rocks, which partake to
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OF VOLCANIC ROCKS.                             41
great extent in the structure of that mountain range, cannot possibly have been
ejected prior to the Jurassic epoch.  The texture of these rocks is, notwithstanding
this comparatively recent origin, that of all true granite, and the prominent varieties
cannot be distinguished from some European kinds of granite, as for instance those of
the Adamnello and the Cima d'Asta in the Southern Alps, which are among the most
receint in age on that continent. Volcanic rocks are widely distributed in the Sierra
Nevada, and are of the same or similar age as in Europe.  Quartzose porphyry occurs
to some extent in WTashoe, under circumstances which make the exact determination
of its age difficult, but render it certain that it is intermediate in this respect between
granitic and volcanic rocks.  These relations would appear to be an exact counterpart
of those observed in Europe, with the one prominent difference, that the commencement
of the eruptive action was much later in America.  Very recently, however, additional
observations have been nmade, which give a somewhat different aspect to these relations.
Mr. Clarence King observed granite, covered by Palkeozoic rocks and antecedent to
them in age, near the Colorado River; while Prof. Whitney and myself discovered true
quartzose porphyry in the county of Plumas, in northern California, associated with
rocks proved by the former to be of Triassic and Liassic age, in such way as to leave
little doubt about its cotemporaneous origin.  Farther east, in the Great Basin,
Paleozoic granite is of no rare occurrence, and it is among the prominent features in
the geology of the Rocky Mountains; while the discovery of porphyritic rocks may
have to await further examination, they having been in most countries the last eruptive rocks to be detected.
However these facts may affect the theoretical conclusions in regard to the
origin and mutual relations of granitic and porphyritic rocks, which had been made on
the strength of former observations, they appear to confirmn the separation, fromn a geological point of view, of both classes of rocks. There has been in the Sierra Nevada and
adjacent countries, it appears, an ancient granitic era corresponding to that of Europe,
followed by a porphyritic era which was nearly or quite coincident with the European.
But, while the manifestations of subterranean agencies almost ceased in Europe during
the following ages, they recommenced With great intensity on the western coast of
North America, and gave rise to a second granitic, followed by a second porphyritic era.
The volcanic era commenced in both countries in the Tertiary epocl, but it appears to
have been in an advanced stage in Europe while it was still in its birth-throes in
America.
Correlation of Age and Composilion.
This point of view is not inferior in interest to the foregoing.  The most
noteworthy fact is tis, that quartzose, and in general highly silicious rocks prevail
among those of ancient origin, basic compounds among those of later ages.  Granite
and syenite are overwhelmingly predominant among ancient eruptive rocks. Diorite
and diabase are generally associated with them, but remain alwvays quite subordinate
in bulk.  The relative proportion is different with porphyritic rocks.  So little
attention has been paid to these, outside of Europe, that general conclusions in
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42                        RICHTHOFEN-NATURAL SYSTEM
regard to them  should be drawn with care.  In the middle part of Germany,
and in southern Tyrol, where they have been repeatedly studied, subjected to
chemical analysis, and described in numberless treatises, quartzose porphyry is
rather predominant.  But porphyrite, melaphyr, and augitic porphyry, are, in the
aggregate, little subordinate in bulk.  The volcanic offers the complete reverse
of the granitic era, respecting the proportionate quantity in which the different compounds have come to the surface.  Andesite and basalt compose as large a proportion
of the aggregate bulk of volcanic rocks, as granite and syenite do of those of the
granitic era.
Some minor differences in age may be noticed among the different orders composing the three classes of eruptive rocks.  Extrusio(ns of granite and syenite appear
to have been almost the exclusive feature of the eruptive activity during granitic eras.
and to have been succeeded only towards their close by the emission of diorite and
diabase, or of other rocks of limited occurrence, such as gabbro and hypersthenite.
Such at least has been observed to be the case in several countries, in regard to the
chief outbreaks; but if we enter into the details of the mode of succession of the
rocks belonging to the different orders, we perceive that it has not been so definite
as with volcanic rocks  granite and syenite bearing evidence, in many localities, of a
more recent age than some neighboring masses of basic rocks.  Yet, in keeping only
the main features in view, we may easily see, that the general order of succession of
granitic rocks has been conformable to a gradual decrease in silica. Syenite is usually.
more recent in origin than granite; and even among the different varieties of the latter, true granite, containing the highest ratio of silica, has generally been anterior in
age to G. Rose's granitite.  There may be some connection between these relations
as they are exhibited in any single granitic district, and the fact that the granitic rocks
of the Sierra Nevada, belonging altogether to a mnore recent era, contain no true granitetheir chief bulk consisting of rocks intermediate in composition between granitite and
syenite.  The porphyritic era, in Germany and on the southern slope of the Alps, was
inaugurated by eruptions of quartzose porphyry, and has terminated in the Alps by
those of augitic porphyry.  The intermediate epoch has been distinguished by rocks
intermediate in composition.  The mode of succession of the different orders is more
distinct than with granitic, but less so than with volcanic rocks.  Melaphyr and porphyrite interchange frequently; but, at many places, the former appears to have preceded the latter in age, in a similar way as andesite preceded trachyte.  They form
together one epoch, which, in the commencement, was occasionally interrupted by
an outbreak of quartzose porphyry.-The laws of the periodical succession of the five
orders of volcanic rocks have been developed in the foregoing pages. Their epochs
are much more distinctly separated than those of the ancient rocks, but the mode of
their succession is more complicated.  With granitic rocks, silica as a component part
decreases with the age; with porphyritic rocks the same is true in a broad sense —the
precedence in age of melaphyr and porphyrite forming the only conspicuous deviation; while in reference to the volcanic era, no rule at all may be discovered at first
sight.  Considering however, the predominant rocks of that era, which are propylite,
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OF VOLCANIC ROCKS.                              43
andesite, and basalt, we perceive that the former two were the precursors of the latter,
containing at the same time silica in a proportion superior to basalt.  The singular
phenomenon of trachyte and rhyolite being ejected at a time intermediate between
the epochs of those rocks, is a notable deviation from the mode of succession peculiar
to granitic and porphyritic rocks. We will try to consider its probable causes in another
chapter.
Correlation of Eruptive Rocks in regard to their Geographical Distribution.
Another point of view offering in the contemplation of eruptive rocks, and which
has a close bearing to their natural system, is the correlation among them in regard to
their geographical distribution.  This is, however, a vast subject, and volumes might
be written in collecting evidence for final argument of general value.  At this place
we can give it only a 0ursory notice.  We will consider, first, what is the mode of
distribution peculiar to each class of rocks, and then trace the correlations perceptible
among them in regard to their different modes of distribution.
Granitic rocks are scattered widely over the globe.  Wherever its surface is
conlposed of ancient sedimentary rocks, and these give evidence of disturbances of
some intensity, by the plication of their strata, we may be almost certain to find granite
entering into the geological composition to some extent.  In the diversified structure
of the European continent, geological maps show the existence of granite ill nearly
every prominent mountain-range.  Considering among them the range of the Alps and
Carpathians, we find granite scattered over its whole extent, from  Savoy to Transylvania, particularly on the southern slope.  It forms some prominent summits, but
occurs also in subordinate positions. The geological relations of several of these
places have been examined, and careful investigations made of the mineral and chemical composition of the rocks.  They have resulted in proving an individuality of
granite such as is peculiar to no other kind of eruptive rocks; different granitic
masses are of different age, and exhibit a corresponding diversity in regard to the
composition of the predominating rocks.  Sometimes, it is true, several neighboring
masses are similar in respect to their petrographical character, and bear evidence of
being nearly of the same age (for instance those of the Adarnello, the Cilra d' Asta,
and Brixen), but others, next adjoining, will be found differing from them in nature
and in age. The length of geological time during which they have been ejected, has
never been established d; but the period appears to have been one of immense duration.  A similar individuality as to age and mineral character may be noticed in
respect to the granite of other mountain ranges on the European continent.  Altogether, the mode of distribution of the ancient granitic rocks, as far as they enter into
the structure of the surface, nlay be said to be in numaerous small districts, which are
independent of each other in regard to their epoch of ejection and petrographical character.  Writh reference to the latter, each separate district shows a great preponderance of rocks belonging to one of the families of the granitic order, which are usually
accompanied by syenite in smaller proportion, and by some subordinate eruptions of diorite and diabase.  These districts are principally scattered along the present lines of eleva(si)




44 4IRICIITHOFEN-NATURAL SYSTEM
tion.  But this may be due in part to their concealment, in the spaces between those
lines, by sedimentary rocks. More signs of the separation of distinct " regions of eruptive
action," as they may be called, are exhibited in the porphyritic era.  Eruptive activity has been intense within them, but appears not to have spread beyond certain boundaries.  Each of these regions embraces a number of the former granitic districts,
while it leaves others excluded.  Some resemblance with the peculiar features of the
granitic era is afforded, inasmuch as each porphyritic region has been independent
from others in regard to the epoch of its eruptive activity.  We mentioned before
that one of the porphyritic regions comprises the middle part of Germany, while
another stretches along the southern slope of the Alps and Carpathians.-If we
proceed to the volcanic era, it presents to us the reverse of the individuality
peculiar to granitic districts, in the wonderful unity exhibited in regard to time
and space over the whole area of extensive belts.  In reference to unity of time
alone, we might call the greater part of the continent of Europe, and even the
entire surface of the globe, one great region of eruptive action, during the volcanic era, since the first emission of rocky matter has been nearly cotemporaneous
in widely separated countries, while its culmninating epochs have probably varied but
little in them  separately, and the rocks have been ejected everywhere in a similar
order of succession.  In regard to local distribution, however, we have to distinguish
certain belts, far exceeding in area the porphyritic regions.  Each of them extends
over a number of preexisting mountain ranges, and the eruptions in each have followed, in their distribution, chiefly the lines of former elevation and ancient seacoasts.  But there is uninistakably to be recognized a tendency of the agencies which
caused the eruptions, to connect these separated ancient belts of elevation; either longitudinally, when ranges superior in extent to the preceding ones would be formed, as
appears to have been the case in the Andes; or, as it were, in a lateral sense, when
the connection of neighboring mountain ranges into table lands would be either
initiated or promoted.  One of these belts, consisting of parts which had previously
been disconnected, may be traced firom Armenia to the Rhine, though I will try to
show in the sequel that it is only a part of another belt which is of faLr greater extent.
We mentioned before, that porphyritic rocks are encountered chiefly in those
places where granitic rocks had preceded them. As regards the volcanic belts, eruptive
activity has been particularly violent in certain portions of them. It is worthy of
note that, whlerever this has been the case, either granite or both granitic and porphyritic rocks had been ejected before. This fact leads us to consider the correlation of
the three classes of eruptive rocks in reference to their peculiar modes of distribution.
Little information exists in regard to this subject. Only one instance"6 shall be related,
which is highly suggestive for the existence of such a correlation, though it is of slight
value as long as it is not corroborated by corresponding facts observed in other countries.  A survey, on a geological m1ap of the Alps and Carpathians, of the southern
boundary-line of those highly metamorphosed forzmations which preceded the Trias in
16 Referred to in Richthofen, Geognostische Bechreibng dcer Umgeg~end von Precdazzo, Sauct Cassian unodl ier
Seisser Alp in Siidcl-Tyrol: Gotha, 1860.
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OP VOLCANIC ROCKS.                             45
age, and chiefly compose the central portion of the Alps, shows that it is directed from
west to east in Lombardy, but in the vicinity of Lugano bends suddenly to the northeast, then turns as abruptly back to its former direction from west to east. After
having passed the granitic mass of the Adamello, the same change is repeated on a
grander scale.  The boundary is turned again in a perfectly straight line to the northeast, and then resumes its former course, which it follows in an equally direct mallner, and in which it continues, exceedingly distinct at first-less distinct, by the
encroachment on it of more recent formations, farther east —until it turns a third time
to the northeast at the sudden termination of the Alps near Vienna, and continues in
this direction for a long distance.  Finally it re-assumes, in the Carpathians, a similar
course to that which it had on the southern slope of the Alps.  Three very distinct
reentering angles are thereby formed. The first of them encloses the country of Lugano; the second comprises the vicinity of Predazzo and Fassa in southeastern Tyrol,
and of Belluno and Vicenza in Venetia; while the third, which is by far the most
extensive, comprehends all northwestern Hungary. Each of them has been a center
of eruptive activity, commencing with the granitic, and continuing through the porphyritic down to the volcanic eras, and all three are among the most classical countries for the study of eruptive rocks. There is, however, a conspicuous difference in
the mode of manifestation of the eruptive activity in each of the three eras. Little
connection exists apparently between the granitic masses of the three countries. They
are portions of the generally scattered granitic outbreaks, and differ among themselves
probably as much in age as they do in regard to the nature of their rocks. In the
porphyritic era, eruptive activity was contemporaneous in the three localities, but
scarcely extended beyond them. In the volcanic era, when the southern slope of the
Alps and Carpathians formed only a portion of a much more extensive belt, the
countries adjoining those three places were chiefly distinguished by the intensity of
eruptive activity.
Great as have been the interruptions between the different eras, the continuance of the selection of those three nooks at the foot of a prominent mountain range
for the manifestation of subterranean energy, from Paleozoic down to modern time,
is evident. Similar instances, though less striking, might be mentioned from other
parts of Europe, such as the porphyritic region of middle Germany. Reverting to
other parts of the globe, it appears to be a general experience, though it is far from
being absolutely proved, that all the principal accumulations of volcanic rocks are
encountered in the neighborhood or immediate vicinity of granitic masses. These are
scattered over areas where no volcanic rocks occur; but the distribution of the latter
within any of the volcanic belts appears to have been dependent, in a great measure,
upon the vicinity of the channels which had in preceding time afforded vent to granite.
The general law deduceable from these relations is this: that, with the growing
thickness of the earth's crust, the systems of fractures which were formed in it at certain epoclhs, and partly gave vent to the emission of rocky matter, increased in depth
as well as in length, and were more and more concentrated to definite portions of the
crust, which are recognizable upon the earth's surface by the partly coincident areas of
K;                                                     (83)




46                       RICHTHOFEN-NATURAL SYSTEM
eruption; that further, simultaneously with the increase in extent, each area manifested
a growing complexity in regard to the distribution of eruptive activity within it, and
that this distribution depended in a great measure upon that of the outlets of more
ancient eruptive matter. The first recognizable stage of this development, which,
however, was probably a far advanced one, is the individualization of those numerous districts of fractures, the narrow limits of which are made manifest by the
mode of occurrence of Azoic and Paleozoic granite. A growing development from that
stage, in the different directions mentioned, is conspicuous in the porphyritic era in
Europe; when, besides the greater extent of the regions in which subterranean agencies
manifested themselves by the fracturing of the crust, every such area had become
more definite concerning its boundaries towards those which were not fractured during
the same era. On the other hand, however, each porphyritic region was more complex
than the granitic districts had been, inasmuch as the former were composed of certain
areas of greatest activity, which, as we had occasion to remark, appear to have been
chiefly dependent upon the distribution of the granitic districts enclosed within each
porphyritic region, while intermediate portions were contemporaneously affected by
disturbances merely, but not by any eruptions. A more advanced stage in this gradual
development seems to be exhibited by the Jurassic granite of the Andes. The belt
distinguished by its eruption appears to have been distinctly bounded, and the area
over which contemporaneous manifestations of like character took place, to have been
even more extensive than the porphyritic regions of Europe. All these features, however, are conspicuous on a much grander scale in the volcanic era. The growing
definiteness of the boundaries of the volcanic belts towards large areas which were
entirely free of eruptive activity, the increasing complexity of their interior arrangement,
and its dependency upon preexisting lines of elevation, and particularly on the
distribution of granitic and porphyritic rocks, are evident from what has been said before.
WTe may even trace a development from the andesitic to the basaltic epoch. It is
known how far superior in extent is the range of basaltic outbreaks to the area occupied
by other volcanic rocks. If considered with attention, it will be found that there is a
tendency in the former to connect within each belt the subordinate ranges of the latter,
longitudinally as well as laterally. The only connection, for instance, between the
volcanic districts of Hungary and those on the lower Rhine, is occasioned by the chain
of isolated basaltic hills which extends through the central part of Germany.
ON THE ORIGIN OF VOLCANIC ROCKS.
The facts established in the foregoing chapters, and the inferences drawn therefrom, may assist us in considering some questions of wider bearing. We have seen that
the volcanic rocks of several, and probably of all parts of the globe, are connected by
simple and definite relations, which together comprehend the main features of their
natural system; and we found that correlations of a similar nature ally the volcanic
with all the ancient eruptive rocks, justifying, to some extent, the supposition that the
eruptive rocks of all ages and places form one harmonious whole, and that we may be
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OF VOLCANIC  ROCKS.                                               47
able to discover the laws of the natural system embracing their totality. In order to
arrive at a more perfect understanding of this system, we must now attempt to examine into the causes of those mutual relations. It would naturally occur to us that they
would be implied  in the circumstances which attended  the generation of the eruptive
rocks, and in the conditions in which they have been before arriving at the places they
occupy at present.   We have, therefore, to investigate the following questions: What
was the nature of volcanic rocks before they arrived at the places which we now
see them occupying?  Where did they originate?  By what agencies did rocks connected in widely separated places by simple and definite relations come to their
positions among others which bear no such relations either to them  or among each
other?   The  importance of these questions for our  subject, the attention which  they
have attracted through the whole history of geological science, and the great diversity
of opinion  prevailing  in regard  to  them, will make it necessary to  treat them  more
fully than might otherwise appear consistent with the objects of this paper.
Volcanic action  and  massive  eruptions, notwithstanding  the  similarity of the
material produced by both, would appear, from the most cursory review of the
phenomena connected with either of them, to differ to some extent, not only in regard
to the causes to which they owe their origin, but also in regard to the position the
matter occupied before its ejection. We have for this reason to keep again distinctly
separated  these  two modes of manifestation  of subterranean  energy.   In  regard  to
massive eruptions, which will first occupy our attention, we can scarcely draw  up any
argumentation without enlarging on the entire range of eruptive rocks, and extending,
at least partially, our views to them.l'
1.   On  the  Origin  of Massive  ErzGPions.
In  order to establish  some  positive  premises available for drawing  conclusions
in regard to the origin of the massive eruptions of volcanic rocks, we reiterate the
following  facts,  of  which  mention  has  partly  been  made  in  the  foregoing  pages:
1st. The eruptive (including the volcanic) rocks offer a great diversity of chemical
composition; but all the compounds represented by them  are mutually connected by simple and definite arithmetical relations as regards the figures which
17 The following considerations are given notw ithout some hesitation, partly on account of the uncertain ground on
which they have to move, and partly because some of their main features are, of necessity, only well known theories reproduced, though, perhaps, under a somewhat different form. Yet, the establishment of the relations detailed in the preceding
chapters, and other observations made of late years, may allow us to arrive at more satisfactory conclusions in regard to
some weighty problems than could be done before, or, at least, to determine more precisely the only direction in which we
have to look for their solution. It should be borne in mind that among the theories recently proposed upon the subjects
specified above, there is not one which has not already had its prototype in the phantasmagorias of the time of the dawn of
geological science, and that it is these which have been constantly reproduced, enlarged, diversified, remodeled according to
the advance of science, and supported by continuous accumulation of evidence. Propositions which had been accepted as
being beyond the necessity of proof, and which are still occasionally reproduced as axioms in popular works on geology, have
been weakened, and not unfrequently overthrown, when facts newly revealed would withdraw their chief supports, but have,
after some time, revived under new forms. In treating on a topic where the degree in which the results of our speculation
appear satisfactory to us depends upon the degree of probability which we think we see in the theories arrived at, and of
their faculty of explaining observed facts, and where we are in constant danger of making incorrect deductions from  imperfect premises, not enough can be done in the way of weighing the evidence by which different doctrines are supported; and
this is particularly necessary in reference to those theories which we are too much accustomed to consider as matters of fact,
upon which further conclusions may be safely built.
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48                       RICHTIIOFEN - NATURAL SYSTEM
express the value of the different ingredients entering into their composition.  The
law of Bunsen, in which are embodied these relations, though true -for all eruptive
rocks, has never been applicable to those of sedimentary origin, nor to the metamorphic sedimentary rocks.  2d. The order in which massive eruptions of volcanic rocks have taken place in different countries, is by no means conformable to
a regular succession of gradations in chemical or mineral composition, but shows
the existence of several distinct groups, each of which comprises, clhemically, a certain
portion of the entire range of compounds intermediate between the two extreme types,
and has, besides, its own peculiarities of texture and mineral composition.  It is for
the latter reason that rocks, belonging geologically to two different groups, may be
identical as chemical compounds, and yet differ petrographically. We may repeat
here that these groups, wherever their mutual relations have been made an object of
study, occur in the-same order of time, and that the rocks belonging to each of them
present generally a great similarity in character wherever they are encountered.
3d. The massive eruptions of volcanic rocks are distributed over the globe in certain
belts, differing in extent and width as well as in direction.
There are, among those of a general bearing, two manifest conclusions at which
we may arrive by the aid of these facts.  The first relates to the origin of the matter
ejected. It may be said that equality of physical and chemical properties, and of the
mode of occurrence, will commonly involve equality of origin. We have, therefore,
to infer, that the source from which volcanic rocks have derived their origin has
been similar in nature in every locality, that the definite numerical relations existing
in the chemical composition of the matter ejected must exist similarly at that source,
and that they must there pervade matter equally in different parts of the globe; and
we may further add, that at the same source the different kinds of matter which correspond to the different passages in composition among volcanic rocks, must be arranged, at every locality alike, in a definite order of position in reference to the
distance from the center of the earth, since such relation in space can alone explain
the definite order of succession in time in which those rocks have been ejected. If
we consider the relations existing between volcanic and ancient eruptive rocks, there
can be no doubt that all of these have derived their origin from that same source, and
that it is of general distribution under the surface of the globe. The second general
conclusion relates to the cause and mode of ejection. As like effects imply like
causes, and the similarity of the phenomena connected with the massive eruptions of
volcanic rocks is conspicuous, we may infer that the agencies to which they have been
due, were, in the main, similar in all cases. However accidental and local ci rcumstances may have caused minor differences, the prominent features in the mode of
geological occurrence and geographical distribution cannot be due to them. In how
far we may be justified to ascribe the ejection of the granitic and porphyritic rocks to
similar agencies, will depend upon the degree of similarity of their part in the structure of the surface of the globe with that of the volcanic rocks. The conclusion must
be drawn froni those correlatious which have already been mentioned, that the funldamental causes of all eruptive activity are alike, and implied in general planetary processes which are closely connected with the evolution of the globe.  These general
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OF VOLCANIC ROCKS.                            49
conclusions appear to be the only ones which are fully justified. If we venture to
inquire into the nature of the agencies which caused eruptive activity, and to explore
their fundamental causes, we have to transgress the limits of legitimate speculation
and enter the realm of hypothesis, the only way of proceeding in which consists in
weighing probabilities.  Further experience, it may be hoped, will extend the limits
of induction, in the same measure as it will furnish increased evidence.
I will attempt, in the first place, to consider, what must be the nature, and
what the position of the source at which the volcanic and all eruptive rocks have
originated, in order that their physical and chelnical properties and their mutual relations may be explained, and then to examine what have been the agencies that have
had, in all probability, the most immediate influence upon the facts connected with
the mode of distribution and succession of eruptive rocks in. general. We have to
start in these considerations by a well-known argument.
If no changes had ever taken place on the surface of the globe, and no sediments had ever been deposited on it, but that state was still preserved which the
globe must have exhibited when the substance of the rocks was liquid on its surface,
then the matter nearest to this would probably be of a nearly uniform chemical composition, and, consequently, of a uniform specific gravity. If we imagine the globe to
consist of concentric layers, it is exceedingly probable, from physical laws, that then the
substance of each successive layer would be, too, of a nearly uniform composition
throughout its extent.  Proceeding, however, from those layers nearest to the surface
to those at greater distance from it, the specific gravity of the matter composing them
severally would necessarily increase gradually towards the interior. This would be
effected by the tendency of the heavier elements to predominate as the depth became
greater, while the lighter would increase in as gradual a ratio towards the surface.
If the lavers were infinite in number, then the passage, in chemical composition as well
as in specific gravity, from the surface towards the center of the earth would be one of
infinite gradations. This condition is eminently the most probable, if physical laws
are taken into consideration. If the crust was then allowed to solidify, to any depth
required, the same interior arrangements would continue to exist unaltered, regardless
of any changes on the surface.  That a condition similar to this actually exists at the
present time, is rendered evident by the well-known fact, that the specific gravity of
the mass of the globe greatly exceeds that of the average of the matter which composes its exterior crust. Eruptive rocks have been carried to the surface from places
beneath it. The depth from which their material has been derived, the way in which
it was rendered liquid, and the cause and mode of its ejection, these are the principal
points of conjecture in regard to them. The most probable place of its derivation
are those imaginary layers which are beneath the theater of external changes, and still
occupy their primeval position. This appears to be manifest from the two facts to
which we have constantly to refer: the ejection  of identical chemical compounds in all
countries and ages, and the exhibition by eruptive rocks, in each country, as well as
over the entire globe, of a series of compounds the specific gravity of which increases
in inverse ratio with the amount of silica. The suggestion that eruptive rocks, in
virtue of these facts, represent the arrangement of nmatter in the interior of the globe,
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50                       RICHTHOFEN - NATURAL SYSTEM
those which are rich in silica and of little specific gravity being derived from  places
nearer to the surface than those in which a smaller proportion of silica is attended by
a higher specific gravity, was first distinctly made by Sartorius von Waltershausen,
who attempted to prove this theory by mathematical calculation, and has at least succeeded in demonstrating its adaptation to the nature of eruptive rocks. The assumption, hypothetical as it may still appear, has an eminent degree of probability, and is of
the highest importance for the geology of these rocks, for the reason that it is capable
of explaining numerous observed facts which have failed to be satisfactorily explained
in any other way.
We may still draw a line of negative argument in corroboration of this theory.
It is perfectly clear that the source of eruptive rocks must either have been below the
lowest depth at which rocks of sedimentary origin occur, or above it. We found that
the former assumption gave a satisfactory explanation of prominent facts, and it is
just as conspicuous that the second fails to give any explanation at all. If eruptive
rocks had had their original seat within the crust of sedimentary rocks, and had been
generated from their substance, they must be analogous to them in chemical composition, that is to say, they would vary in this respect within very wide and complex
limits in any single country, and we should encounter no lesser differences when comparing the totality of eruptive rocks in one part of the globe with that in other parts.
The fact that no arithmetical relations can be recognized as connecting the various
sedimentary rocks in their composition, would hold equally good among eruptive
rocks, since it is impossible to conceive a progress from the indefinite to the definite,
from that which is void of any recognizable relations in respect to the composition of
matter to that in which such definite relations are plainly evident, by the mere
influence of such agencies as.would cause fluidity, (that is, the combined action of
heat, pressure, and water.) As no agency, indeed, is known to which such a result
may be ascribed, this argument removes beyond the limits of probability the
assumption, that the original seat of eruptive rocks has been above the foundation of
the gradual accumulation of those rocks which have been produced by external
changes, and therefore allows only the other alternative, that it has been below that
boundary.
While the theory of Sartorius will explain the causes of the prominent mutual
relations existing among eruptive rocks in regard to their composition and general
similarity in all countries, we have still to trace those causes which effected their ejection from a deep-seated original place to the surface. This question is more abstruse
than the foregoing, inasmuch as the manner in which forces have formerly been acting
beneath the earth's crust, is a subject more involved, and allowing a larger range of
hypothetical explanation, than we met on a field where the observed properties of
matter offered a comparatively safe guide. Complicated as the processes appear to
have been, to the coiperation of which massive eruptions were due, it seems that
they may severally be traced back to one common cause of a higher order, of which
all, or most, of them are but different modes of manifestation.  Probabilities accumulate to point out as this fundamental cause, the process of the gradual cooling of the
earth and the solidifying of its crust toward its interior.
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OF VOLCANIC ROCKS.                           51
Since the early day of the ingenious speculations of Descartes, this great and
general cause has been considered as the main agency to which the disturbances
on the surface of the globe are due; and, though having been more or less in
favor at different times, the doctrine has at no time been completely abandoned.
The various aspects which it has periodically assumed, the latitude given to it
on one side, and the objections raised against it on the other, mark one feature
of the phases of the gradual progress of science. It has been applied in different
forms to explain the mode of origin of ancient eruptive rocks; and, since Dolomieu and, in a more elaborate way, his pupil Cordier, have assigned the same
original source to volcanic activity, the contraction of the interior of the globe, by
radiation of its heat into space, has been considered as offering a sufficient explanation for the majority of the phenomena which are often united under the term
"vulcanism."   But, giving all due consideration to the vast effects of which it has
undoubtedly been the cause, the conception of the modus operandi of this agency
(contraction) alone meets with considerable difficulty. An outward tension might,
indeed, result in the formation of fractures on elevated places; and, supposing for a
moment that these fractures would descend into regions where matter was in a liquid
state, then the latter might possibly be ejected, and caused to accumulate on the surface. But such would hardly be the effect of an inward tension. It may, too, cause
the rending of the crust and, possibly, the filling by liquid matter of those rents
which are in the lowest places; but it does not explain the extrusion of this matter
to the surface, nor the fact of its particular accumulation on elevated parts of the
same. It would take too much of our space fully to detail the numerous mechanical
difficulties which occur, if one attempts to explain all the phenomena comprised in the
name " vulcanism" by the exclusive assumption of the contraction of the interior of
the globe. Some of the more obvious objections against this theory will be briefly
mentioned in the course of the following considerations.
A number of facts point towards the existence of some unknown force below
the earth's solid cr'ust, which counteracts in a considerable measure the.permanent
subsidence of the latter by contraction. It is perfectly evident that the secular rising
of parts of the surface of the globe above the level of the sea cannot be merely the
apparent effect of the different degree of its general subsidence, as has been maintained
by very distinguished geologists; but that elevation, that is, the periodical increase of
the distance of parts of the surface of the globe from its center, must be a reality.
Considering the amplitudes of the changes of level that have taken place in historical
time, they will be found to sum up to such figures as, if reduced altogether to subsidence, would indicate a far greater shortening of the radius of the globe within that
time, than is compatible with astronomical calculation.  It is true that the retardation
of the rotation of the earth by the tidal wave must counteract, in some measure, the
acceleration caused by any shortening of the radius of the planet; but this retardation
is insignificant if compared with the amount of the changes of level.  The reality of
elevation is forced more directly upon the mind, if those cases are taken into consideration where certain portions of continents are rising above the level of the sea at a
more accelerated rate than neighboring regions, which is of very frequent occurrence
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52                                 RICHTHOFEN - NATURAL SYSTEM
in volcanic countries, and conspicuous in those  numerous instances where elevation is,
or has been in  former times, proceeding  more  rapidly  along the  crests of mountain
ranges than at either foot of them.   Adherents  of the  theory, that all oscillations of
the earth's crust are  only due to  its contraction, consider elevation  as real in  these
cases, and have tried to explain it by the assumption, that folds must be formed by the
subsidence of the ample shell on the contracted nucleus; that these folds would increase
in amplitude in consequence of a lateral pressure caused by further subsidence, and
thus an absolute rise would be effected. It is, indeed, very probable that this process
is of vast importance in the formation of mountain  ranges; but it cannot explain the
total amount of the changes of level.  If Fourier's calculation, that, taking the present
loss of heat by the globe as standard, the radius of the latter should have shortened
seventeen centimeters in twenty-five  centuries, is correct, the  diminution  would  have
been  1,700  centimeters in  250,000  years, and  about six hundred feet in one million
years. It is manifest how utterly insignificant would be the corresponding diminution
of the earth's circumference, when compared with the vast changes of level which the
surface must be supposed to have undergone during such a length of time.
We are bound, for these reasons, to consider, not alone the process of elevation
of mountain ranges, but also the secular rise of extensive regions, as realities which
cannot be exclusively explained by the contraction of the globe. But if so, there must
exist another force, the effects of which oppose those of contraction.  The united action
of both, and  the  periodical prevalence of either of them, would  then  be  capable  of
explaining the alternation of elevation and subsidence at every single place, and the
conltemporaneous action of both in neighboring regions. This antagonistic force consists, probably, in an increase of volume attending the slow and perfect crystallization
of matter, by cooling down, during immense periods, from a viscous state. With a
great number of bodies, contraction by loss of heat appears to continue to the very
moment of solidification, and to increase during the latter when no time is allowed for
crystallization, but to be diminished, and finally reversed, in the same measure as opportunity is given  for a slow  and  perfect crystallization.   In  the  special case  of rocks
made up of silicates, this must remain a supposition which  is not proved, but is erninently probable.l8
18 An increase in volume by crystallization has been found, by experiment, to take place in numerous instances, in fact
in most cases when a perfect crystallization from a molten state has been obtained, provided that the volume of the substance
experimented on could be determined immediately before the act of crystallizing. In respect to those silicates which make
up crystalline rocks (not taking into account the water entering into their composition), experiments could hitherto not
be made, because it is impossible, with our present means, to allow the viscous mass sufficient time for crystallizing. Yet,
there are some suggestive facts which have, in some measure, the value of experiments. We mention among them particularly one which was observed by Ferd. Zirkel in analyzing, with the aid of the microscope, the texture of the
minerals participating in the composition of eruptive rocks. He found that " glass cavities " contain ordinarily several
of those vacuities which are also exhibited by the cavities filled with water, and have been supposed by Sorby
and others to have originated from  the contraction, by cooling, of the enclosed matter. If the substance contained
in the glass cavities bears signs of an incipient crystallization, by having partly a "lithoid" texture, the vacuities are
of rarer occurrence, while they are entirely wanting in the so-called stone-cavities, when the lithoid texture pervades the
whole mass filling the cavity. It is scarcely possible to make an experiment more convincive than this natural occurrence,
which is more open to subtle observation and measurement than is ordinarily the case with experiments on cognate subjects.
It needs hardly to be mentioned, in connection with this question; that those experiments which have been made by Bishof
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OF VOLCANIC ROCKS.                                      53
Wte nmlay then distinguish as eminently probable, the following immediate effects
of the cooling of the globe: 1. Contraction of the liquid portion of the interior, by
cooling down to the temnperature required for solidification uinder the respective pressure. 2. Expansion by slow and perfect crystallization. 3. Contraction of the crystallized masses by further cooling.  We may add to  these as a secondary effect of
greatly inferior importance, the flow of heat or change of chthonisothermal planes, at
those places where subsidence causes an accumulation, or elevation an abrasion of
matter on the surface. This process, to which a truly wonderful importance has been
ascribed by some, must be going on continually and everywhere. Being itself the
result of motions of the crust, its proximate cause must be the processes attending the
cooling of the globe, while in its totality, it may have a further, though very insignificant, effect ill shifting the areas of elevation and subsidence.
Other agencies, with which we are yet unacquainted, may probably result from
the same common cause. But taking into consideration only those already mentioned,
it may appear difficult to form  a clear conception  of the mode in which  they must
cooiperate, in order not to counteract each other, and thereby to result in a general
movement of the crust in one direction  only, but to bring  about that variety of
effects which manifests itself chiefly in the change of elevation and subsidence, and in
other modes of dislocation of the crust.  It is perfectly evident, that the aggregate
effect of those agencies is different under different portions of the crust of the globe,
contraction  prevailing in some parts, and expansion in others; the former causing
subsidence and deposition of sedimentary matter, the other elervation and denudation.
But if we consider how gradual is the passage in the state of aggregation from the liquid
interior of a current of lava to its solid crust, and if we bear in mind how immensely
vaster in volume are those masses which constitute the interior of the globe, and what
imumensely longer time is given for their cooling and crystallization, the conclusion is
irresistible, that the passage in the states of aggregation from solid to liquid must
extend there over an immensely greater space.   Crystallization will very probably
take place next to those portions of the crust which are already solid. If it is attended
by an increase of volume, and this increase produces tension, it is very likely that the
nest adjoining masses, though not yet crystallized, will offer too much resistance to
allow  this tendency for expansion  to find immediate  relief by yielding to the general tendency to contraction which may prevail in liquid  masses at greater depths.
It may thus be explained why, in different parts of the crust, a motion, independent
and in opposite directions, may result from the two-fold tension attending changes
of volume which take place at different distances beneath the surface, and which
and others with a view of ascertaining the increase in volume whiclh different rocks undergo by melting, do not affect our
supposition..In the first place, the molten state may be quite different from that in which the same substance would W~e
immediately before crystallizing, and the passage from one to the other may be attended by a decrease in volume which Is
not counterbalanced by the subsequent increase taking place by the passage of the substance into the crystallized state.
Then, the rocks on which the experiments were made had been solidified under a great pressure, while, when in a molten
state, they were only exposed to the pressure of the atmosphere. Finally, it has not been ascertained whether all the water
which is contained in the rocks does escape on melting or not. If snot, then it will probably have the effect of inflating the
molten mass.;( i'J)




0`4                           RICHTHOFEN- NATURAL SYSTEM
would oppose, and, to a certain degree, destroy each other, if the state of aggregation
of subterranean matter allowed of a free conduct of motion.
These slow and continuous agencies, chiefly those among them which give origin
to the process of elevation, must have been, too, instrumental in causing the massive
eruptions of rocks. At least, no other force supposed to act beneath the surface call
account as fully for certain facts connected with them, such as the gradual changes
in the mode of their geographical distribution, or the connection of the manifestations in any certain system  of fractures, or the fact that ages of comparative
repose have been interrupted by parosysmal actions of great violence which have
taken  place during  certain  eras in the history  of each separate country.  The
latter circumstance points clearly to the assumption, that during the eras of repose,
or at least their later portion, a constantly increasing amount of potential energy
must have been accumulated under the crust of the globe; since the subterra.
nean agencies called into existence by the cooling of the globe did of course never
rest, nor can they be reasonably supposed to have been more intense in the Tertiary
than they were in preceding periods.  It is self-evident that the increase of tension
must have been much more considerable under areas of elevation than under the
more extensive regions of subsidence.  In the latter case, when a downward tendency
is caused by contraction, the weight of the crust must come in its aid; the essence of
the resistance to that tendency may therefore be concisely expressed as cohesion minus
weight; while weight added to the cohesion will give approximately the resistance to
elevation by expansion.  The tension under a crust of great thickness must therefore,
in the latter case, increase to a stupendous intensity, until it is sufficient to overcome
the resistance, and will then be able to result in such paroxysms as that by which
the volcanic era has been inaugurated, and the main feature of which consists in the
formation of fractures which, by their mode of association, constitute distinct systems
or belts, separated by areas in which the existence of fissures in the crust cannot berecognized on the surface.
Although the processes suggested may furnish us the cause for the prime condition to the emission of rocky matter from below. namely, the periodical opening in the
earth's crust of such fissures as widened with the approach to the surface; yet
they fail completely to explain, by themselves alone, the more immediate causes and
the mode of that emission.  It may be presumptive to extend speculation upon this
topic beyond those limits which we have reached; but a safer guide to conjecture oil
the same than had been known at any former time was given during the last few
years by the experiments of Daubree, and the microscopic examinations of the texture
of rocks by Sorby. It has been held quite generally till of late, that the opening of
a fissure to a liquid mass below, would be sufficient by itself to cause the ascending
of the liquid through it to the surface;19 but this supposition is utterly irreconcilable
15 It must here be remarked that some of the most eminent writers on the subject of volcanoes, chiefly Poitlett Scrope, Prof. DPana and others, have suggested long ago, that the ascendling of lava in a volcanic channel must be
due to both the fluidity and expansion imnpartedl to it by the globular state of the water which finds ingress to the channels of the lava and enters into its composition. They have anticipated, by this suggestion, in some rneasure, the results of
(932)




OF VOLCANIC ROCKS.                                    5
with physical laws.  Supposing for a moment that theory to be correct which considers all the changes of level as originating in contraction alone, then it is quite probable that fissures would be formed in the center of the areas of subsidence. But as
they must be closed near the surface, and open in their lower parts, it is difficult to
see in what manner the liquid matter could ascend through them to the surface, while
no better account could be given for the occurrence of eruptive rocks on high tablelands.  In order to explain it, recourse has been had to the most arbitrary assumptions.  It may even be read in our time in various geological books, that the crust below
might give way from  the overload, and the whole be "plunged" into the semi-fluid
mass beneath, causing it to overflow.  Mountain ranges of a thousand miles in extent
have been assumed to subside suddenly upon the liquid mass, and to " splash it out "
through fissures. Leaving out of consideration these fantastical theories, there remains
a number of others, according to which the weight of portions of the crust would
cause the ejection of liquid matter. If the substance composing the crust exceeded
in specific gravity the liquid matter below, then this mode of ejection would be probable, and we should be indeed surprised that large portions of our globe were not
flooded over repeatedly by molten masses from below. If the specific gravity of the
crust and the fluid matter below was the same, then it would require the most extensive fracturing of the crust and uplifting of its firagments, in order to make the liquid
mass overflow the latter.  But all that we know in regard to the subject goes to show
that the masses below the crust are of greater specific gravity than those composing
it. To suppose the weight of the crust to cause the protrusion of liquid matter
through fissures, is therefore to suppose an action which is mechanically impossible.
We arrive at no more satisfactory conclusion in regard to our present problem,
if, besides the contracting forces, we assume the existence, beneath the crust of the
earth, of others which have their origin in the increase of volume by crystallization,
and thus produce expansion. I attempted to show that they furnish the most probable
agent to which may be ascribed the opening of the systems of fissures, which partly
served as the channels for the extrusion of rocky matter. These fissures would have
to be open at the surface and to decrease in width below, because formed by an outward tension on areas of elevation. It cannot be assumed that they would descend to
any greater depth than the lowest limits of solid rocks; they would, therefore, not
reach down to any matter sufficiently liquid as to be capable of being forced up
through them. And if it should be able to ascend, then it would solidify within the
fissure, almost instantly, by loss of heat, and long before reaching the surface. There
is, however, even a more forcible argument to demonstrate why it should not have
been calpable of ever entering the fissures.  For, if our supposition that the silicious
masses beneath the crust increase in volume by crystallization is correct, the relief
from pressure by the formation of fissures must have the immediate effect of rapidly proexperiment established by Daubree. But the supposition was only made for the case of lava, and not for that of rocks which
were ejected without volcanic action proper. Yet even in regard to volcanoes it did only explain the extrusion of lava to
the surface from a place at a limited distance below it, and failed to give a clue to the manner in which the constant supply
of matter to those places was kept up.
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56                                   RICHTHOFEN - NATURAL SYSTEM
moting the crystallization of vast masses which had been held in a viscous state before,
by the existence of the tension itself.  It needed, for all these reasons, another agency
which would not alone force up molten matter through the fissures, but also cause it to
arrive at the surface in that particular state of aggregation which it has had, according
to the observations of Sorby.  This agency is indicated, by the experiments of Daubr6e,
to have been water, the descending of which into the fractures is indeed a necessary
consequence of their formation.   There  it would  convert the  state of aggregation of
the masses surrounding  the lowest parts of the  fracture into  that called  by Daubre
" aqueous fusion," which appears, indeed, to have been the state in which all eruptive
rocks have been immediately before their consolidation. The process of aqueous fusion,
as has been shown by the same eminent geologist, is attended by a very considerable
increase in volume of the masses affected.   It would, therefore, give rise  to processes
totally different from those which had preceded. For this expansion would inimmediately cause a motion of the masses rendered liquid, in the direction of least resistance, that is, upwards  in  the  fissure, and  would, if continued  for a  sufficient time,
mnake the same overflow on the surface of the crust, even if unassisted by other ejecting agents, such as the vapor of water.20
We  may  carry these deductions still further, if we revert to our previous conclusion, that the relief from  pressure by the fracturing of the crust would have caused the
crystallization of masses below it, which had been held before in a viscous state by the
tension itself. This process would extend in depth as well as laterally, and gradually
affect the viscid masses beneath an entire belt of fissures.  It would have had again to be
attended  by an increase of volume.   But those crystallizing masses not being in a state
of aqueous fusion, the resistance could in this case not be overcome by the extrusion
of that portion of them by which they were increased in volume, and the effect would
be, as in the case first mentioned, accumulation of potential energy. I will attempt to
20 If we consider the geological features of all large accumulations of eruptive rocks, such as the great andesitic
ranges of Hungary, or the quartzose porphyry composing a plateau of great dimensions in southern Tyrol, or those granitic
masses which, by overlying the edges of stratified rocks, give evidence of having been ejected to the surface in a liquid stateit would appear that their emission has been a slow and mainly a quiet process of long duration, hardly attended by those convulsions and paroxysms which form the prominent features of volcanic action, and should have been no less characteristic of
massive eruptions if they had been due in any large measure to the expansive force of vapor. The process of the emission
of the rocky matter has, it is true, been evidently intermittent in most cases, as may be inferred from the occurrence of vast
accumulations of breccia, and it appears that an extensive solfataric action has frequently taken place through neighboring
fissures; but the manner in which the matter was protruded through the main fissures and deposited on the surrounding parts
of the surface, had evidently no similarity to the mode of ejection of scoria, ashes, and lava from most of the active volcanoes. The mode of action described, which may be inferred from geological observation, is perfectly in accordance with what
we should expect it to have been by reasoning it priori on the basis of our previous suppositions.  For, if G. Bishof's calculation is correct, that the elastic force of steam is at its maxirnuln when it has the same density as water, which it would
acquire under a pressure of 8,300 atmospheres, the loftiest column of lava (taking its mean specific gravity to be 3) which
should be supported by it, would be, according to Jukes, 88,747 feet. The original seat of those eruptive rocks which were
protruded without being accompanied by volcanic action, must necessarily have been at a much greater depth, and we should
therefore, also from this point of view, be led to suppose that the expansive force of steam has had only an insignificant part
in their protrusion from greater to lesser depth. It must, of course, have come into action when the liquid masses arrived
near the surface, but will have caused hardly more than an ebullition, even in viscid masses, on account of the extent of the
openings. The formation of conglomerates could thereby be vastly promoted, but their final deposition and consolidation
must have been quite different from the manner in which similarly subdivided matter would be deposited around a volcanic
orifice.
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OF VOLCANIC ROCKS.                              57
demonstrate in another chapter that the changes of level which have been connected
with the ejection of rocks, appear to confirm these suppositions. Suffice it here to
draw the necessary consequence, that the increasing tension must finally have had the
effect of rupturing the newly consolidated masses.  If in the intermediate time the
masses filling the fissures of the first epoch had, at least partially, been consolidated,
then a new system of fractures would be opened, within the limits of, though not coinciding with, the first. These fractures of the second epoch would descend to greater
depth than those of the first, and in allowing the access of water to masses situated in
lower regions and being of a more basic composition, would open for these the way to
the surface. By the repeated occurrence of this or similar processes, the theater of
action from which the rocky masses were conducted to the surface, might descend, by
steps, into considerable depth within a comparatively short period, and thus there
could be produced a great diversity among the rocks emitted through one systemn  of
fractures, though this diversity would be regulated by definite relations in regard to
the nature and succession of the rocks ejected. The process would come to an end
when the solidification of matter and the formation of fractures had descended to those
masses, the state of aggregation of which was such as no longer to allow them to crystallize when the pressure was diminished, and in this way any further increase of
volume would be prevented.  The matter filling the fractures would now solidify, and
the communication of the interior with the surface be cut off, with the exception of
the volcanic channels. The resistance offered by the crust of the globe would hence
be greater than it had been before, and there would follow another era of repose,
longer than that which had preceded the era of eruptive activity.
The application which may be made of these processes suggested by theory, to
the explanation of the actual correlations of eruptive rocks in regard to their age,
chemical composition and geographical distribution, is obvious. We may, indeed,
venture to deduce d priori the history of eruptive action, in its main features, from the
hypothesis of Sartorius, and the assumption that silicates will increase in volume by
perfect crystallization.  We should have to conclude that in a remote period, when
the crystallized crust and the sedimentary shell of the globe were inferior in aggregate
thickness, fractures and eruptions of rocky matter would have been of frequent occurrence, and that highly silicious compounds should have prevailed among the ejected
masses.  Little, if anything of them, is probably visible at the present surface, as the
rocks of the Azoic and Paleozoic formations are probably the monuments of an already
far advanced stage of the development of our planet. All the distinguishing features
of the eruptive rocks of these periods (including of the Paleozoic age only the Silurian
and the first part of the Devonian), such as the great number and individualization of
the granitic districts, the independence of each of them in the subtler differences
regarding the commencement and further development of the eruptive activity as well
as the peculiar nature of the rocks ejected, the great preponderance of highly silicious
compounds, the common association with them of small quantities of basic rocks, and
the slight increase of the proportion of the latter in the Devonian period-all these
phenomena are easily understood, without further explanation, in the light of our
hypothesis.
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58                        RICHTHOFEN -— NATURAL SYSTEM
Let us now direct our attention at once to the volcanic era. The conditions
of the globe must have been very different in the Tertiary from  what they had
been in the Palkeozoic period. A longer time of comparative repose had in most parts
of the globe preceded the inauguration of the violent manifestations of vulcanism in
the Tertiary period than had ever before elapsed between any two eras of eruptive
activity.  The globe had cooled down.  Volumes of sedimentary matter had accumulated, and added externally to the thickness of its crust, while it had increased in a
vastly greater measure by the crystallization of liquid matter below. Those silicious
compounds especially, of low specific gravity. which had formerly yielded the material
of the vast accumulations of quartziferous eruptive rocks, would have been consolidated, and the limit as it were between the solid and the viscous state of aggregation
receded into regions where the matter would be of a less silicious composition and of
greater specific gravity.  The similarity in distant countries of the rocks first ejected
(propylite and andesite) goes to show that the recession of that limit into greater
depth must have proceeded in a nearly equal ratio in all those regions where volcanic
rocks are distributed. When the tension below had increased sufficiently to overcome
the resistance, it would now no longer manifest itself in the formation of small and
differentiated systems of ruptures.  In the direct ratio of the increase of the resistance the fractures would have to be of greater extent, and those elongated belts of
them would be formed which even now are partially distinguished as the belts of volcanic activity. The first rocks ejected would necessarily be of a more basic composition
than the predominant rocks of the granitic era, while the repetition, at a later
epoch, of the process of fracturing would give rise to the ejection of rocks in
which silica would be contained in a still lower proportion.  The greater portion
indeed of the ejected rocks consisted of propylite and andesite, in the first, and of
basalt in the second half of the volcanic era. A notable but only apparent anomaly
in the regular order of succession has been the emission of trachyte and rhyolite
between the andesitic and basaltic epochs. But if it is considered that these rocks were
ejected partly from the same fractures through which andesite had ascended, and
partly from others in their immediate vicinity, while the distribution of basalt has
been independent, to a certain extent, of all foregoing eruptions, it is evident that
the occurrence of trachyte and rhyolite is closely dependent on that of andesite, and
bears only a very remote relation to basalt. It appears that after the ejection of the
chief bulk of andesite, when other processes ending in thile opening of fractures into
the basaltic region were being slowly prepared in depth, the seat of eruptive activity
ascended gradually toregions at less distance from the surface.  There is, within the
limits of conjecture based on physical laws, no lack of processes which could cooperate
to that effect. The consolidation of the ejected masses within the fissures would probably proceed simultaneously, by loss of heat, from the surface downwards and, by
pressure, from below upwards.  The opening of new  branches from  the main fractures, the remelting (by the aid of the heat of the molten mass within the latter, and
of water finding access to it) of solidified matter adjoining the fracture, the emission
of that remelted matter through those branches: all these are secondary processes
depending on the first almost necessarily. The supposition that to these is due the
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OF VOLCANIC ROCKS.                              59
order of time in which trachyte and rhyolite have been ejected to the surface, is
corroborated by the fact that these rocks occupy generally a subordinate position in
regard to quantity, and have had, to a great extent, their origin in volcanic action.
When treating about the latter, we will come back upon this subject.  There remain
some peculiar features of volcanic rocks which cainnot be satisfactorily explained at
the present time.  We mention, among them, the fact that the three modes of texture
of rhyolitic rocks are often severally limited to certain localities; the mode of formation of the laminated structure of rhyolitic and trachytic rocks; the occurrence of the
compounds of hornblende and oligoclase in that threefold form  to which we have
repeatedly referred; the fact that basalt has been followed only to a very limited
extent by rocks bearing to it a similar relation, as trachyte and rhyolite do to andesite.
As regards the long lapse of time intermediate between the Devonian and the
Tertiary periods, the mode of occurrence of eruptive rocks in the same shows in
nearly every respect a gradual transition from that which was peculiar to the granitic to
that whichwe just described as being characteristic of the volcanic era. This intermediate period Inay be designated as the porphyritic era, though this name appears to
apply more properly to its first part only. Quartziferous rocks were not so predominant in it as in the granitic era, porphyrite and melaphyr having nearly equaled
quartzose porphyry, in point of quantity.  Augitic porphyry was ejected in a
much larger proportion to the aggregate bulk of the porphyritic, than diabase to
that of the granitic rocks.  Where it occurs, it was the last in the order of rocks
erupted, while quartzose porphyry was generally the first among them, though this
place is sometimes occupied by porphyrite. In reference to the general features of
their geographical distribution, porphyritic rocks occupy no less distinctly an intermediate position, as may be seen by what we have said on this topic on another page.
There are exceptions to the order of general development as here specified.
They regard chiefly the texture, and are almost exclusively to be found among the
rocks of the porphyritic era, though the recurrence, in propylite, of the properties of
ancient diorite, is a phenomenon of a no less exceptional nature.  Leaving this rock
(propylite), or rather only some of its varieties, out of consideration, the volcanic rocks
have their peculiar characters, by which even the most basic rocks are to be recognized when seen in large accumulation.  In the rocks of the granitic era, if we
consider its end to be within the Devonian period, the characteristic features of porphyritic and volcanic rocks are probably never to be observed. Among the exceptional occurrences within the porphyritic era, may first be noticed the fact that perfect
granitic texture is still occasionally encountered, as, for instance, near Predazzo and
on the Monzoni in southern Tyrol, where some subordinate masses of rock resembling
granite and syenite have been ejected in the Triassic age, togetherwith the well-known
porphyritic rocks of that region. There are similar instances known from other places
on the European continent, but they are scattered, and the respective rocks always
quite limited in extent.  The grandest exceptional instance that is known up to this
time, is the recurrence in the Jurassic period of perfect granitic texture in the eruptive rocks of the Sierra Nevada.  But as regards their mineral composition, these rocks
belong to the family of syenitic granite, containirng horlblelnde as a very characteris(97)




60                                  RICHTHOFEN -— NATURAL SYSTEM
tic ingredient, while silica enters into their average composition probably in a
much lower proportion than it is contained in ordinary granite. The varieties known
as granitite are of rare occurrence.   In another respect, namely, the mode of geographical distribution, the Jurassic  granite of the Andes marks an advanced  stage, if compared with the Permian and Triassic porphyries of Europe, inasmuch as the area of
its distribution excels the  porphyritic regions of the latter continent in regard to their
extent and the unity of their inner relations, while it does not come up, in these
respects, to the properties of the great volcanic belts.
2. Origin of Volcanic Action.
I  have tried  to  demonstrate in  another chapter, that volcanic  activity is intimnately connected with massive eruptions in a three-fold way, namely, in respect to
the epoch of its commencement, the mode of its distribution, and the nature of the
rocks ejected. Basaltic volcanoes occur in ranges built up by the massive eruptions of
basalt, or in their vicinity, in such connection as to make obvious the nearly conternporaneous origin of both. Andesitic volcanoes are found in the neighborhood of those
masses of andesite which had  been ejected without being attended by volcanic action;
while volcanoes which have emitted rhyolitic or trachytic lava are so situated as to
justify the inference of a close connection between the first opening of their vents
and the origin of neighboring accumulations, either of rocks of similar character or of
andesite, but which are due to massive eruptions.21
No distinct line of demarcation can be drawn between the two modes of mani21 This affinity in regard to petrographical character and geographical distribution is probably the cause why both
agencies have ordinarily been confounded. It has been a current notion, strongly advocated at times, that all volcanic
rocks have come to the surface in a similar way to that in which lava is being ejected from volcanic vents, and that the
cause of the ejection has been the same in all cases. In regard to the fact that extensive mountain ranges are completely
built up of volcanic rocks, it was argued that it is by no means necessary, and in fact erroneous, to suppose them to have
originated in events surpassing in magnitude those of the present day, as the length of geological time would explain how
they could be built up by the gradual accumulation of innumerable currents of lava. Where traces of former volcanic
vents could not be found, the easy destructibility of the matter of which the sides of craters are usually composed, afforded
a convenient and apparently just argument for explaining their absence by denudation. The same doctrine was applied for
explaining the mode of formation of ancient eruptive rocks where we see mountain masses made up of them, while others
have arrived at the conclusion that they were the'" roots " or " cores " of volcanoes; granite itself is by them considered
to have had formerly, and to have now this function, even in' the case of those volcanoes the lava of which consists of basalt.
Both these doctrines have been adopted the more readily, as they appear to be in harmony with the favorite hypothesis, that
at no time have any changes on the surface of the globe been more violent than those going on at the present day, nor
different from them in mode. However ably this theory, which contributed so much to the advancement of science by
checking the phantasmagorias of former time, has been advocated, an unbiased comparison of the grand manifestations
recorded in the geological structure of an andesitic mountain range, with the mode and degree of activity of present volcanoes, must lead to different conclusions. In the endeavor to sustain the t priori assumption of the equality of force in all
ages, too little stress has been laid on the circumstance, that, however nearly equal the aggregate amount of force acting on
the globe may have been, its modes must have undergone a change, chiefly by the partial conversion of the heat of the
globe into other forces. At the same time, the degree of the intensity of the manifestations of subterranean forces must
have varied in a two-fold way. With the continuous increase of resistance, the aggregate motions of the crust must have decreased, since a growing amount of these forcestwas required for overcoming the resistance. On the other hand, the manifestations of the same forces had to become periodical and paroxysmal, and periods of violent action had to be separated by
others of repose. The violent activity of the Tertiary period has passed, and our present volcanoes appear to mark the transition into another period of repose. I will try to demonstrate that the causes of their action are utterly inadequate for explaining the grander phenomena and correlations of massive eruptions.
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OF VOLCANIC ROCKS.                               61
gestation of subterranean energy, as some of the monuments of the grand volcanic
action of the past indicate the former existence, at certain places, of a stage ilntermediate between both.  But in those numerous instances where the nature of either of
them can be distinctly recognized, some conspicuous differences may be noted between
them.  Volcanoes are provided with a, channel connecting the seat of volcanic action
with the surface. The matter which they eject consists either of stratified layers of
ashes and scoria, or of currents of lava in the shape of flat sheets superimposed over
one another, or of alternating layers of both kinds of material, the latter structure
being most frequent.  A low dip, verging on horizontality, of all planes dividing substances of dissimilar nature, or marking successive deposition, and homogeneity of
material parallel to those planes, together with a structure of the mountain masses
distinguished by the radiation of all mineral matter from one common center, may be
considered as those features by which even extinct volcanoes, or fragments of them,
may unmistakably be recognized.  The center may shift, or have shifted, within narrow limits, or a series of centers may follow each other in close succession, but this
will obliterate the true character only in a slight degree.  Similar rocks, when they
came to the surface by massive eruptions, do not present these distinguishing features.
They usually compose ranges of small width in proportion to their length, and in the
place of one or mlore distinct centers an elongated axis may be detected, from which
the structural and morphological features originate. As regards the interior structure,
there may be observed a certain massive character of the rocks. which is partly produced by the prevalence of their compact varieties, and partly by the circumstance
that homogeneity of lithological character may be traced to a great distance in a vertical direction. In sections, masses are frequently found thousands of.feet in height,
which do not vary perceptibly in character, and show no horizontal structure.  If the
slopes of the ranges are examined, the rocks will be found preserving a homogeneous
character chiefly in a direction parallel to the axis, while it is less persistent at right
angles to the latter.  The planes dividing dissimilar rocks are inclined at all angles,
and have very frequently a steep or nearly vertical position.  Breccias even, which
sometimnes occur in very large masses, are bounded in this way towards the adjoining
compact varieties; they are of irregular shape, and do not often occur in stratified
layers or in elongated currents, as is the case when they are produced by volcanic
action.  The ranges made up by massive eruptions show no signs of craters.  Yet
they are frequently the foundation of volcanoes.  Oftener still do volcanoes occur on
the lower portions of their slopes, or they may form a series parallel to the axis of
the main range, and even greatly exceed it in elevation.
Notwithstanding these points of difference, there are not only stages of transition between the distinct geological features resulting from either mode of action, but
a similarity in character may be produced under certain circumstances, which makes it
difficult to decide what was the mode of origin of an accumulation of volcanic rocks.
In the first place, the matter extruded through fissures may have been so liquid as to
expand at once in thin sheets. This is very frequently the case with basalt, in the
great accumulations of which, whether they be due to rnmassive eruptions or to volcanic
activity, thie promiinenlt differences wrought   by b)ollth nlodes of action in the external
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62                        RICHTIHOFEN-NATURAL SYSTEM
features are generally least conspicuous.  This basic compound is not only more fusible than the more silicious rocks; but, it appears that the admixture of superheateic
water will increase its fluidity more than that of the other compounds, while those
unknown influences which cause the mass to solidify in the particular form of basalt
must be still more potent iu increasing its fluidity, since lavas consisting of dolerite,
leucitophyre, and trachydolerite, which are quite or nearly identical with the former
in chemical composition, are never so liquid as those of basalt, and are not unfrequently
quite viscous. These rocks, together with all those of a more silicious composition, exhibit more distinctly the differences in origin, and this is probably clue, in a great measure, to the peculiarity just mentioned.  The great fluidity of basalt, which is also made
evident by its frequent occurrence in very thin and yet very extended dykes, causes
it, even when confined in the narrow space of a volcanic vent, to let the vapors of
water escape, in quiet ebullition, in its orifice. as Dana has so beautifully illustrated in
his description of Kilauea, while, at intervals, it will break out and cover the surrounding country with flat shleets of lava.  The action connected with the ejection of
the same rock fromn larger fissures, in former time, appears to have been similar to
this.  Yet, the numerous instances of the occurrence of basaltic cinder cones, and, on
the other hand, of large accumulations of solid basalt with no perceptible horizontal
structure, go to show that also in the case of this rock the i-nodes of occurrence may
be different when they are the result of different modes of ejection.  Other instances
of a similarity of the manner in which the matter has been deposited, when due to
either mnode of ejection, are frequent on the flanks and at the ends of the andesitic
ranges of Hungary, where currents of andesite as well as of rhyolite have been emitted
through fissures in andesite, at little elevation above the foot of those ranges. They
appear to be due to processes intermediate in kind between both niodes of ejection.
We may, finally, mention those cases where massive eruptions were sub-aqueous, and
layers of fine-grained tufa formed, alternating with coarser conglomerates, between
which may be intercalated solid layers of the same kind of rock of the firagments of
which those are composed.  The similarity of this kind of depositions with the sedinents of submarine volcanoes is often very great.
The principal point of difference between massive eruptions and volcanic action
appears to be the depth of their source under the surface, and all the mninor differences
are probably dependent upon that.  The region from which the former have derived
their material, is, as we tried to prove, at a great depth beneath the deepest sedimentary rocks.  The seat of volcanic action appears to vary within wide limits in regard to
its distance from the surface, but to be, on an average, at much less depth than that of
the massive eruptions; though there are circumstances which render it probable that
it is in all cases beneath the shell composed of sediments. Evidence has been gathered
by Prevost, Dana, Scrope, Hopkins, and others, in favor of the assumption that volcanoes are not connected with the molten interior of the globe, and are therefore not
to be considered as safety-valves. The comparatively little distance of the seat of volcanoes beneath the surface is rendered particularly evident by the small area of the
earthquakes attenllding their activity, wlhen conlpared  with the wide extent of others
which must be dependent on some deep-seated action, but have no recognizable con(100)




OF VOLCANIC ROCKS.                              63
nection with any particular volcanic vent.  It is no less obvious from the fact that two
neighboring volcanoes may not only eject, contemporaneously, different kinds of lava,
but also be, to some extent, independent of each other in their manifestations.
AVe may base further conclusions in regard to the origin of volcanoes upon tile
following premises, which we repeat from the foregoing pages: First, several facts
appear to indicate that the source of volcanic action is at a comparatively limited
depth; second, all volcanoes, whether active or extinct, are intimately connected with
massive eruptions; third, this connection is of such a character as to establish, in the
majority of cases, the close similarity and chemnical identity of the mineral matter
ejected by the volcano in its first epoch of activity with that of neighboring hills or
of its own foundation, which had been accumulated by massive eruptions;while in
other cases these neighboring hills or the foundation of the volcano are composed of
volcanic rocks not identical with tile lava, and then the latter will belong lithologically to
a kilid of rock which, according to the order of succession of massive eruptions, would be
of a more recent origin than the former; that is, trachytic or rhyolitic volcanoes were
-frequently opened at those places where only andesite had been accumulated before,
and basalt where either andesite or trachyte had preceded; but the reversed order
appears not to occur, no basaltic volcano having been succeeded, in its own neighborhood, by massive eruptions of trachyte or andesite; fourth, many of those volcanoes
which have been active through a long period, have undergone a periodical change
in regard to the character of the mineral matter ejected by them, and this change is
in general (though with exceptions) conformable to the order of succession observed
in regard to massive eruptions.
F'rom these facts may be inferred the complete dependency of volcanoes upon
massive eruptions. The latter, as we attempted to show, were due, firstly, to the opening of systems of fissures which extended throughout the solid crust; and secondly, to a
quiet outflow, which was caused by the expansion attending the change of aggregation
of solid or highly viscous matter around the lowest part of the fissures into that of
aqueous fusion.  The relations of volcanic activity to massive eruptions are indicative
of a process by which the elongated and extensive vents of the latter wvere gradually
differentiated into isolated and narrow channels feeding isolated orifices on the surface.
It has been observed that cinder eruptions marlk generally the last stage of volcanic
action.  We may go a step further back, and say that volcanic action is the last stage
of massive eruptions.
In order to arrive at a conception of the manner in which the change from one
mode of action to the other could be effected, let us suppose that a main fissure was
filled with matter from below, and mnountains of volcanic rocks accumulated above it
by the long continued overflow.  Solidification would at once set in, and proceed
downward whenever a cessation of the extrusive action occurred, independent of
the question whether it would not simultaneously proceed upwards from  the depth.
Its progress would not be equal in all parts of the fissure, since this must be
wvider at some places, and more contracted at others.  In this condition we should
have one of the causes for the isolation of centers of action, for the length of time
duringc which different portions of the matter would remain in a fluid condition, must, of
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64                        RICHTIlOFEN -NATURAL SYSTEM
course, depend in a great measure upon the width they would severally occupy within
the channels, and those filling its wider parts must for a longer time remain susceptible
of renewed expansion by accidental circumstances. Another cause which would have
the same general effect, is the localization of the ingress of water.  Some of its channels would, to all probability, become obstructed, and the rate at which new ones
would be opened in their place would probably diminish in nearly equal ratio with the
total amount of tile manifestations of energy connected with the phenomlenon of ejection
in its different stages.  As it appears that the slupply of water, which may be either
constant or intermittent, is, next to an elevated temperature, the chief condition for
entertaining volcanic action, it may be inferred that the cause mentioned would contribute greatly towards the isolation of certain portions within the mailn fissure, by helping
to keep the matter within them in a liquid state.  If this second cause was coincident
with the first, by the restriction of the ingress of water to a place where the fissure
expanded, then both circumstances would combine to prolong the state of liquidity at
that point. The connection between the bottom  and the surface may have been
kept open in a certain part of the mouth of the fissure, while solidification was proceeding over the rest of it.  An isolated vent would then gradually be formed, and nlarrowed down to the size of a volcanic orifice.  Obstructions of the outflow, by periodical
consolidation, would become more frequent, and thus would proceed a slow change of
the mode of action of massive eruptions to that which is peculiar to volcanoes.
This is probably the simplest mnanner in which volcanoes can originate.  It will
apply particularly to a number of those which have undergone no change in regard to
the character of their lava, and the lofty cones of which rise over mountain ranges
consisting of the same material with their own lava and cinders, though owing their
origin to massive eruptions. TWe must now consider a third cause which would aid in
promoting volcanic action, and probably come very often into play.  It is indicated by
the frequent occurrence of series of volcanoes extending in. lines parallel to the axis of
the main outbursts.  Their orily possible cause is the formation of fractures, parallel to
the main fissure, and branching off from  it.  In order, however, that these could be
formed, solidification must have proceeded downward in the main fissure, without any
communication with the liquid portion in depth having been kept open.  This process
would necessarily imply a temporary cessation of the process of extrusion. That this
could take place may be the more readily understood, if it is taken into consideration,
that the liquid masses filling the fissure in its whole extent must contract considerably
by constant loss of heat, and that any additional expansion produced by the promotion
of aqueous fusiomi at certain places had first to equalize this loss of volume, before
it could manifest itself in a rise of the whole mass.  This twofold action, which is
probably one of the main causes of the intermittent character of volcanic activity, must produce an alternating motion of matter within the fissure, and
there would be given ample. opportunity, during a period of its subsidence, for
the consolidation of the upper portion to a great distance down fromn the surface.
If a period then followed in which, by dislocations of some kind, a change took
place in the conditions subterranean, and expansion began again to prevail within the
fissure, the lnew supply of force would mlanifest itself at the upper limit of the liquid
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OF VOLCANIC ROCKS.                              65
portion, where the least resistance was offered, and new fractures would be opened
branching off at that depth fromn the main fissure.  The liquid mllatter would ascend
througyh these secondary fractures, and, if these were of suficient,tdimesions,  give rise
first to the formation of parallel ranges of volcanic rocks by massive eruptions, and
then only to a gradual isolation of channels of volcanic action, as in the case first explained; or, if the fracture consisted of a series of smaller ruptures, cause at once the
formation of a series of volcanoes.  The activity in these secondary fissures could continue long after any manifestations had ceased over the miain fissure, and even after the
consolidation of the nmatter contained in this, with the exception of the volcanic hearths,
had proceeded into far greater depth.  The formation of secondary fractures, branching off firom the main fissure, might be repeated at different depths, and fissures of a
third order be formed, branching off from those of the second.  Subterranean reservoirs of liquid matter, which may either be isolated or connected, would thus be formed
at differeit depths, and be arranged after a siinilar plan below ground, as we notice
among the active and extinct volcanic orifices above ground.  The hypothesis of
the existence of such subterranean seas of melted matter, as they have been called, has
also, though in a very different meaning, been arrived at by the adherents of the theory
of a metamorphic origin of volcanic rocks.  This coincidence increases the degree of its
plrobability.  But unless the nature and distribution of those reservoirs is made dependent on grander plenoniena having collnnection with the interior of the globe, they
will not be capable of explaining the harmony prevailillg either in one volcanic region
or between all these regions.  We have been led, by arguing on our suppositions, to
the same conclusion at vwhich we arrived before by induction firom observed facts, naLmely,
that the seat of volcanic action must be at a comparatively limited depth. Yet it appears that this depth is in all cases below the shell of sedimentary rocks.  Aumong the
reasons supporting this assumption, we mention only one.  This relates to the chemical
composition of lava.  The volcanic is typically the era of andesitic and basaltic compounds.  The occurrence of trachyte and rhyolite among the ejected rocks goes to
show, that the seat of action receded at certain places from the andesitic regions to
those of the comlpounds corresponding in chemical composition to the two kinds of rockl
named.  But if it had been partially above those regions where matter is still in its
prinmtval position, then we should expect that there would be volcanoes the lavas of
which, being derived frorm sedimentary rocks, would, as a whole, deviate in composition
from the law of Bunsen.  No such volcanoes are known, and it is, therefore, not probable that the seat of any of those the lava of which has been analyzed is within the
shell of sedimentary rocks.  It is true that subordinate deviations fromn the composition
as required by theory occur; but it has been found sufficient to ascribe them, as in the
case of Vesuvius, to a mecllhanical destruction of the rocks surrounding the channel of
ejection by the ascending lava.
Active volcanoes tlherlselves firnlisl an illustration in evidence of their own
origin as hlere advocated.  It is well ]known that small cones are freqluently met with
on the slopes of larger volcanoes.  If tlhey occur in larger nulmber,  as on Mount Etna,
they are usually situated in lines which radiate firom the crater.  Each of them is built
up of layers of scoria anld ashes sloping away firom the center, wh]ere a crater is im(103)




66                          RICHTHOFEN-  NATURAL SYSTEM
mersed, and such cones will occasionally emit currents of lava, and be in fact the repetition on a small scale of the mother volcano.  The usual and probably correct explanation of their mode of occurrence is this: That, through crevices or by passage
through porous rocks, water gets access to glowing lava, and, by its action on the samne,
causes the opening of a fracture, and, in imlmediate succession, the repetition of the
same phenomena which the mother-volcano presents when active.  Just as these parasitic volcanoes have their roots in the glowing lava, volcanoes in general must, as is
demonstrated by their mode of occurrence be considered as parasites on certain subterranean portions of the material of massive eruptions, which still possess a high temperature and are kept in a liquid state by the molecular combination with water which
finds access to them. This mode of origin of volcanoes, however, is only a repetition
on a small scale of the manner in whiich massive eruptions themselves originated, inasmuch as volcanoes bear a similar relation to the latter as these do to the primeval substance composing the interior of the globe, to which the fractures descend. These
main fissures, which are probably very few in number in every volcanic belt, form the
great arteries in this harmonious system.  Their common origin will furnish an explanation of the general similarity of the phenomena presented by different volcanic
belts; while the varied and possibly very intricate. mniode of their ramifications towards
the surface, together with the different conditions of tile rocks which they intersect, the
various rate and local diversity of the access of water, the different circumstances which
may determine the depth in which the expansive force of vapor can be brought into
action (among these may be the relative proportion in which chlorine, fluorine and sullphur are present), and other influences unknown, would give ample means for explaining the diversity of all the phenomena of vulcanism. within each separate volcanic belt:
such as the apparently intricate, and yet to a certain degree harmonious, mode of distribution of the rocks ejected; the correlations existing among the latter in regard to
their composition; the dependency of volcanic action on massive eruptions; the mode
of distribution of hot springs, solfataras, geysers, and other phenomena which were apparently associated with both modes of action; the different phenomena connected with
the occurrence of earthquakes; the small size of their area of disturbance when connected with volcanic action, its varying, and sometimes very great extent in other
cases where no connection with any one distinct volcano can be discovered; the singular correlations, finally, which have been observed to exist between different volcanic
vents situated on the same belt.  Some hints may even be got in regard to the remoter
correlations which apparently exist between the phases of volcanic action on neighboring belts, though it must be conceded that in respect to the latter rnlumnerous facts have
been observed which cannot be satisfactorily explained in the way here proposed, and
rather appear to indicate the influence of the phases of magnetic currents onl the
manifestations of vulcanism.22
22 An extremely valuable contribution to the elucidation of these correlations respecting the phases of volcanic activity was lately given by Dr. Emil Kiluge, in his work, Ueber den Syochrlonismtus used Aoteg~omismtns von v/lconischen
Eruptionen', Leipzig, 1863. The clear and able compilation of facts will be of lasting interest, though grave objections may
be raised against the author's views on the origin of volcanic action, which are not based on any inferences drawn from the
correlations demonstrated in the same book.
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OF VOLCANIC ROCKS.                            67
The foregoing considerations may explain what appears to have been the most
probable way in which volcanic activity was developed at and near the places of nmassive eruptions.  They apply without difficulty to those cases where volcanoes have undergone no change in regard to the character of their lava.  We have still to consider
those more intricate cases where the latter has periodically changed. There are volcanoes which exhibit a regular succession of andesitic, trachytic, and rhyolitic lavas, followed in a later period by the outpouring of basaltic lava, either through the same, or
through other vents in the immediate vicinity.  There are others in which only a part
of this series can be observed, such as the succession of basalt to trachyte or rhyolite,
which appears to be of the most frequent occurrence, or of rhyolite only to andesite.
As the order of succession is generally the same as that exhibited by massive eruptions,
it would appear that it must be due to the same causes in both cases. If we take it
for granted, that those extensive reservoirs of melted matter, from which was either
continued its quiet protrusion, or volcanoes were fed, have had their seat in those concentric layers of the crust composed of silicious masses which correspond to trachyte
or rhyolite in composition, then there is little difficulty in explaining the protrusion
of' portions of them.  For there must be a limit to the expansion of a substance such
as a given mass of andesite, by aqueous fuision, and thereby a limit to its ejection.
The eruptive activity might then either come to rest or continue.  As it is very inaprobable that water takes originally a part in the composition of those masses below
the shell of sediments which have crystallized from a molten state, its access to them
at places contiguous to a source of heat, such as must be given by a fissure filled with
molten matter from  below, must be attended by a powerful influence on them.  It
would exert itself in aqueous fusion and expansion. But the viscidity peculiar to these
highly silicious substances would not allow their extrusion runtil after tile mnore liquid
andesitic masses had been ejected.  Supposing the reservoir in which this first
change was effected to have been in the trachytic region, further action from the same
could be cut off by the cessation of the ingress of water to it.  Another reservoir, situated in the rhyolitic region, might then be isolated within the educting channel, filled
now with trachytic matter, and the same process repeated, as before, ending with the
change of trachytic into rhyolitic rocks.  As regards the succession of basalt to these
silicious rocks, we refer to the fitet established before, that the fissures which gave vent
to basalt, have all been formed at a much later epoch fhan those through which andesite
had ascended, and that they were only partly coinciding with them.  It would appear
that subterranean reservoirs of liquid matter, connected with the surface by channels of
ejection, should have offered, in many cases, the places of least resistance.  It may,
therefore, be inferred, that basalt, the great comparative liquidity of which is a wellknown fact, would enter many of those reservoirs, and be emitted through the same,
or through newly-formed channels, in preference to any matter of a more viscous consistency; and it will not be difficult to understand whiy basalt should, in many instances,
have again been followed by, or alternated during long epochs with, lavas of a rhyolitic composition.
These considerations, which may be equally applied to the order of succession
of massive eruptions and to that of volcanic lavas, are not given with a view of explain(105)




(38                       PICIITHOFEN- NATURAL SYSTEM
ing this intricate subject, but only to show that natural occurrences mnay, with the aid
of the theory here advocated, be explained without having recourse to any forced assumptions.  There is no one of the processes pointed out which is not within the limits
of those we are accustomred to consider as hilghly probable in regard to that part of
volcanic action which is removed only a little way beyond immediate observation, and
therefore more accessible to well-founded speculation than is the remrioter connection
between volcanic action and the fissures throughl which the massive eruptions of volcanic rocks took place.
3. Other  7heories respecting the Origin of Eriuptive Rocks.
The various theories which have been proposed in regard to the origin, not
only of the volcanic but of all those non-foliated crystalline rocks which are made up
of silicates, diverge in different directions.  Mlost of them, however, leave unnoticed
the most essential features of those rocks such as their nature in regard to the details
of chemical composition, their similarity in character in distant countries and different
ages, the laws of their mnode of succession and distribution, and the fact of their periodical emission after long periods of repose; and no one undertakes to account for
all of them.  There may be distinguished two classes of these theories: the first conmprehends tllose which assume the original seat of eruptive rocks to have been beneath
the sedimentary rocks, while the second embraces those which would have it to be
within the shell composed of the latter.  It was the purpose of our foregoing theoretical considerations to point out, that it is exclusively in the direction followed by the
theories of the first class that we may at all look for a satisfactory explanation of the
relations presented by the eruptive rocks.  But, though the views here advocated
belong altogether to this class, the leading theories embraced in it have a very different
scope.  That form of them which was held by Buch, Humboldt and others of the most
prominent geologists, and is still quite largely adopted, starts from the assumption that all
eruptive rocks were ejected in thle same condition in vwhich they are supposed to have
been when at their original place in the earth's interior, that is, molten by dry he:lat
while the contraction of the globe by loss of heat is regarded as the sole cause of their
ejection.  Among the weighty objections which may be raised against these theories,
may be mnentioned: that the eruptive rocks on their arrival at the surface have evidently not had a temnperature which would be suffcient for their dry mlelting; that
they contain a certain proportion of water enclosed, which was formerly not brought
into account; tlhat the ejection fromi the depth to the surface of mnasses molten by dry
heat is a process impossible of explanation, and that, if it was possible, the rocks
should have a different texture fromn that exhibited by granite, diorite or propylite;
that, finally, contraction alone is as little capable of furnishing an agent for the rending
of fissures opening towards the surface, which is the prime conditiori of eruptive
activity, as is the cooling of the globe of giving the conditions requisite for the process
of ejection itself.  The theories mentioned have been longest maintained on the
European continent, vwhere they are even nowv advocated by many.  Though approachinlg nearest of all to the inost probable modle of origin of eruptive rocksS, the reasons
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OF VOLCANIC ROCKS.                             69
why they are untenable, with our present state of knowledge, are too obvious to be
made an object of a more detailed explanation.
More numerous and iriore obvious objections may be raised, chiefly from a geological point of view, against those theories of the second class, according to which all
the rocks under consideration, with the exception of those lavas which we actually see
being ejected from volcanoes, would have derived their origin exclusively from metamorphism izn situ; be it that the change is supposed to have been imainly of a chemical
nature, and effected on or beneath the surface, by the action of water alone containing
certain substances in solution, or that refuge be taken to the cooperation of superheated water and pressure at a great depth. A large amnount of positive facts as well
as of sagacious reasoning have been applied in their defense; but viewed in the light
of those observations which present themselves continually to the geologist in the
field, in evidence of an essentially intrusive and extrusive nature of those rocks, the
premises on which argumentation is based must appear extremely deficient.  Also
will the reasons which we shall adduce against the leading theory of our day, be applicable afortiori against the assumption of an origin of our " eruptive " rocks only by
metamorphism in situ.  Before entering upon that leading theory, we have still to
mention the existence of a number of others, which, though acknowledging the probability of all origin of all lava and " trap-rocks" from the liquid interior of the globe,
assume that granite, syenite, diabase, diorite and certain porphyries are so-called hypogene rocks, that is, have originated by metamorphism of sediments in situ.  Against
these theories may be raised the collective objections which apply severally to the
others.
The obvious objections which may be made to the theories hitherto mentioned
have given more and more ascendency to another doctrine, which we may designate
as the metamorphic theory of eruptive rocks, and which owes its great influence upon
modern geology to the fact of its starting from a certain number of established geological facts and lithological observations, and from the results of experiments.  It is
eminently a theory of the second class. No arguments against it will be more potent
than those which prove the fallacy of the basis of all the theories of this class, as
they will show that we must look for the origin of eruptive rocks altogether in a different direction from that followed by them.
The metamorphic theory is essentially to the purport, that all eruptive rocks,
whether of recent or of ancient age, whether ejected by volcanic action or carried into
their present position without any sign of the latter, were originally sedimentary rocks
rendered liquid by the cooperation of heat, pressure, and water. It is supposed that
these rocks, by the continued superposition of immense masses of sediment, had arrived at a great depth under the surface, where the agencies mentioned would co6perate to modify and transform their state of aggregation, resulting either in a molecular
change alone, or in their conduction into a state of fusion. In the former case, the
sediments would be simply metamorphosed, while in the latter, they would either
crystallize in depth, with a total loss of their original structure, and form  "plutonic,"
or " hypogene," or "indigenous " rocks, or be forced upwards through fractures, and
solidify partly in the conducting channels and partly on the surface, when they would
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70                              RICHTHOFEN -  NATURAL SYSTEM
form " trap rocks " and " lava.'" One comlnon cause is thus assigned to both metamorphic and eruptive (including volcanic) action, the latter being considered anll advanced stage and ultimate result of the former.  The immense action of metamorphism
is an undeniable fact.  But while formerly, on account of the violent agencies which
were supposed to have been required for it, and the growing conviction that heat alone
could not produce effects oI1 such au enormous scale as had been suggested, the subject had to be treated with caution, and any extreme assumption was received with
doubt, if not with a certain repulsion, the  condition of things, in  this respect, has of
late undergone a great change, as scientific experiments, and especially those of Daubree, have demonstrated the extent of the influence of water and pressure in producing
metamorphic action.  They have proved  the remarkable fact that, at a comparatively
low temperature, and with the aid of pressure, the effects of water, if continued for a
sufficient time, particularly when it is charged with alkaline substances, will be able
to produce changes in  the nature of rocks which  surpass even  the  most audacious
assumptions of former time. An apparently safe foundation was now given to the
widest generalizations, and the consequence is, that an almost unlimited action is at the
present time brought to the account of metamorphism.23   The least founded conception, however, of the faculties ascribed to it, we consider to be its supposed sole instrumentality in the production of volcanoes and the eruptive action of former ages
from portions of the shell of sedimentary rocks. It is true that a great additional
degree of apparent probability has been given to the metamorphic theory of eruptive
rocks by the results of the microscopic examination of rocks so successfully instituted
by Sorby, since they prove a remarkable similarity, in the minutest texture, of the
minerals constituting granite and some  cognate non-foliated rocks, with the main ingredients of certain foliated rocks made up of silicates, such as gneiss and micaschist.
These had been observed long before to form the last link of a series which may be
traced, by slow gradations, from  unmletainorphosed sedimentary rocks, through those
which are undoubtedly metamorplhosed, up to the crystalline foliated rocks mentioned.
The newly discovered facts appeared to indicate that granitic texture is the most advanced stage in a progressive series of molecular changes, and, carrying the argumentation still farther, the assumption was apparently justified that a corresponding origin
must be ascribed to other rocks, such as those of volcanic origin, which are connected
with granite by another long and gradual line of passage.
In drawing up our argument against this theory, (including all those doctrines
23 It may be frequently noticed, that those eminent men who, by creating a firm basis for induction, have pointed
out the path through regions in the field of science the knowledge of which had consisted before of a confused accumulation of facts and suggestions, did themselves apply the newly acquired views within moderate limits, while sweeping generalizations on the same basis were usually made by others. It is so in the present case. The limits within which Daubrde
himself has applied his ingenious conclusions from his experiments on the action of superheated water and pressure upon
silicates, will probably never be drawn any closer. No discovery, however, could have been more opportune to those who
had advocated before the origin of eruptive by the remelting of sedimentary rocks. Against the form of this theory which
was first proposed by Hutton, and enlarged by Lyell, Babbage, Herschel and others, similar objections could be raised as
against that form of the theories of the first class which was held by Buch. With the aid of the new views acquired,
however, it was remodeled and brought into its present shape. A far greater extent is thus being given to the conclusions
firom Mr. Daubrde's experiments than their author ever intended.
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OF VOLCANIC ROCKS.                               /[
which assume either all or a part of the massive crystalline rocks composed of silicates
to have originated by the metamorphisml  of sediments) we will first point out some
reasons against its general tenor, which may be conclusive in regard to those rocks
the eruptive nature of which is almost generally conceded, while we will have to
bring some additional arguments against its special application to those which are often
designated as " hypogene " or " plutonic."
The metamorphic theory starts from the assumption of an alternating progression fromn the center of the earth, and recession towards it, of the chthonisothermal
planes, the former being caused by sedimentary deposition, the latter by denudation.24
It is then argued that, by the progression of these planes, sedimentary strata would
acquire a more and more elevated temperature, and, being permeated by water, would
be metamlorphosed, and finally rendered liquid.  It is demonstrated that heat, generated
by the plication of the strata in the lowest and central part of an area of subsidence,
would aid in promoting these changes, which would end in the rupturing of the crust
and the protrusion of liquid matter.  Eruptive activity should, according to these
views, be confined to areas of subsidence, in particular to their central portions.  Geological observation does not favor this conclusion, since the emission, at least of the
volcanic rocks, has taken place on the borders of those areas, on high table lands, and,
in general, in. places which have undergone elevation before and since the time of the
first commencement of the eruptive activity.
It would appear that the experiments of Daubree should not be too freely
applied to reasoning on processes within the shell of sedimentary rocks.  They have
been made with relatively large quantities of water, such as can scarcely be expected
to be present in solid rock at some distance below the ground.  Supposing that that
quantity naturally enclosed in it would', under great pressure, cause its fusion, then
no reason can be adduced why there should not prevail a liquid state of all matter at a
limited depth below the surface, and over extensive regions, if not over the whole
globe.  That such is not the case, is evident from the want of any signs of subterranean
tides.  Leaving this difficulty out of consideration, another presents itself concerning
the periodicity of eruptive activity.  If processes such as those suggested by the
adherents of the metamorphic doctrine were its cause, then it would be impossible to
give anl explanation, why there have been long eras of rest intervening betw7een others
of violent eruptive activity, or why the latter was of general distribution over the globe
during the Tertiary era, and preceded by a period of rest which was probably nlo less
general.  The phenomena of vulcanism might have manifested themselves at a certain
time mnore in one country than in another, if metamorphism had been their cause, since
their principal theater would be constanltly shifted to the places of the most violent
metamorphic action; yet they should, at least in the aggregate, have been continuous.
These objections, however, against the metamorphic theory of eruptive rocks
are of little weight when compared with another, respecting their nature as chemical
24 It is needless to enter here upon a discussion of the causes atnd effects attributed to these phases in the " flow of
heat " by the adherents of the metamorphic theory, since this topic has been made lately the subject of a paper by the skillful
hand of Professor Dana.
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7 2                                   RICHTHOFEN - NATURAL SYSTEM
compounds. It is from this point of view, as we have repeatedly remarked, that are
offered the chief and most deeply-founded  differences  between  the  eruptive  and  the
stratified and foliated rocks.   Definite numerical relations on the one side, complete absence of them  on the other, and a transition between both, marked by the gradual disappearance of those relations with the passage from granite, in which they are very
distinct, to gueiss and micaschist-these are  in short the  prominent characters of the
great divisions of those rocks which are accessible to the observations of the geologist.
We  alluded  at another place  to  the  corollary of this distinction, namely: that it is
impossible that eruptive rocks are remelted sediments, because, if they were, they
would necessarily have to participate in the varied and indefinite chemical composition
of these.   This important argument appears to have been  completely overlooked  by
the adherents  of the  metamorphic  doctrine, and  it would  be  sufficient by  itself to
make the latter appear to be in contradiction with the true state of facts. 25
If we descend  from  this general tie  embracing  the totality of eruptive rocks,
to those relations which either separate or connect distinct groups of them, in  regard
to the time at which  they came  to the surface, we are unable  to find  any explanation
of the peculiar uniformity of these relations, if we adopt the metamorphic doctrine.
Rocks of great mutual similarity might have been occasionally ejected at different
places; but the repetition  of the same order of succession inl distant regions would be
just as inexplicable, and contradictory to the  nature of sedimentary rocks, as the perfect chemical identity of rocks which are widely separated in space, but occupy a
similar position in the order of succession.
Another geological consideration may be mentioned which appears to weaken the
metamorphic theory.   It is known  that the  most ancient rocks are distinguished, in
general, by a much greater simnilarity among each other in respect to chemical composition than is the case withl those of more recent origin, and  that one promninent feature of the  majority of them  is the presence of silica in a similar proportion to that in
which  it is contained  in  granite.   The  formation  of sedimentary  rocks having  been
due, at all times, to the disintegration of rocky matter antecedent to them  in age, it is
obvious that ancient sediments would have participated in the silicious nature of the
25 It can hardly be comprehended that it should have been maintained, and be believed, with our present state of knowledge, that clayslates were, by metamorphic action, converted into granite and syenite, and sandstones into porphyry; since
granite and (quartzose) porphyry are chemically identical, while clayslate and sandstone differ in this respect not only among
themselves, but each of them represents a large number of different accidental compounds, without any law of mutual connection. The supposition that " the presence of the sandstone formations affords the conditions required for the occurrence
of the great porphyry-masses," gives evidence of an interpretation of natural occurrences by preconceived ideas. There are, it
is true, numerous instances of sandstones having acquired, by metamorphic action, a more or less perfect porphyritic texture,
so as to be hardly capable of being distinguished, in specimens, from true eruptive porphyries; but geological observation
will seldom leave any doubt in regard to their true mode of occurrence; and, as a generality, it is not difficult to distinguish
these metamorphic rocks with porphyritic texture from " the great porphyry-masses," such as those of southern Norway, or
middle Germany, to which the above-mentioned supposition has been especially applied. It is evident that the eruptions of
the porphyritic rocks were there subaqueous, and that the true relation is exactly the reverse of that suggested: the ejection of great masses of quartzose porphyry afforded all the conditions requlired for the formation of sandstone beds.  Qolartzose
porphyry did overflow lvast regions, and, by its immediate disintegration into tufaceous matter, gave origin to those red
sandstones by which it is so often accompanied. These cover the porphyry often in horizontal beds, and bear to it similar
relations of gradual passage and interstlatification, as trachytic tufa does to trachyte, where the eruptions of the latter were
subaqueous.
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OF VOLCANIC ROCKS.                             73
rocks by the destruction of which they originated, and should have had a tendency
towards an average between them in regard to chemical composition. If such sediments were afterwards rendered liquid by metamorphic processes, and either carried to
the surface as eruptive rocks, or solidified beneath it, so as to form. part of it only after
agoes of denudation-if they were then again partially disintegrated, re-deposited and
reerupted, and so forth, down to our time, the final result should be an ever-increasing
uniformity in composition of both eruptive matter and sediments. It may be objected,
that this tendency towards uniformity must have been checked by the formation
of chemical sediments, as by them a portion of certain elements was eliminated, and
must have become almost irrecoverably lost for further participation in the supposed
revolving process. The substances prevailing in chemical deposits are lime, soda and
magnesia.  Recent eruptive rocks should, therefore, besides showing a uniformity of
composition, be somewhat deficient in those substances, and contain in proportion
more of silica, alumina and potassa.  The facts are exactly reverse. Not alone has the
variety of eruptive rocks rather been constantly increasing; but, among those most
recent in origin, such are greatly predominant as contain lime, soda and magnesia
in larger proportion than ancient rocks, while thley are relatively deficient in silica,
alumina and potassa. We arrive by this last line of argument at the conclusion, in
the first place, that the present variety of rocks, especially of those of eruptive origin,
can in no way be explained by, but is contradictory to, the assumption that they originated by the repeated destruction and reformation of the material which constituted
the surface of the globe in the most ancient time. From this it follows, in the second
place, that, even supposing that the theory should be admissible for the majority of
eruptive rocks, entirely new matter must have come repeatedly to the surface, in order
to replace the constant loss, as it were, of substances such as lime and soda. Eruptive
rocks ascending from sources beneath the deepest sediments are the only means imaginable for the performance of this function. The enormous extent to which matter
must have been supplied from that source is especially evident, if it is considered that
there must have been a time when no sediments existed on the face of the globe, and
that the disintegration of preexisting matter is the prime condition to their formation.
The entire mass of the sedimnents which make up the exterior shell must, therefore,
have been derived from the destruction, partly of the original crust which solidified on
the globe, and partly of those rocky masses which protruded through that crust to the
surface.  It is eminently probable that the mass of matter derived from the latter
source exceeds very far that which is due to the first.  This consideration shows
that it is hardly possible for us to form an adequate idea of the vast importance which
the periodical emission of rocky matter from places beneath the primordial surface must
have had in the history of the formation of the crust of the globe.
We may thus start from any point of view within the range of positive
knowledge, and we shall find that there is not one line of argumentation which does
not go to show that the doctrine of the origin of eruptive rocks by the metamzorphism
of sedimentary matter is irreconcilable with the most prominent established facts. Not
only does it fail completely to explain them, but with most of them it is in obvious
contradiction.  But as a mnetamorphic origin has been proved to evidence in regard to
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7 4                       RICHTHOFEN -NATURAL SYSTEM
certain foliated rocks, and as it appears no less safe to conclude that those rocks, the
intrusive or extrusive character of which is generally admitted, originated from below the sedimentary rocks, we have still to examine where is the limit to which the
theory of such an origin may be safely applied.  A line of gradual passages connects
the foliated rocks with gneiss, and through it with granite, and another line of
gradual passages can be traced from the volcanic through the porphyritic to the
granitic, and through them again to the gneissoid rocks. The affinity of granite and
gneiss may therefore be said to be the central point from which all other lines of
passage in the nature of rocks diverge; and it must be ascribed to this relation,
that many geologists, while admitting the origin from below the crust of the globe
of " trap-rocks " and " lava," still consider granite, syenite and cognate rocks to
have been generated by the metamnorphism  of sediments, and to occupy now  the
same position which these sediments had before being metamorphosed.  The peculiar character of granitic texture, which is nearer allied to that of foliated crystalline than to that of volcanic rocks, the geological occurrence of granite in intimate
connection with those and its apparent independence of position in reference to the
latter, the  singular order of solidification of the minerals constituting granitic
rocks-these and other reasons of a similar kind would indeed appear to support
the assumptiorn that granitic and volcanic rocks differ in regard to their origin; while,
on the other hand, additional probability is apparently given to the connection of
granite and foliated rocks in regard to their mode of origin, if it is taken into consideration that many varieties of gneiss resemble granite closely in chemical compositionl, although others deviate from it in this respect and do not conform to the
law of Bunsen.
According to the theory here advocated, all those rocks will be the true representatives of the primordial mass of the globe, which are subject, in regard to their
composition, to the law of Bunsen, and occur, besides, in positions which place
beyond doubt their intrusive or extrusive origin, so far as they no longer occupy
their original place. As regards the first distinguishing mark, it is known that granite,
syenite, diorite, diabase, and nearly all those porphyritic rocks which have been considered as of hypogene origin are subject to the law of Bunsen, as far as their composition has been ascertained.  In this respect, therefore, they are fundamentally different
from sedimentary and metamorphic rocks. Although believing this to be conclusive
evidence of their origin from below the shell of sediments, we must enter into a short
discussion of the chief objections which have been raised against the necessary corollary
front this mode of origin, which is the eruptive character of those portions of granite
which we see on the surface, and the assumption of the existence of this as the fouidation of all sedimentary rocks.
It is argued by the adherents of the metamorphic theory of eruptive rocks, in
the first place, that granite cannot  be the  fundamental rock in the crust of the globe,
on which those of sedimlentary origin are resting, since even very ancient masses of it
are frequently found to overlie stratified rocks.  But this fact, which was known long
ago, shows only that it is impossible, on account of the great thickness of sedimentary
rocks, to krnow by ocular observation of what nature is the fundamental rock.  It is
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OF VOLCANIC ROCKS.                              75
logically absurd to assume the non-existence of a foundation consisting of rocks differing in origin from sedimentary rocks; and tile conclusion is inevitable, that it must be
composed of rocks which were generated by the solidification of such mrasses as formed
part of the primordial substance of the globe, next to the surface.  Probabilities
accumulate to i)oint towards the assumption that very silicious granite composes that
foundation, together with such gneissoid rocks as must be supposed to have been formed
in an incipient sea of very high temperature, and under the pressure of the superincumbent atmosphere of aqueous vapors, at a time when the cooling of the globe had
advanced far enough to allow of their first condensation, and when pressure, water,
and heat could coioperate to produce on its surface similar effects to those which the
same agencies are supposed to have wrought in later periods at an ever-growing distance from it.  The most probable mode of these ancient processes has been pointed
out by Daubree.  The true nature of the foundation rock can, of course, not be
positively known, but must remain a matter of conjecture.  No solution of this
problem will, however, be more satisfactory than that which is based on the hypothesis of Sartorius, because it is in harmony with all the phenomena of vulcanism.
The probabilities in favor of a granitic foundation are, therefore, very great, while
no valid objection has yet been raised against its having the nature indicated.
It is further argued that, the position of granite being always in the midst of
sedimentary, or of such foliated crystalline rocks as are connected with the former by
gradual passage, and often conspicuously above such rocks, the only material froin
which it could be generated are the sedimentary rocks themselves. The arguments
brought in evidence of these assertions are among the most potent which can be
adduced against them.  Granite, in Norway, is superposed on Laurentian rocks turned
upon their edges, and, as no channel can be found by which it ascended, it is argued
that it must have been generated by metamorphism in situ.  But as, on the other hand,
it has justly been remarked, that no position is more favorable for metamorphic action
than that of upturned strata, it is difficult to comprehend why the Laurentian rocks,
which were evidently nearer to the source of heat, should not have been metamorphosed in a much higher degree than the overlying granite.  The same objection may
be made in other instances, as in the case of the Huronian and Laurentian rocks in
Canada, which were found to be overlain by granite thousands of feet in thickness.
The negative evidence, apparently afforded by the fact that no channels have been discovered through which the granite could have protruded, is of very slight value, if it
is taken into consideration how rare are the instances where the conducting channels
can be seen, in the case of overflows of volcanic rocks, or even in that of currents of
lava. There are a few more instances known where granite can be distinctly observed
to overlie the upturned edges of stratified rocks, and must, therefore, have overflowed
the same in a liquid state. In some other cases, as in those of the extensive granitic
areas of Bessarabia and Western Australia, the true geological position of granite
escapes observation; while, in a number of others, it has the appearance of an intrusive
mass.
The strongest objection which may be made to the eruptive nature of
granite relates to its lithological characters, which are different from those of the
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T76                       RICHTHOFEN   NATURAL SYSTEM
eruptive rocks of the present era.  The metamorphic theory suggests that enormous
volumes of those masses which it supposes to have been rendered liquid by metamorphism, and small portions of which were occasionally emitted to the surface, did
solidify below ground with a total loss of their previous structure, and may arrive at
the surface by elevation and denudation.  Granite is supposed to have been in all
cases a hypogene rock, generated by metamorphism  in those same positions in which
it is found at the present time, and to be still formed continually in the same way;
though it is admitted that the tension which may have attended its former state of
liquidity, may have caused it to ramify into rents and fissures of the overlying rocks.
We have, of course, to assume the existence of an immensely greater amount of hypogene matter corresponding in composition to all the diferent eruptive rocks, but buried
so deep beneath the present surface, that, possibly with the exception of a few granite
masses, it never does, nor ever will, form part of it.  We have, for this reason, to consider all non-foliated crystalline rocks made up of silicates, and which are visible on the
surface, as having been removed far from their original seat, and therefore as being
eruptive. It is a matter of course that only a very small portion of all matter protruded from the depth would be ejected to the surface, as by far the greater portion would
solidify within the channels of ejection.  Granite occupies frequently the latter position quite distinctly.  But if we consider those large accumulations of the same rock
covering hundreds of square miles, we can only explain them  in two ways.  Either
they must occupy their primeval position, as is supposed by the adherents of the
metamorphic doctrine, or they must have been ejected and spread in a liquid state
over the underlying rocks. The objections against the first supposition are, the adaptation of granite to the law of Bunsen, and the fact already noticed, that it overlies
the upturned edges of strata; the objection against the other is the peculiar mineral
character of granite.  But we should always bear in mind that our knowledge in regard to the conditions required for producing any certain kind of mineral character
within a comnpound of silicates of any certain composition is very limited, as is illustrated, among others, in the cases of propylite or dolerite; and the inferences based
on subjects which are imperfectly known to us should, in geology, always be made
subordinate to those which we can positively establish by observations. It is not on
the ground of the latter that a hypogene origin is ascribed to all granite, but on the
ground of its mineral characters. It is said that granitic and volcanic rocks offer quite
generally a different appearance, and that the texture of the former indicates crystallization under great pressure, while the volcanic rocks were evidently solidified on the
surface. As regards the first assertion, such rocks only should be compared together
as have a similar composition, that is, granite with rhyolite, or diabase and augitic
porphyry with basalt; not, as is ordinarily done, granite with phonolite or basalt. mWe
referred already to the similarity in character between certain varieties of rhyolite and
granite. It is corroborated by the microscopic examination of these rocks by Ferd.
Zirkel, from whose memoir26 I quote the following passages:'"Quartziferous trachytic
26 Dr. Ferdinand Zirkel, Mikroslkopische Gesteinsstudien, in Sitzungsberichte der Kais. Acad. der Wissenscicaflen zu
Wien, vol. 47, 1863.
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OF VOLCANIC ROCKS.                             77
porphyry (rhyolite) cannot be distinguished under the microscope in any respect whatever from felsitic porphyry. If one dare not doubt the eruptive origin of the former,
one cannot but acknowledge the same mode of origin in regard to the latter." And
again: "Let us here remark, that the microscopic structure (texture) of trachytic
quartz does not differ in any respect firom that of granitic quartz; there is no diffeirence
between these two families of rocks so widely separated in time, either as regards the
number or the appearance of the water and glass cavities." Considering the second of
the before-mentioned assertions, it is true that Mr. Sorby has, by his sagacious observations, come to the conclusion that granite solidified at a temperature of about 6000
Fahr., and that the various granites have been formed under a pressure equivalent to
a depth of from 40,000 to 69,000 feet. The calculation by which these figures were
obtained, is made upon the basis of the proportion between the bulk of the water
contained in the microscopic cavities, and the size of the vacuity, the latter being supposed to indicate that the water had formerly filled the entire cavity and contracted
within it after the solidification of the surrounding rock. The laws of hydrostatic
pressure, however, as Daubre justly remarks, are in such a case not applicable in the
same way as they would be in a column of water ascending through a fissure; temperature and pressure may, in a mass of rock solidifying from a viscous state, be preserved, as in a closed vessel, to within a few feet from the surface; "it is, therefore,
possible that many processes, such as the crystallization of granite, may have been
going on under pressure, though at a very limited depth." Considering that a mass
of granite, after the solidification of its exterior portion, will indeed be enclosed within
walls that rmay offer a strong resistance equivalent to a great pressure, it may be inferred that it will assume, in crystallizing, a similar texture to that which it would
have obtained if the same process had been going on at a great depth below the
surface. It can hardly be suggested what must have been the texture of the crust of
a granitic mass. But the age of the granite is sufficient to justify the conclusion that
the crust of those masses which solidified on the surface of the globe must have been
completely abraded, and only those portions be preserved to observation which consolidated under considerable pressure.
Though these arguments may show that the difference in the character of a
large mass of rock need not necessarily be proportionate to the depth under the surface of the globe at which it solidified, and that granitic and volcanic rocks are nearly
related as regards their microscopic texture; yet, the conspicuous external differences
between these two classes of rocks must remain a problem difficult of solution. The
proportionate rapidity with which volcanic rocks, on account of their usually small
volume, must have cooled, may be among the reasons. But it is not the only one. There
are differences in the nature of rocks, which escape our present means of explanation.
Hornblende-propylite, though no doubt at all can be entertained in regard to its subaierial solidification, has completely the character of the so-called Plutonic rocks, and
yet lacks their supposed principal distinguishing mark: crystallization in depth. Considering that certain varieties of propylite, andesite, and trachyte are modifications of
the same group of chemical comnpounds; and, though having been solidified on the surface of the globe within a short period, yet exhibit marked external differences, we canO                                                        (115)




7 8                       RICHTHOFEN-NATURAL SYSTEM
not be surprised to see, that other rocks composed of silicates, which are so widely separated in time as is the case with granite and rhyolite, should offer even greater differences.  Some further clue to a better knowledge of this subject may be expected from
the study of the nature of volcanic rocks.  We see the same chemical compound
forming highly viscid lava in one volcanic crater, while it is quite liquid in another.
There, it solidifies to dolerite or leucitophyre; here, to basalt. Similar influences
appear to work greater differences in more silicious compounds. We should therefore
put little value on the doubts entertained in regard to the solidification of granite, as of
a rock ejected on the surface and expanded over it, until we are better acquainted
with the causes of the lithological differences among volcanic rocks; and the evidence
regarding the mode of origin of granite should, till then, be mainly taken from its
geological occurrence and its chemical composition.
RELATION  OF THE DISTRIBUTION OF THE VOLCANIC ROCKS TO THE CONFIGURATION OF
THE SURFACE OF THE GLOBE.
If we embrace in a broad review all those relations mentioned in the foregoing
chapters which regard the history of eruptive action, we arrive at the general conclusion that, in reference to the entire globe, it is one homogeneous and harmonious
whole, and has been attended by such gradual changes only as were necessarily
occasioned by the progress of the physical development of the globe itself, while, in
regard to every different part, it presents a series of distinct phases intimately connected by mutual relations. It would be a subject worthy of the closest investigation,
to trace the effect which the events of these phases have had severally upon the structure and the configuration of any separate country. A comparison of these effects
as they are manifested in different regions would then aid in establishing some of the
chief causes of the differences of structure peculiar to each; and the knowledge of
these would make us better acquainted with the laws governing the evolution of the
globe, and prepare the way for a more thorough understanding of its physical geography.  I will only attempt at this place to trace the effects referred to in regard to
the last of the phases of eruptive activity, the only one which was nearly contemporaneous in all countries. It occurred at a comparatively recent date, and we can view
the results in a clearer light than we can those which are remote in time and partly
obliterated by the vast changes which since then have revolutionized the surface.
We have, in the first place, to trace more in detail than we have done before,
the peculiarities of the distribution of volcanic rocks.  It has been observed that active volcanoes are chiefly situated along the lines of the present sea-coasts, especially
at the foot of mountain ranges parallel to them; or that they follow elevated submarine ranges, when they will either remain submerged beneath the sea, or protrude
above it, forming chains of islands. Some of these appear to mark the lines of high
mountain ranges bounding submarine continents, if we may use this expression for
those areas of a shallow sea-bottom which are separated from  others of great depth
by wall-like elevations, as are indicated for instance in the main ranges of the vol(116)




OF VOLCANIC ROCKS.                                      79
canoes of the Indian Archipelago.  Active volcanoes have also been found to be particularly numerous in those regions where the narrow  terminations of two continents
verge towards connection, as is the case in Central America, between Alaska and
Kamitschatka, and between Australia and Farther India.  The mode of distribution of
extinct volcanoes has been little  investigated.  But as they are far more numerous
than active vents, and as volcanic action has completely ceased in extensive regions, a
perfect understanding of the distribution of volcanic activity on the globe call only be
acquired when they shall be fully taken into consideration.  The general laws of their
distribution appear not to differ from those relating to active volcanoes. They, too,
have been evidently dependent upon the course of the sea-coasts which existed at the
time of their activity, and were in many places quite different from  those of the present epoch.  But we must note, besides, this additional feature of extinct volcanoes,
that among the theaters of their grandest and most extensive displays are certain
countries which were formerly covered with large inland basins filled with salt water,
and form  now mostly elevated table-lands on which may still be seen the remnants of
those former inland seas. Volcanoes so situated form  often crowded groups occurring
at distances of more than five hundred miles from  the sea-coasts which existed contemporaneously with their activity.  To them' belong the numerous extinct volcanoes
on the plateau between the Sierra Nevada and the Rocky Mountains, a region which
has probably had no equal in regard to the extent in which volcanic activity has taken
place, as it comprises the States and Territories of Oregon, Idaho, Montana, Utah, Colorado, Nevada, California. Arizona, New Mexico, and continues southerly to the high
lands of Mexico.   Other volcanoes of high table-lands are those of the  Mongolian
Desert, the Thian-Shan, Armenia and, probably, eastern Africa.  The volcanoes of all
these regions are extinct, with the exception of a few in the southern part of the plateau of Mexico, which are in action, and some others which still exhibit solfataric pheonomena.
Interesting as would be a mnap  embracing all active and extinct volcanoes, it
would give only the outlines of the distribution of those larger accumulations of volcanic
rocks which were produced by massive eruptions, and have ordinarily attracted little
attention.27 Geological maps being often imperfect as regards them, it is not yet possible to distinguish any other features of their distribution than those mentioned in
reference to volcanoes, as they can always be found in the vicinity of these. We may
however add, that massive accumulations may eventually be found near the summits
of mountain ranges, where volcanoes do ordinarily not occur.
27 This may be exemplified in regard to the Andes of North and South America. Humboldt (Cosmos, vol. iv)
distinguishes five groups of volcanoes, separated by non-volcanic regions. They are mentioned by him as follows: 1st, volcanic group of Mexico, nearly five hundred miles in length, but occupying scarcely more than one degree of latitude, on
account of its direction from east to west; 2d, three hundred and fifty miles free from volcanoes; 3d, group of Central
America, having a length of over eight hundred miles; 4th, seven hundred and thirty-five miles non-volcanic; 5th, group of
New Granada and Quito, five hundred and fifty-five miles in extent; 6th, the longest interval without volcanoes, being 1,133
miles; 7th, glroup of volcanoes of Peru and Bolivia, extending over five hundred miles; 8th, a non-volcanic space of six
hundred and twenty miles.9th, the group of Chile, the most extensive range of volcanoes of America, being 1,143 miles in
length. Summing up. Humboldt takes about 3,000 miles to be the aggregate extent in length of the volcanic regions, 2,838
miles that of the intermediate spaces which are free from volcanoes. If all the volcanic rocks occurring in the Andes
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80                           RICHTHOFEN - NATURAL SYSTEM
If we turn our attention front the mode of the geographical to that of the geological occurrence of volcanic rocks, we may first consider from this point of view the
relations just mentioned.  The two modes of distribution which are most conspicuous,
namely, on the foot of mountain ranges and along sea-coasts, are nearly identical in a
geological aspect.  For most, if not all of those ranges the flanks or vicinity of which
are distinguished by the occurrence of volcanic rocks on a large scale, have been contiguous to sea-coasts in the Tertiary period, or they are so at present, or they have been
so in the intermediate time.  This is true of active and extinct volcanoes, as well as for
massive eruptions, in the Carpathians, on the southern foot of the Alps, on the borders
of the plains of northern Germany, on the slopes of the central plateau of Asia, in their
whole extent from Armenia and the Caucasus, passing Lake Issikul and Lake Baikal,
to the vicinity of Pekin, in the coast ranges of California, in the Cascade Mountains of
Oregon, and in numerous other countries. As regards the prominent occurrence of
volcanic rocks on high table-lands, it is analogous to their situation on sea-coasts, inasmuch as the salt lakes occurring on them, which were of much larger size in the Tertiary period, would be an equivalent of the vicinity of the sea. Those larger regions
not covered by salt lakes, which are situated in the interior of continents, and have
been so since the commencement of the Tertiary period, were generally not the theater
of outbreaks of volcanic rocks.  This circumstance renders alnost certain the influence
which the neighborhood of large quantities of salt water has had upon the comnmencement and main phases of the eruptive activity, though its continuation in the latter
phases, which are those of volcanic action proper, may in numerous instances have
been maintained by fresh water.
The relations of the distribution of the volcanic to that of the granitic and porphyritic rocks have been considered in another chapter. There is probably no place distinguished by the accumulation of the first where the previous eruption either of granitic
or of both granitic and porphyritic rocks may not be observed, if the conditions are such
that observations ill this direction are possible. But as there are porphyritic regions outside of the volcanic belts (for instance, the quartzose porphyries of southern Norway, or
the Triassic trap-rocks of the Connecticut Valley), and as the samre is true of a great
number of known granitic districts, and probably of a far greater number of others
which are not accessible to observation, it is evident that the existence of ancient channels of ejection did not necessarily imply their reopening in the Tertiary period, while
there can scarcely be any doubt that the once shattered places where they were situated offered less resistance to the formation of new fractures, than other portions of
those regions in which the greatest disturbances took place in the Tertiary period.
We have, finally, to mention the connection which apparently exists between the
occurrence of volcanic rocks and the regions where ancient formations have been
were laid down on a map, it would hardly show such great intermissions. The extensive fields of lava found by Capt.
Fitzroy, in southern Patagonia, the volcanic rocks of the Desert of Atacama, and of northern Peru, are instances of their
extensive distribution in the non-volcanic spaces of South America, while on the northern continent they are one of the
main features in the structure of the great western Inountain ralnges, probably throughout their whole extent from P'ananma
to the peninsula of Alaska, and are accompaniedl probably by thousands of extinct craters.
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OF VOLCANIC  ROCKS.                                               81
highly disturbed and subjected to an extensive metamorphism. It appears that the
eruptive action was in a great measure, perhaps absolutely, limited to regions which
offered this condition. But this connection by no means justifies the conclusion that
the existence of metamorphic foliated rocks was among the first causes of the ejection
of volcanic rocks. There are not only countries, such as the mountains of Scandinavia,
the Ural Mountains, the Appalachians, and other ranges distinguished  by the  records
of a very ancient metamnorphism, where no trace of volcanoes has been found; but this
applies even to a few, though particularly grand instances of mountain ranges which
were the theater of metamorphic action on a grand scale within the most recent periods,
such as the Alps, the Himalaya, and the Pyrenees.
The two last named relations, both  conspicuous, yet both  secondary in importance, are evidently nearly identical, since granite enters, probably in all cases, into the
structure of those  same countries which have been the theater of an  extensive metamorphic action.   It is, however, worthy of note that, of the  regions so distinguished,
part of those  only appear to have  become the theater of eruptive activity in the Tertiary period, where granite must, by the mode of its occurrence, be assumed to have
formerly been extruded to the surface, as is the case in the Sierra Nevada, on tlie
southern foot of the Alps, in middle Germany, and in central France, while no volcanic
rocks arrived at the surface in other mountain ranges where granite occurs only in the
shape of wedges surrounded by foliated rocks, and bears a merely intrusive character,
as in the central range of the Alps.28
28 Another peculiarity in the distribution of volcanic rocks may be noticed. It is their outbreak at places of dislocation or faulting of mountain ranges, if we may apply these expressions for the immense displacements which some of the
latter have undergone, along lines either at angles with or parallel to their direction. A remarkable instance of the first
kind is afforded in Hungary, where a fracture, along the line Eperies-Kaschau, crosses the Carpathians in a nearly meridional
direction. To the west of it, ancient formations, along with granitic, porphyritic, and volcanic rocks, make up the surface,
and are elevated to high mountain ranges culminating in the tIigh Tatra.'hey extend eastward close to the fracture, and
there terminate abruptly. East of it, the main chain of the Carpathians continues as a range of little elevation, and consisting chiefly of Cretaceous and Eocene rocks, while the high foothills of the western part are replaced by the Hungarian
plains.  Immediately out of the fracture rises the Eperies-Kaschau range, of nearly one hundred miles in length, which
consists exclusively of volcanic rocks. The subsidence of the land on the eastern side of the fracture must have amounted
to several thousand feet in this case. A transverse dislocation of similar magnitude, by which the western side was sunk
thousands of feet, is observable on the eastern bank of the Rhine, along the boundary of Switzerland and Vorarlberg.  No
volcanic rocks however are visible at that place, which fact appears to be in accordance with the absence of any ancient
eruptive rocks in the neighborhood, and the wedge-like shape of granite in the nearest places of its occurrence. A peculiar
instance is afforded by the three porphyritic regions on the southern slope of the Alps, each of which indicates a deep depression, bounded by longitudinal and transverse fractures, of that area which has in each case been the seat of eruptive
action.  Prof. J. D. Whitney and I had occasion to observe another instance, more forcibly striking for its grandeur than
any of the foregoing. Following the crest of the unbroken range of the Sierra Nevada, from south-southeast to northnorthwest, one arrives in the northern part of the County of Plumas at a sudden change of scenery and rocks.'I'he metamorphic rocks of the Sierra Nevada are broken off and terminate in a line apparently at about right angles with the direction of the crest of the former. The rugged and wild scenery which they present, particularly in this county, gives way
abruptly to one equally remarkable for its beauty and geological interest. The high volcano, Lassen's Peak, rises in the distance, amidst an intensely volcanic group of hills. Low forest-clad ridges built of lava, and separated by meandering plains
covered by meadows, stretch southward from  it, gradually sloping down towards the line of dislocation, and there abut
against the wall of metamorphic rocks. The whole surface, though lower than the summit range, is entirely volcanic.  Seveuty miles northwest from Lassen's Peak rises Mount Shasta, more sublime even than the other. Between both is a deep
depression, through which the Pit River takes its course from  northeast to southwest, at little altitude above the sea.
This depression is situated at the very place where the crest of the Sierra Nevada should pass, and is directed transversely
to it.  Northwest of Mount Shasta, Prof. W. Ii. Brewer found the continuation of the Sierra Nevada, with similar char(119)




82                                   RICHTHOFEN- NATURAL SYSTEM
These appear to be the most essential geographical and geological relations
regarding the mode of distribution and  occurrence of volcanic  rocks.   It is perfectly
evident that no one of themn singly was the chief cause of their occurrence, nor were
all of theni together   but each of them  had a marked influence, either upon the direction  and  the  location of the  orifices of the fractures, or upon  the  mode  of ejection.
Keeping in view  these different relations, I shall dwell, in the rest of this chapter, more
particularly on the connection between the occurrence of volcanic rocks and the configuration of the surface. This subject may appear to be beyond the scope of this
paper.   But the examination of every question  which  relates to the inner connection
between the phenomena attending  the  ejection of volcanic rocks will aid in disclosing
the true nature of these, and promote the knowledge of the principles of their natural
system. We will endeavor to answer the following questions: Was the particular
structure of certain portions of the crust of the globe, which is indicated by the situation of elevated regions on its surface, among the causes of the eruptions of volcanic
rocks? or were the inequalities of level on the surface, in the volcanic regions, due to
the processes attending and the agencies causing the eruptions?
The answer to both questions must be in the affirmative. The peculiar structure
of the earth's crust, at those places where mountain ranges and highlands rise on its
surface, appears to have influenced in a great measure the distribution of the volcanic
rocks, because those among them  which are accompanied by the latter, had been
elevated before  the  time of the  eruptions, and  the  adjoining  lowlands  were  not the
theater of eruptive activity.   But, on the other hand, it is evident that the ejection of
volcanic rocks, or rather those subterranean  processes of which  they were one of the
acter in rocks and scenery to that which that range has in other parts. It appears that a gap of more than one hundred
miles in length has been formed in the region of the two volcanoes, by the displacement of a portion of the Sierra Nevada,
which was probably bounded by two lines of fracture transverse to the direction of the mountain range, and has subsided
thousands of feet, and that then an immense accumulation of volcanic rocks filled up the gap, and closed in building up the
two giant volcanoes.  Other lines of dislocation which have given vent to volcanic rocks, and which have more frequently
been noticed, are directed parallel to mountain ranges. Of such nature appears to be the abrupt descent of the Sierra
Nevada towards the Great Basin, which has been the theater of violent eruptive action; and probably the relations on the
western slope of the Rocky Mountains are of a similar nature.  The Vihorlat-Gutin Range, in Hungary, offers a striking
illustration of an extensive accumulation of volcatlic rocks along the foot of a preexisting mountain range, though the dislocation is not conspicuous in that country, on account of the deposition of recent sediments which filled up the Hungarian
Basin. There may be some affinity between these modes of occurrence of volcanic rocks and the manner in which they are
met with in certain areas of flat or hilly countries, surrounded by ranges composed of ancient rocks. The best illustration
is afforded by the Basin of Transylvania. The undulating country of the interior is encircled by high ranges consisting of
ancient formations, which are lined on their inner side with volcanic rocks. Hungary itself affords a similar instance, though
less regularity-is perceptible; and the same structure is somewhat approached in the geological relations of Bohemia.  We
may also mention, as recalling that mode of occurrence, certain depressions between the two summit ranges of the Sierra
Nevada, such as the Basins of Sierra Valley and Lake Tahoe, which are encircled, first by a ring of volcanic rocks, and
then only by the metamorphic and granitic rocks which form the bulk of the Sierra Nevada.  Or the " Parks " of the
Rocky Mountains.  Their geology, it is true, is almost unknown. An interesting description of the San Luis Park, the
greatest among them, recently published (see American Journal of Science and Arts, November, 1867), shows that the
elevated rim of its basin, which is estimated at eighteen thousand square miles, is made up of ancient formations, and that
volcanic rocks encircle more immediately the extensive plain forming its bottom, which is itself composed of volcanic
sediments, thus completing a structure that reminds of that of Transylvania, in more than one respect.  If we look for
instances on a grander scale, we may find some analogy with the mode of occurrence of volcanic rocks as just described,
in the volcanic ranLgres encircling the Basin of the Pacific Ocean. And it may not be out of place if we call attention to the
similarity with these circular basins which is presented by the configuration of the surface of the moon.
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OF VOLCANIC  ROCKS.                                              83
resulting events, exerted a powerful reaction upon the further promotion and acceleration of those motions of the crust by which the irregularities of level of the surface were
produced.   This  may be inferred, in the  first place, from   the  fact, that in and along
volcanic belts, elevation and plication of sediments have taken place, probably in every
instance; both  modes  of disturbance  have  affected, partly sediments anterior in age
to the eruptive action, and partly such as were contemporaneous with and posterior to
it; and there may be frequently  noticed  a diminution  of the  intensity  of these  disturbances, in an inverse ratio with the distance from   the  volcanic belt.29   But besides
this fact, observation has placed it beyond doubt, that in the second half of the Tertiary period, and after it, a greater aggregate amount of relative, and probably too of
absolute elevation has taken place generally over the globe than had been the case for
ages before.   The altitude of the rocks composing  the summits of the  highest mountaini ranges and  table-lands of the  present timne, exceeds that which  they had in the
29 This evident connection, in volcanic regions, of elevation and plication, as cause and effect, appears to be one of
the gravest objections against the general validity of the theory of Mr. Hall on the mode of formation of mountain ranges,
though the incongruity of an explanation of the granitic wedges by metamorphism in situ with the chemical nature of
granitic rocks, the total insufficiency of a gradual sedimentary deposition for accounting for a foot-for-foot subsidence of
large areas, and the difficulty of finding any natural cause of the assumed fact that the belts of greatest amplitude within
each area of subsidence should have been elevated to mountain ranges in preference to other neighboring parts, are objections of scarcely less weight. It is a well-established fact, that the Alls existed as a mountain range, at least from the time
of the Jurassic period (though probably long before it), and have gained in height in later periods. Yet the Jurassic and
Cretaceous strata are as much contorted as are those of more ancient age, and the same is true in a still greater measure
of the Tertiary Flysch. If, therefore, Mr. Vose, in his Orographic Geology (p. 68), arrives at the conclusion: " So far as
we have examined the facts, no other hypothesis than that of the slow sinking of vast masses of yielding sediments can at
all satisfactorily account for the plication and other evident effects of a compressive force so invariably exhibited in mountain districts," the more recent formations of the Alps confirm this view as little as the above-mentioned example regarding
the volcanic belts. It may be mentioned, in this connection, that, while Mr. Hall's theory would necessarily imply an
increase in the intensity of plication towards the axis of greatest accumulation, which is considered by him to have been
coincident, after the elevation, with the axis of the mountain ranges, this conclusion is not confirmed by the structure of
the Alps. In Vorarlberg and northern Tyrol, which offer probably, of all parts of the Alps, the most normal and regular
structure, all strata, commencing with those of Triassic age, are least disturbed where they are nearest to the axis, and the
plication increases towards the foot of the range. Superposition, by contortion, of older formations on those of more recent
origin, is encountered with increasing frequency of instances and growing distinctness, in crossing the parallel waves from
those nearest to the axis, towards the northern foot of the range. Another objection, nearly related to the foregoing, is
this, that, granting that a trough-like depression would be able to cause plication by the settling of the yielding strata
towards the axis of greatest subsidence, the waves thereby formed would have their steeper inclination towards that axis,
and their flatter slopes directed outwardly, since a greater resistance would oppose the motion of the lower strata than
would be offered to the higher ones. The analogous instance, to which reference has been made, of a wave rolling towards
the beach, shows plainly the effect of the retardation sufficed by the lower portion of the water by friction, in the steepening
of that side nearest to the place towards which motion is directed, and the final throwing over of the top in the same direction. A familiar instance, which repeats more exactly, though on a small scale, the conditions suggested by Mr. Hall's
theory, is afforded when a viscous mass, such as resin or pitch, moves downward on a slightly inclined plane. If the necessary conditions are given for the formation of waves, their steeper sides will always be on the lower end, or, on that which
would be nearest to the axis of a trough-like depression. It is obvious that this shape is the reverse of that of the waves
of plication which Mr. Rogers and others have observed, and on which even Mr. Hall's theory is partially founded. This
theory, which was first proposed for a mountain range in which recent formations do not occur, and where, therefore,
not all of these objections could be raised, may, if somewhat modified, have its limited applicability for partially explaining
the mode of formation of certain mountain ranges; but numerous arguments appear to preclude its general applicability,
especially so in the case of the European Alps, and probably not less in the case of the Himalaya.  It appears, too, to be
quite inadmissible in the case of the Rocky Mountains; and as regards the Sierra Nevada, only a chain of the most arbitrary assumptions would be able to give it an apparent validity for the explanation of any features in the structure of that
range.
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84                        RICHTHOFEN — NATURAL SYSTEM
Cretaceous period by thousands of feet. This is not only true for those ranges which,
like the Andes and Rocky Mountains, have been intensely volcanic, but also for such
as show a less immediate connection with eruptive activity in the volcanic era, as is
the case with the Pyrenees, the Alps and the Himalaya. The elevation which succeeded the Cretaceous period appears to ha've everywhere been slow, intermittent, and
partly retrograde, in the Eocene epoch, while its chief phase is traceable to the Miocene
and subsequent epochs, by the relative altitude above the level of the sea to which
sediments of different ages have been raised.
There can scarcely be any doubt that this acceleration of the changes of level,
and the extrusion of volcanic rocks, have had an intimate connection. But the mode
of this connection has by no means been ascertained, and appears to be quite an intricate subject. It was formerly believed that the eruptive rocks were themselves the
upheaving agents. their intrusion and ejection having been the mechanical means of
the elevation of mountain ranges. But geological observation and argumentation have
conclusively demonstrated that such could not be the case, and made it probable that
both elevation and eruption are the most conspicuous symptoms of different agencies
which were dependent on one common cause. When that first theory had to be abandolled, another was put in its place, to the effect that the ejection of rocks must have
been attended by subsidence; and, although we shall show in the following pages
that the rise of the crust greatly predominated in the vicinity of the eruptions, there
can be no doubt that subsidence was probably in all cases among their intricate effects.
But the cause generally ascribed to it is not at all capable of explaining it. The
assumption that eruption must be attended by subsidence is usually made on the ground,
that the removal of a certain volume of matter by ejection, from a deep-seated place,
would cause the formation of a vacuity, corresponding in extent to the volume
removed, and that consequently the overlying portions of the crust would settle down.
This assumption appeared to be corroborated by the observation that the region surrounding a volcano subsides during its activity, while it rises in periods of rest. But
it is evident that the size of a current of lava is not proportionate to the settling of an
area of hundreds of square miles to the amount of several feet. If, moreover, the
views here advocated, regarding the cause and mode of eruptive activity, are correct,
that is, if the ejection of rocky matter is the effect of the increase of volume which it
undergoes on passing into the state of aqueous fusion, then the eruption is only the
discharge of the surplus matter which has no room within the space of the fissure; no
vacuity could therefore be formed, and the commonly adopted cause of the subsidence
would not exist. There are, however, two other causes which would produce subsidence, and have probably been acting in every case of nmassive and volcanic eruptions. The first of them is the contraction of the liquid mass in the conduits, by cooling. Its amount must be considerable in proportion to the space of the fissure, but
small when compared with the volume of a mountain range on the surface. Its effects
will be local and abrupt. In the case of volcanoes, it is the probable cause of the
familiar phenomenon of the subsidence of the bottom of craters, and of those less frequent cases where whole portions of the cone are suddenly engulfed. As regards
extensive accumulations of rocks by massive eruptions, there are certain features of
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OF VOLCANIC ROCKS.                            85
the configuration of their surfalce which cannot be mnore plausibly explained in any
other way.  To these belong the sudden breaks in the continuity of their surface,
which consist sometimes in elongated and steep wal]s, thousands of feet inll height, and
miles in length, or in crateriform or semi-circular basins, and in other more or less
abrupt depressions, which may be chiefly noticed where large regions are uniformly
covered with granite, porphyry or volcanic rocks. This cause will affect the mass of
the eruptive rock itself, but not perceptibly the surrounding country.   Yet, this is
subsiding when a volcano is active, and it canl be definitely proved in the case of the
andesitic ranges of Hungary, that during the epochs of the massive eruptions, the general rise has been repeatedly interrupted bythe subsidence of the suroundings of the
theater of activity. To these changes applies probably the second of the causes alluded
to.  Whenever a fissure is filled by liquid matter injected from below, its surroundings
must necessarily become heated, and, by their expansion, produce a slight increase of
the rise of the surface.  This heat escapes, in the case of a volcano, chiefly in the epochs
of its activity, by the emission of lava, vapors and boiling water, and other phenomena associated with volcanic action.  Massive eruptions will have been attended by the
escape of heat on a much larger scale.  They appear to have been often accompanied
by an extremely violent emission of hot water, as may be inferred fiom  the great
accumulation of deposits of silica in some volcanic countries, or from  the immense
overflows of extensive regions by volcanic mud: this occurs in Hungary and on the
western slope of the Sierra Nevada on so grand a scale as to almrnost exclude the possibility of its having originated merely from volcanic action, especially as no volcano is
visible from which they could have escaped.  These processes must of course have had
the effect of lowering the temperature of the mnasses surrounding the fissure to some
distance from it, and of producing subsidence during the eras of activity.  Yet they
are not sufficient to explain the extent to which it has often taken place, though it had
ill no case, in the vicinity of the theaters of eruptive action, more effect than to reduce
locally the amount of elevation.
A few examples will suffice to show how vast are the cllhanlges of level which
have taken place since the commencement of the volcanic era, and to demonstrate their
connection with the other manifestations of vulcanism during the same era.  An instructive instance is furnished by the country situated betwveen the Pacific coast and
the Rocky Mountains.  The labors of several distinguished geologists have made us
acquainted with some of the main features of its complicated structure; but it is only
by the detailed examination of some of the most important portions, together with the
accurate determination of the age of several of the sedimeltary and metamnorphic formations, made by and under the direction of Professor Whitney, that the foundation has
been laid for anl exact exploration of the entire western part of North America, which is
now proceeding with rapid steps, and promises to give important contributions towards
the solution of the questions discussed in this essay. There appears to have existed, as
we mentioned before, an ancient grallitic era in that region.  But it is as yet impossible to recognize the relation of this ancient granite and the metamorphic rocks by
which it is accompaniel to the ancient or to the preselpt conlfiguration of the surface.
Is the western countries, those folrmationls are concealed by the imrmen-se overlyin-g
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86                         RICHTHOFEN -NATURAL SYSTEM
accumulation of Paleozoic and Mesozoic sediments, and the subsequent denudation of
these, though grand in the extreme, has only been sufficient to expose to view the
ancient granite in some localities, among which we mentioned that on the Colorado
River.  Other masses of ancient granite are visible at the surface in numerous places in
the Great Basin, but their exact relations still remain to be determined, while in the
Rocky Mourltains they are one of the prolmlinent features, partaking largely in their
structure.
The next period of interest, in regard to the occurrence of eruptive rocks, is that
of the deposition of Triassic and Liassic sediments in great aggregate thickness, which
were found by Whitney to extend from the Pacific coast far into the Great Basin. They
prove that all this country was then still submerged beneath the sea, while the Rocky
Mountains formed probably a broad belt elevated above it.  It is in this period that
were ejected the quartzose porphyries of the County of Plumas, in northern California,
almost contemporaneously with those of the southern Alps. Whether this event, which
was probably not limited to the region mentioned, was attended by any changes in the
configuration of the surface, cannot yet be decided. After it, however, they must have
taken place on a grand scale, reminding one of the emergence of the central body of the
Alps from the sea after the deposition of the infra-Liassic limestones.  The ejection of
granite found, it appears. almost the whole country embraced between the western
slope of the Sierra Nevada and the eastern slope of the Rocky Mountains lifted out of
the sea.  But changes greater than those which had preceded them, appear to have
attended and followed those gigantic outbreaks. They were probably due, in a great
measure, to the intense metamorphism which was connected with them, and has conpletely changed the petrographical character of the preceding sedimentary deposits.
When the volcanic era commenced, Which was probably in the Miocene epoch, all the
ancient formations of the Sierra Nevada were nearly turned upon their edges, and the
depressions of the surface in the Great B asin were filled with salt water. But the altitude of the entire plateau above the level of the sea was probably insignificant at that
time compared with what it is now, as may be inferred firomn the fact, that rivers did then
flow on the present western slope of the Sierra Nevada, parallel to its crest, which they
could not have done if that slope had had its present inclination.  We must imagine
that where the great mountain range raises now its lofty summits, a hilly country extended then, ascending slightly to the east. The first outbreaks of volcanic rocks found
those rivers still flowing in their beds, as is proved by the higher sediments in the old
river-channels, which consist of volcanic tufas. But great changes occurred after the first
commencement of the volcanic era, changes whiich contributed probably more towards
imparting to the western portion of North America its present features than ally which
had preceded them.  The volcanic belt extending along the entire western coast of
America, had its greatest breadth between the coast of California and the Rocky Mountains, and the eruptive activity has been violent over that vast region. It appears that
to that era is clue the principal part of the elevation of the high table-lands.  Such, at
least, is the case in their western portion.  The crest of the Sierra Nevada rnust have
been elevated at a clquicker rate than its western foot, as distinlc traces are left ill the
gravel deposits that those ancient rivers which were ilowvin-g pacrllel to its crest have
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OF VOLCANIC ROCKS.                              87
been gradually turned from their channels, overflowing their banks at several places in
succession, and taking a course down the slope, until an entirely new system of watercourses was created, at right angles to the crest, the steep ravines and gorges of which are
now one of the characteristic features of the Sierra Nevada.  It appears that the changes
of level since the inauguration of the volcanic era have also been progressing on a grand
sclie in the eastern part of the Great Basin, where the eruptive activity had probably
even more gigantic proportions than in the west. The central portion of the Rocky
AM/ountains was raised, according to the estimate of Dana, about seven thousand feet since
the Cretaceous; and its eastern part from one to two thousand feet, since the Miocene,
according to the observations of Hayden.
These figures render evident the great elevation which the mountain mass
between the Sacramento and the Missouri must have undergone since the commencemient of the volcanic era. Twice in the history of that country, since the Triassic
period, may there be recognized an extraordinary intensity of all those changes which
we must ascribe to subterranean agencies.  The first instance was in or about the
Jurassic epoch, when strata, which appear to have been quietly deposited during preceding eras, vere elevated, and a gigantic intrusion and ejection of granitic masses was
attended by an intense and wide-spread metamorphism, by which disturbances and
plications were promoted; the second, in the volcanic era. It is probable that similar
phases to those mentioned will be recognized throughout the entire range of the Andes,
the geological structure of which appears to offer much similarity in different parts. To
the events of the volcanic era, chiefly, will have to be ascribed the connection of both
parts of the continent, though it may have been prepared by that preceding era of intensified actions, which manifested itself in the ejection of the granite of the Sierra
Nevada, and appears to have left no less distinct traces in other portions of the range
of the Andes.  We may still note a peculiar difference in the mode of the changes of
level if we proceed across the continent from west to east. It appears that the narrow
strip of land adjoining immediately the western coast has been subjected rather to
periodical oscillations than to any lasting changes of level, while the great elevation of
the mountain ranges and highlands must be chiefly ascribed to the circumstance that
all changes have acted there essentially in the same direction, producing the elevation
of extensive regions.  This would explain why the western descent of the Andes has
been periodically increasing in steepness and the strip between them and the coast is of
little width.  The latter appears to correspond to the boundary between an area of
elevation to the east and an area of subsidence to the west, one of which was especially
subjected to the manifestations of vulcanism, while in the other it has left no recognizable signs. It is quite different on the eastern side of the range of the Andes, where,
in both parts of the continent, a slow rise has taken place, which has connected, during
the volcanic era, that mountain range with other ranges farther east, by those extensive low-lands which are so important a feature as regards the extraordinary productivity of both parts of the continent.
Similar to these are the relations presented by the European continent, in regard
to which we will only mention a few prominent facts.  During the porphyritic era and
the time immediately succeeding it, great changes of level had taken place on that
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88                       RICHTHOFEN-NATURAL SYSTEM
continent. It appears that in Middle Germany the Triassic period designates the commencement of anl era of repose, while in the Alps, where the porphyritic outbursts took
place in the Triassic age, the Liassic strata still participated in the disturbances and
elevations, which partly attended the porphyritic era and partly succeeded it. Thereafter followed another period distinguished by the comparatively small amount of disturbances which took place in it. Subsidence appears to have prevailed in its first part,
while towards its enid, during the second half of the Eocene epoch, that renewed rise
commenced which contributed so much towards imparting to that mountain range its
present configuration. In surveying the general features of the geology of Europe, it
will be found that the changes of the boundaries of the continent and the sea, as produced by rise and subsidence, have, in the period intermediate between the porphyritic
and the volcanic era, been inconsiderable in proportion to the length of time; and,
though grand in the totality of their results, they arle probably far surpassed by those
which have been produced since propylite reopened the eruptive activity. None of
the prominent mountain ranges, it is true, have been called into existence since then.
Sonie of them, of little importance and consisting completely of volcanic rocks, have
been formed, and sedimentary formations have been folded quite extensively, so as to
form hilly regions; but the main ranges had existed before, and only underwent an increase in volume, though one of considerable mnagnitude, and the rate of elevation appears
to have been greatest with those mountain ranges, during the period indicated, which
have the greatest altitude at present. The amount of elevation which Eocene and
Miocene strata have experienced in the Alps, Pyrenees, and others of the prominent
mountain ranges justifies this conclusion, while the mode of occurrence of these formations in the Alps allows us to infer that their central portion was elevated at a more
rapid rate than either the northerly or the southerly part. It is worthy of note that
not alone in the Alps, but quite generally in Europe, the Jurassic and Cretaceous
strata have undergone only little more disturbances than those of Eocene age. In the
belts elevated during the volcanic era, all three formations participate ordinarily at
nearly equal rates in the structure of the low ranges of the hills stretching between
the main ranges and girting their foot. The conclusion appears therefore to be justified that in the volcanic belts, at least, the elevation of the Jurassic and Cretaceous
has chiefly taken place during the volcanic era.
There may be distinguished a twofold mode of elevation in the volcanic era: it
mlanifested itself either in a secular rise of continental areas or in the elevation of
mountain ranges, as may be exemplified by referring to the difference in the mode of
elevation between the Andes and the countries adjoining them to the east.
The distinction is not so definite if we refer to the European continent. Among
the regions elevated during the volcanic era may be chiefly noticed a broad belt estending from  the Alps through the Turkish Peninsula, Asia Minor, Armenia, and
Persia to the Himalaya, and continuing less distinct westward to the Pyrenees. The
three main ranges have experienced the greatest amounit of elevation within this belt.
The rest of it, which is distinguished by the folding of the Ntlummulitic strata over its
entire area, marks the region of the next greatest intensity of the elevating forces within
much more extelnsive area of continental rise. The iincrease of altitude has been com(129)




OF VOLCANIC ROCKS.                              89
paratively inconsiderable over this area; yet it is, in its totality, of greater import for
the configuration of the surface of the planet than the production of the mountainous
and hilly regions mentioned.  For it had the effect of connecting regions which were
before separated by the sea, and of increasing thereby the area of continents in certain directions.  It is well known that the sea, which in the beginning of the volcanic
era extended in the valley of the Danube to above Tienna, bathed both slopes of the
Carpathians, and reached eastward to the foot of the highlands of Central Asia, has
since then considerably retired, and that the secular rise of northern Africa, Arabia,
and Asia Minor has, also in these countries, enlarged the continental areas.
So much time has elapsed, in most countries, since the main phases of the volcanic era, that the influence of the events connected with them  upon elevation and
subsidence has probably relaxed.  It appears that po(rtions of the areas of elevation
have suffered a periodical subsidence, not only since that time, but also during the
volcanic era. But its aggregate anmouLnt having been in most of them less than that of
elevation, the final effect manifests itself generally as a rise; though it appears that
in recent times some regions, chiefly those which are situated on the northern and
southern boundaries of the great area of continental elevation, are undergoing a more
marked subsidence.  Even now, however, those countries chiefly are rising which
were distinguished by eruptive activity in the volcanic era.
These few instances will suffice to show how great have been the changes in the
configuration of some portions of the surface of the globe during the relatively short
geological time which has elapsed since the commencement of the volcanic era, and how
much superior they appear to have been to those which had been going on during
preceding periods of much longer duration. Those facts which are known in relation
to this subject, allow to infer, that the changes of level were proceeding at an accelerated rate, during, and probably also in the time next preceding, the volcanic era, and
that the elevation was limited to certain belts of great extent which were, in the
Tmajority of cases, distinguished firom neighboring regions by eruptive activity.  But
this is not true for all cases, since there are some regions the elevation of which was
especially grand, and in which very few rocks, or none at all, were ejected.  In examlmining into the causes of the connection of elevation with the other events of the
volcanic era, we must therefore keep in view the distinction of those cases where it was
attended, and those where it was not attended, by eruptive activity.
As regards the first cases, ve mnay refer for their explanation to our previous
theoretical considerations of the causes and effects of the formation of fractures.  If
the increase of volume by the slow and perfect crystallization of viscous masses below the crust was the cause of the rending of fissures, a slow rise must have preceded
this process.  That this was so on the European continent, is verified by the fact, that
the Eocene strata had been elevated quite considerably when the first of those sediments, which caRn be proved to have been contemporanleous with eruptive activity,
were deposited. If then the relief from pressure caused the extensive crystallization,
at the depth to whiclh the fissures reached, of masses which had been held before in a
viscous condition by the pressure itself, an accelerated rise would be the effect; and if
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90                       RICHTHOFEN- NATURAL SYSTEM
the repeated rupturing of the crust caused a repetition of this process, in the way indicated before, epochs of an accelerated rise would alternate with those of a retarded
elevation.  This theory will explain why a very considerable rise, and of greater extent than that which had preceded, attended the phenomena of the basaltic epoch.
Although we rmust consider this subterranean process as the chief cause of conltinental elevations, so far as they attended the phenomena.of eruptive activity, it was
certainly not their only cause.  There can be no doubt that the process of metamorphism  must have promoted those changes.  As far as we understand the conditions
required for its production, it appears that they could not be created by any other
known processes in a degree similar to that in which we must necessarily suppose them
to have attended the ejection of rocks, whether its mode be that of massive eruption
or of volcanic activity.  Even on the surface there are no places where we can observe,
at the present time, metamorphic processes of greater intensity than near the orifices
of volcanoes, or in solfataras, or at the theaters of other processes attending or succeedling volcanic activity; nor is there at any other places more evidence afforded of subterranean metamorphic processes. The latter will exceed those visible on the surface in
nearly equal proportion with the increase of temperature and pressure in depth;
while we may conclude theoretically, that the neighborhood of fissures filled with
heated  substances from  below  would not only have a higher temperature than
other portions of the crust, but also allow of a comlparatively free circulation of water,
on account of its shattered condition. In respect to metamorphic action, as in all
other respects, the grandeur of the phenomena of nmassive eruptions would make us
presuppose that they would have given rise to a far more extensive and intense metamorphism than the insignificant processes of volcanic activity.  The occurrence of
ancient eruptive rocks in belts of highly metamorphosed sediments removes it almost
beyond doubt that they are connected in the relation of cause and effect. It is assumed
that metamorphic processes, on account of the entering of water into the composition
of rocks, and their crystallization, must be attended by an increase of volume, and
will have the mechanical effect of elevating the surface.
We may thus obtain a few hints as to the causes of the changes of level which
those portions of the surface of the globe which were the theater of eruptive activity
in the volcanic era, have undergone since its inauguration.  The agencies to which
they were due must be even more intricate than the motions of the crust, because the
movement in one direction may be the resultant of several forces working in opposite
directions.  Increase of volume by crystallization below the crust, in those regions
where the formation of fractures relieved the masses from  pressure. is probably
the most potent of these agencies.  Metamorphism would work in the same direction,
while loss of heat would cause subsidence.  All these agencies have their first and
common cause in the formation of fractures by an upward tension, and all other
phenomena of vulcanism  are the immediate or mediate effects of that same cause,
which itself results from the gradual cooling of the globe.
There remain those cases to be considered where some of the most prominent
mountain ranges, in the structure of which volcanic rocks take no part, were elevated
durling the volcanic era, at a much greater ratio even than those in which these rocks
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OF VOLCANIC ROCKS.                              91
occur. The Alps and the Himalaya are the most striking instances of this kind.  The
first of these, and probably, too, the other, had, as we mentioned before, undergone a
considerable elevation ill and after the porphyritic era, but have probably changed their
altitude very little in the next following periods.  The acceleration of their elevation
during the volcanic era appears, however, to have greatly exceeded that of the Andes.
Tile coincidence in time of these events, and of all the phenomena characteristic of the
volcanic era, justifies the supposition that they are connected in their origin.
If we contemplate, in its relations to the volcanic era, the great elevated belt of
which the Alps and Himalaya form the axis, we find it to consist of three parallel
zones.  The central one comprises those two mountain ranges and the broad mountainous counltry which connects them, and which owes its configuration chiefly to the
events of the volcanic era.  The Alps and Himalaya are free from volcanic rocks. This
is also true as regards the central portions of those chains lwhich branch off from the
Alps in a southeasterly direction, and of those (as far as their geology has been explored) which extend westerly fromn the Himalaya.  The farther one proceeds in the
Turkish peninsula towards the east, the more frequent are the monuments of eruptive
activity of the volcanic era; they are known in Servia and Bulgaria, in the north, and
in Macedonia, Thracia and Epirus, in the south. They increase in similar ratio as
one proceeds westerly from the Himnalaya, through the highlands of Armeneia to Asia
Minor. If it is considered that active volcanoes are particularly numerous in those
regions where the elongated portions of two different continents have, as it were, a
tendency to connection, the analogy with the mode of distribution of volcanic rocks
in the regions mentioned is conspicuous. The system of the Alps with its southeastern,
and that of the Himalaya with its western braLnches, were disconnected before the Tertiary period, and were united into one mountainous belt during the same. The eruptive
activity contemporaneous with this slow process was remote from the main axis where
this had existed as an elevated range before, and approached it more and more, from
both sides, increasing, at the same time, in intensity, until it culminated in that
region where the connection of both systems was effected.  Massive eruptions and
volcanic activity have ceased completely in this central zone.
North of this, is another zone, which was distinguished in its entire length by
the intensity of eruptive action in the volcanic era. It stretches fromn Central Asia by
the Caspian Sea, the Caucasus, the Crimea, the Carpathians and, in branches, through
central Germany to central France.  In the European portion of this zone, there are
only recognizable in thermal and mineral springs the last feeble remnants of former
volcanic activity, while they are somewhat more energetic in that portion which is
situated on the Asiatic continent.  A  third zone, not less distinguished for the intensity of eruptive action in the volcanic era, accompanies the main axis to the south.
It traverses India, and, in the Bay of Bengal, is connected with the volcanic belts of the
Indian Archipelago, while it continues to the west through Arabia, Syria, Palestine, to
the MSlediterranean, where it comprises the active volcanoes of the Grecian Arclhipelago
and Italy, and is connected by Sardinia and southern Spain with the Azores. Volcanic
activity still continues in this belt, but the period of the massive eruptions has passed
long ago.
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92                        RICHTHOFEN -NATURAL SYSTEM
The entire belt, composed of these three zones, has to be considered as one great
area of elevation, characterized, at the same time, over its greater portion, by the remnants of the eruptive activity of the volcanic era.  No portion of it, however, has, as
we mentioned before, experienced an amount of additional elevation during that period
equal to that of those mountain ranges which existed before it, and among which the
Alps and Himalaya are the most prominent. In these cases there cannot have been any
connection between the elevation and the ejection of the rocks to the surface, because
these do not occur.  But this does not preclude the connection between the elevation
and those agencies which are the causes of eruption.  It has been often demonstrated that the changes of level must have been accompanied by the formation of
such fractures as would be closed next to the surface, and, though allowing of an intrusion of plastic nmatter from below, would prevent its extrusion to the surface.  It is
probable that fissures of this kind may have been chiefly formed where high mountain
chains are composed of metamorphic rocks, since they frequently exhibit vertical dislocations, partly parallel to their axis, and partly at right angles to it, by which the strata
onl one side have been moved tlhousands of feet above'the other, and which, notwithstanding, did not give vent to the ejection of rocks.  These faulting fissures appear to
form frequently the lateral boundaries of mountain ranges and the limits of the manifestation of vulcanismn, and both these circumstances allow us to infer that they extend
downward to great depth.  Though one of the main  features in  the geological
structure of the Alps, they have hitherto been little examined.  But it is probable that
their formation was coincident in time with the main phase of elevation, that is,
with the volcanic era. If, then, in that part of the crust over which the Alps are elevated, such fractures were formed as did not open on the surface, then all the agencies below would cooperate towards elevation alone. The expansive force produced
by aqueous fusion would find no vent for the discharge of the masses by which the
volume had increased, nor would there be anlly opportunity given for the escape of
heat by hot water and other means; while, on the other hand, the masses in depth
would be relieved firom pressure by the formation of this kind of fractures in the sarmne
way as by those which would allow the passage of liquid matter to the surface. In the
case of the latter, an immense amount of force is spent in other modes of action, while
in the first case it could be applied almost exclusively to elevation.  Besides this direct
action, however, we must also keep in view, that in those regions where fissures would
not be open at the surface, the conditions required for metamorphic action would be
given on a particularly grand scale.  Gases and water would not reach the surface, but
be employed in depth in pr.omotillg metamorphic action over vast regions, and increasing the rate of elevation.  It may be by processes of this kind that already in ancient
times, contemporaneously with an accelerated elevation, were formed those granitic
wedges surrounded by broad belts of foliated metamorphic rocks, which are peculiar
to certain mountain ranges, particularly to those which also in the volcanic era have not
been the theater of eruptive activity.  The intrusion, from below, of heated masses into
fissures closed towards the surface, would give all the conditions required for the exhibition, on a grand scale, of those processes of hydropyric metamorphism which Daubrie
has rendered probable by his experirnments and theoretical deductions.
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OF VOLCANIC ROCKS.                             93
Leaving aside these theoretical consideration-s, and comparing only the different facts mentiloned in regard to the changes of level which have taken place on the
surface of the globe since the inauguration of the volcanic era, we are forced to the
conclusion that they must be intimately connected, as regards their causes, with the
other phenomena by which this era was distingiuished from preceding periods. Elevation and eruptive activity, even when locally not quite coincident, are coiirdinate effects
of the cooling of the globe; but while the one is its immediate effect, the other results
from it only by the concurrence of other agencies, which by themselves alone would
have been incapable of producing results of such magnitude. As there are distinct
phases in the history of eruptive action, dependent on and marking the evolution of
the globe, so we may recognize different stages in the mode of manifestation of the
elevating forces. In those elevations which took place during the volcanic era, there
are certain peculiar features evident, among which may be mentioned: the increase in
altitude of those mountain ranges included in the volcanic belts; the union into one
main chain of several smaller ridges, the axes of which are situated on a line; the
lateral connection of parallel ranges into highlands; the oblique connection of the
ends of main chains, the axes of which are situated on parallel lines, but are remote
from  each other longitudinally, by broad belts of mountainous regions (of this
character is the connection of the Alps and the Himalaya); the elevation, finally, above the level of the sea, of large areas which had been submerged before,
whereby distant mountain ranges were connected by extensive lowlands, the size
of the continents increased, and their outlines rendered more uniform. The mode
of these great changes of the configuration of the surface of the globe was probably
a repetition on a large scale of similar changes which had been going on in former
eras of eruptive activity. It is not our object here to develop them. But throughout
the course of these changes there appears to be conspicuous a tendency to connect, in
certain directions, what was before disconnected, to increase in size and render more
definite certain areas of elevation, and to separate them from other areas of subsidence,
which appear to have been likewise increasing in extent. We may even recognize an
increase of this tendency during the volcanic era, as the phenomena connected with
the basaltic epoch have been far more widely and generally distributed over the areas of
volcanic belts than those of the preceding (propylitic and andesitic) epochs. There must
be certain general laws which regulate the directions in which the connection of disconnected parts takes place. Their knowledge is still very limited. The admirable manner
in which Dana has laid out the grand outlines of the arrangement of the islands of the Pacific, has indicated the way by which we may look for the solution of this problem. If the
structure of the crystallized crust of the globe gives the most probable cause of the
definite directions which may be recognized in the outlines of continents and chains of
islands, it should be borne in mind, that the direction of structural planes, if they
exist, must vary gradually in depth, in the same ratio with the chemical composition,
and that they may at t3he depth of andesitic compounds be different from what they
would be in the granitic depths. The singular way in which the connections by elevation are effected, and which has been already partly pointed out by Dala, appears to
Q                                                     (131)




94              RICHTHOFEN - NATURAL SYSTEM  OF VOLCANIC ROCKS.
find only an explanation when such changes of the direction of the structural planes
are assumed to take place in depth.
The tendency to simplify beyond their present conditions the general outlines
of the configuration of the surface of the globe, continues probably without intermission, though an era of comparative repose has followed the violent exertions of vulcanism of the volcanic era. The great backbone of the Eastern continent, comprising the
Alps and the Himalaya, and the second great belt which comprises the Andes, and,
more properly, encircles the whole Pacific Ocean, are at the present epoch the main
features in the orography of the globe. Both have derived their prominent position
from the events of the volcanic era.  The conclusions at which we have arrived in
these pages have been drawn chiefly from  observations made in regions of prominent
interest in either belt, and it is for this reason especially that we believe that at least
some of them will be found to be susceptible of general application.
I am fully aware how imperfect is this attempt to develop, in the correlations of
the volcanic rocks, the principles of their natural system.  This system should be, as
we remarked before, not a classification of objects by certain conspicuous properties,
but a classification of objects by their definite mutual relations.  We have not to make
the divisions, but to find them.  We shall be able to do this with greater or less perfection, in the same measure as those relations are studied and firmly established.
Much is needed to arrive at this end. It can only be achieved after evidence has been
accumulated by the combined and harmonious labors, in different countries, of geologists in the field as well as in the laboratory. The range over which both modes of
observation extend from year to year, and the field on which their results meet, is increasing in a surprising degree. Yet eruptive rocks form ordinarily an object of less
exact observation than those of sedimentary origin; and it cannot be denied that harmonious observation, by which alone can be established the principles of comparative
petrology, is rendered nearly impossible, as long as no uniform system of nomenclature
is applied. These pages are the result of an attempt to add one mnore to the many
contributions towards establishing the foundation of both, and to show the value which
the study of eruptive rocks and of those mutual relations which comprise their natural
systemn promises to have for approaching the solution of some of the highest problems
of geological science.
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TABLE OF CONTENTS.
PAGE.
INTRODUCTORY...................1..........................................  1
THE NATURAL SYSTEM OF VOLCANIC ROCKS........................................ 5
ORDER FIRST: RHYOLITE..................                   12
ORDER SECOND: TRACHYTE.............................................                17
ORDER THIRD: PROPYLITE...................................................... 20
ORDER FOURTH: ANDESITE....                                                         25
ORDER FIFTH: BASALT.26
CORRELATION OF THE FIVE ORDERS OF VOLCANIC ROCS.                                   28
Laws Relative to the Age of Massive Eruptions.                         29
Laws regarding the Mutual Relations of Massive Eruptions and Volcanic Activity 32
RELATION OF VOLCANIC ROCKS TO ANCIENT ERUPTIVE ROCKS...........                    35
Correlation of Age and Texture..................................  39
Correlation of Age and Composition.................................... 41
Correlation of Eruptive Rocks in regard to their Geographical Distribution..   43
ON THE ORIGIN OF VOLCANIC ROCKS........................................  46
1. Origin of Massive -Eruptions.................................... 47
2. Origin of Volcanic Action.                                          60
3. Other Theories regarding the Origin of Volcanic Rocks........68
RELATION OF THE DISTRIBUTION OF VOLCANIC ROCKS TO THE CONFIGURATION OF THE SURFACE
OF THE GLOBE...........78
(133)