UC BERKELEY MASTER NEGATIVE STORAGE NUMBER 03-67.60 (National version of master negative storage number: CU SN03067.60) MICROFILMED 2003 UNIVERSITY OF CALIFORNIA AT BERKELEY LIBRARY PHOTOGRAPHIC SERVICE REPRODUCTION AVAILABLE THROUGH INTERLIBRARY LOAN OFFICE MAIN LIBRARY UNIVERSITY OF CALIFORNIA BERKELEY, CA 94720-6000 COPYRIGHT The copyright law of the United States (Title 17, United States Code) governs the making of photocopies or other reproductions of copyrighted materials including foreign works under certain conditions. In addition, the United States extends protection to foreign works by means of various international conventions, bilateral agreements, and proclamations. Under certain conditions specified in the law, libraries and archives are authorized to furnish a photocopy or other reproduction. One of these specified conditions is that the photocopy or reproduction is not to be "used for any purpose other than private study, scholarship, or research.” If a user makes a request for, or later uses, a photocopy or reproduction for purposes in excess of "fair use," that user may be liable for copyright infringement. University of California at Berkeley reserves the right to refuse to accept a copying order if, in its judgment, fulfillment of the order would involve violation of copyright law. Turton, F. E. The petrology of the Franciscan sandstones 1915 BIBLIOGRAPHIC RECORD TARGET University of California at Berkeley Library Master negative storage number: 03-67.60 (national version of the master negative storage number: CU SN03067.60) GLADIS NUMBER: 184789198D FORMAT : BK AD:991013/FZB LEVEL:b BlL7T:am DCrF:a CSC:4 MOD: Fl.:7 UD:030604 /MAP CP: cau L:eng INT: OPC: BIO: FIC: CON: ARCV: PC:s PD:1915/ REP: CP1: FSI: 11.C: 11:0 040 CUscCU 090 SbDISS.TURTON.GEOL 1915 100 1 Turton, PP. EB. 245 14 The petrology of the Franciscan Sandstones /$cby F. E. Turton. 260 $01915. 300 31 p. ;8c29 om. 502 Thesis (B.S. in Geology)-- University of California , Berkeley, May, 1815. 610 20 University of California, Berkeley.$bDept. of Geology and Geophysics$xDissertations. 690 0 Dissertations, Academic$xUCBS$xGeology$y1911-1920. Microfilmed by University of California Library Photographic Service, Berkeley, CA FILMED AND PROCESSED BY LIBRARY PHOTOGRAPHIC SERVICE, UNIVERSITY OF CALIFORNIA, BERKELEY, 94720 DATE: 7/03 REDUCTION: 10 X © PM-1 3%"x4"” PHOTOGRAPHIC MICROCOPY TARGET NBS 1010a ANSI/ISO #2 EQUIVALENT 10 SIE 2 = FE 6.3 i 36 BE fos : Il =e 2 Li fr] ° ow = l= LZ lis pe rl 1) lg [LA AAA hi sl Ee! - - 2 er bit elt alt. tli.olt, le, 18, [&, 9, |g, |¥, [&, [8 [r}™ evn gL cit fot, 6 18, le lo (6, Ib I6, IR) Kh DXs53 TuRToN Geol 1915 Zhe T THE PETROLOGY OF THE FRANCISCAN SANDSTONES. BY F.E.Turton. Submitted as a Thesis for the Degree of Bachelor of Science. University of California. Berkeley,California. May 1915 LIBRARY COPY Table of Contents. Introduction. Fage Description of the Central Coast Ranges. The Franciscan Croup &s & "hole, Ceneral Description of the Franciscan Sandstones. Petrographic Description of the Sandstones. Description of the Franciscan Cherts. J Relation of Bhert,Shale and Sandstone. — General Description of the Bonita Sandstone. Fetrographic Description of the Ecnita. The Marin Sandstone. The Cahil Sandstone. ™ [So yecial Feature of the Ceahil Sandstone. QO] P he Undifferentiated “Sandstone. +3 The Bumboldt Sandstone. reatures of the Teathered Sandstone. Conclusion. 24 o7 28 Introduction. The problem at hand is primarily a discussion of Petrographical features of the Franciscen sandstone as found in the localities surrounding the Bay of San Francisco,including e general description of the sand- stones and discussing some of the criteria which would serve to determine the mode of their deposition. The Franciscan series is named from San Francisco where it was first described and is best shown about the bay at the Feninsulas of San Francisco and Sausalito where it is subdivided inte its different formations. The undffferentiated sandstone is however characteristic at San Pablo Point,Mount Diablo,and various other places in the Bay region. The Franciscan series constitute a part of the Central Coast Ranges,the geology of which is varying and complex. Description of the Central Coast Ranges. The Central Coast Ranges are made up of various types of igneous and sedimentary rocks and they serve as an illustration of the principles of deposition, erosion and warping and of both igneous intrusion and extrusion. The lowest formetion we know is a series of ged- iments making up a basement complex,including quartzites, limestones and schists,intruded by granite rocks, such as the Montara granite of the Sante Cruz Range. The Franciscan series rests on the eroded surface of thesz older igneous and metamorphic rocks and is composed of igneous and sedimentary rocks.The sedimentary rocks include sandstones of various textumes,radio- larien cherts,shales and foraminiferal limestones in- timately mixed.Intruded into the thick series of sed- imentary strata are peridotite and spheroidal basalt, the peridotite being today largely serpentinized. Unconformably on the Franciscan are the Cret- aceous sandstones and shales of the Shasta Chico formations.The rocks comprise a very thick series, much of which has been eroded,having however & varying thickness of from one thousand feet to several miles. Above the Cretaceous are Martinez and Tejon sand- stones and shales of Eocene age being much less dis- turbed than the Cretaceous sendstones and shales. The only Oligocene in this region is the Agasoma CGravida zone found and described by Dr.B.L.Clark at Walnut Creek,lying unconformably upon the Tejon.It is however of but slight extent in comparison to other Tertiary formations. Locally upon the Cligocene but in general upon the Tocene or even series of lower periods lies the Monteray group of the Miocene.This group is characterized by its alternating sandstones and bituminous shales, the latter being of great economic value by reason of being the source of the oil found in California. Above the Monterey are the Flioccene sediments the Merced and Orindan,sandstones,shales and conglomerates and above these a series of acidic and basic lavas. The Franciscan Crour as & "hole. The only part of this series whith which we are coneerned is the Franciscan Crouvn.This group as has been stated includes sandstone,radiolarian chert,a few lenses of limestone containing foraminifera,and & number of igneous rocks,such as basalt,diabase and peridotite and a number of schists. The sandstones are divided by the Ingelside and Sausalito radjiolarian cherts into three horizons, the upper,middle and lower corresponding to the names of Donita,lMarin and Cahil sandstones.These names have been derived from the locality of which these formations are most characteristic;the Bonita from Foint Bonita; the Marin from the lMearin Peninsula:and the Cahil from the Cahil ridge in San lateo. The cherts are closely associated with the sand- stones in many places lying conformably bedded between them, but throughout the larger areas of sandstone are found only as lenses. The igneous rocks are intruded in irregular masses throughout the whole ridge,being in close connection with both the sandstones and the shales. The schists are of various types and are formed by the action of igneous intrusions and by local dynamic action on the other members of the group.By far the most important schist is the glaucophane schist.It is found generally in small areas but the areas are ( very numberous throuphout the Coast Ranges.At Humboldt, in railroad cuts,the glauconhane stands out as deep blue cliffs.The glaucophane is a blue amphibole con- taining a soda molecule.It is associated with basalt, ( diabase and serpentine in the Coast Ranges and is the result of metamorphism on cherts,sandstones,shales . or igneous rocks. The principal sediments of the Franciscan are of course three sandstone members which are separated by the two chert formations,but in only two localities are all five members present.0On the Feninsula of san Francisco and Sausalito the cherts form a definite division between the sandstone so that the three sand- stone members of the series can be accurately separated and named,but at other localities where only lower, intermediate or upper members of the series are present, it is only with difficulty that they can be determined as to their proper horizon. General Description of the Franciscan Sandstones. The sandstones in gemeral so similar in composition © nd texture that there is no basis for differentiation. They are characterized by their massive aspect due.both to the thickness of the beds and the obscurity of the bedding planes. The sandstones are characteristically hard and well ( cemented and where fresh are greenish or blue gray in color due to the chlorite and other colored minerals —l ® including bits of shale which are present throughout the sandstones,though not always prominent in hand spsclrens. Mars the sandstones ares weathered they present & brown face and are easily broken down.The brown color is in part due to the oxidation of the iron of the ferro- magnesian minerals,only a slight amount of which serves as a strong coloring agent.Put both a part of the codor and the relative softness of the rock is due to a kaclinization of the feldspars. The Franciscan sandstone yields an abundant rich soil due to sharp changes of season and lack of vegitation; the soil being richer than soil derived from sandstones of later geolcgical age.This rich heavy soil is one of the causes of earth slumps which are common in the Coast Ranges,and which are commonly supvossd to be caused by minor faulting.The depth of weathering varies in diferent 1localities,but on the average is about fifteen feet. The cementinc material of the sandstone is silica consistine of the recrystallization of the finer ® material which goes to make up the sandstone.Several chemical tests were mede on each member of the series, and in no case was a carbonate found to be cementing meterial,although in many localities secondary stringers and veins of calcite,datolite,pectolite and quartz are found in more or less fractured zone of weathering.The only section where carbonate is known to occur in the sandstone is at Mt.St.Helena,and in this case it is aE N a secondary mineral dye to a close association with intrusive serpentine body. The secondary veining in the shattersd zone is found well exemplified in several localities. . a) . de In the Marin sandstone,to thes west of Fort Point, J there is a lods of datolite and pectolite partly filling a fractured zone along the shore.In the undifferentiated sandstone at Point Richmond there is an exposure of massive highly indurated sandstone altered to a denth of about twenty feet.It is highly fractured,being broken into large blocks and these are fhiirther broken. The entire face presents a shattered appearance as if formed by differential movement,but as the fresh gray sandstone shows no parting and as the fracturing increases rapidly toward the surface,we can almost positively say,they are due to weathering.In all these fractures are revnlacing veins of silica varying from a fraction of an inch to two and a half inches wide. There are also few evidences of stratification. Redding usually is apparent more than intercalation of beds or lenses of shale rather than any differentiation of the sandstone itself.We do however find lenses ofc conglomerate in many parts of the series throughout the region. Another characteristic phenonemon found to a greater or less degree in the two upper members of the group, but more noticably in the upper,or Benita,sandstone is the abundance of carbonaceous shale and mud balls in more or less irregular frarments in the sandstone. Also distinct seams of hard coal along with innumerable smaller particles of carbon are found in both saerdstones. The lower sandstone undoubt @ dly contains some carbon with the shale,but in all the shale analyzed there was not found enough carbon to give a test with niter. The sandstone members of the Franciscan series are characteristically different from sandstones of later age,but in the constituents of the sandstone and the nature and form of the clastic particles. The sandstones ar wm heavy hedded and are also what are called "heavy sandstones" ,that is they contain the heavy primary minerals,such as magnetite,zircon,rutile, titanite and occasionally tourmaline. trographgic Desciiption of the Sandstones. From a field and petrographic study,the similarity of the differentiated and undifferentiated sandstones is readily seen.fxcevt locally they ars hard well in- durated sandstone.They are of medium grain in general, though much fine grained rock is seen,and are pre- vailingly gray or greenish gray in color.0Oneof the most characteristic features is the angularity of grain of the constitusnts.This applies ir general to all,but in particular to the quartz and feldspar grains. These are in nearly every case sharply defined and ex- cepting where highly altered show only slightly rounded corners.Another prominent feature is the rel- ative abundance of feldspar in the rock.While quartz -ry. is of course the dominant constituent,feldspar is very prominent and in some slide examined, seems to make up at least a third of the rock. The sandstone is composed of very fine fragments grading up to the medium grain of the rock in general and these finer particles of quartz,sometimes stained with chlorifle and in many cases cryontocrystalline,have been recrystallized to form the cement which sc firmly binds the rock together.After a careful study of all thin sections no recrystallization of the larger quartz grains was found.In Pulletin No.8 B.S.G.S.Irving and Van Hise describe a recrystallization of quartz grains by a secondary growth or an enlargement of the primary AS rains.Their treatise deals primarily with quartz- and quartziferous schists and they attempt to prove that almost all the ancient quartzites are composed in the main of the original fragmenteal matsrial,un- altered,but cemented torcether by a siliceous cement, of secondary origin,which is aprarently a secondary growth of the primary grains of quartz.This siliceous cement forms the only part of the rock that has cry- stallized "in situ” and its intricate interlocking and optical continuity with the original quartz frag- ments gives rise to a decentive apnearance of complete original crystallization.They describe rolled grains which are enveloped in secondary crystal faceted quartz ccatings,and again,specimens in which the outline of the original grains of sand are perfectly distinct, and the cavities between them filled with clear quartz in crystalline continuity with the contiguous grains, so that the whole is a mass of interfering crystals each having a sand grain as a nmacleus. In the Franciscan sandstones the finer particles are undoubtably recrystallized and fill in betwsen grains of quartz,feldsrar and other minerals but there is no secondary enlargement of the grains as described in Bulletin XNo.=R. The feldspar grains comprise both the potash and the plagioclase,but of the plagioclase all are acid none being more basic than andesine.All are highly altered even in the fresh sandstone presenting a rough clouded surface and some fragments are entirely changed to sericite and kaolin. Another notable feature of ths Franciscan sandstone is the abundance of volcanic material. Fragments of igneous rock are present in all rock examined. They are generally altered and show no defimite structure but in many fragments fine tabular crystals of feldspar are seen and some show perfect diabasic structure. Biotite,hornblende,chlorite and locally augite and epidote are very common.Epidote is not prominent but is found with enough persistence to characterize it as a feature of the sandstones in general. Shale is also a persistent feature of the series being present to scme extent in all slides examined,and * c+ he medium grained hand in general is readily seen in color. specimens.The shale varies in size from microscopic | In thin section they are made up of clear trans- fragments up to lenses several feet in length.Some | parent areas,free of iron oxide and imbedded in an lenses of a few inches extent are perfectly banded opaque matrix which in reflected light is red.The in dull colors.All are angular and prominent in the | transparent areas ars the remains of rediolaria and sandstones by reason of being larger in size than show the outlines and sometimes the structure very ( the other constituents with which they are associated. ’ well.The transparent areas consist of clear chalcedony Carbon is characteristic in the seriss,both as and the mstri® is bhiefly fine grained chalcedony pure carbon and as a mixture in the shale.None has been gtained with irom owxide.The variation in color is due definitely determined,by the author,in the Canil tc the oxidation of the iron oxide. sandstone but it may be present.It can not be different- Relation of Chert,Shale and Sandstone. iated in thin section when intermixed with the shale The relation of these cherts to the Franciscan and several tests were made with fragments of shale sandstones is not always clear but in most cases the from different sandstone members.There was sufficient sandstone is separated from the cherts by a few feet ( carbon in some of the shdle examined in both the 7 of shale.The sandstone ends abruptly giving place ; Bonita and Marin sandstones to give a definite test to a shale which becomes finer grained toward the with niter,but none was ascertained in the lower member. cherts.This shale featurs is not common but ths Magnetite is present in all the sandstones,being reason may be readily seen. very prominent locally,and is accompanied by crystals The shale is naturally soft and nom-resistant and and fragments of zircon and titanite. : being between two harder “ormations is easily and Description of ths Franciscan Cherts. rapidly weathered. The Ingkeside and Sausalito cherts separate the Cf the few contacts ssen there is not sufficient sandstones into the three formations.They are prevailingly evidence to determine the relation between the sand- ( a dark brick red,but locally vary from green to yellow, stone and the chert.There is a fault contact in the wnite and black.Shale is interbedded with the cherts Marin sandstone on the south side of the Golden Gate conformable ; and is usually interbedded with them.The shalz is colored and two contacts in which the shale is a gradient the same as the chert with which it is interbedded. | between the chert and the sandstone as at Fort Baker ( There igneous bodies have been intruded against the and at Twin Feaks. chert the latter is shattered and very brilliant in =10~ General Description of the Bonita Sandstone. The upper or Bonita sandstone is found about the bay at Point Bonita and at Bernal Heights,San Fran- cisco,although some of the undifferentiated sandstone which makes up the bulk of the area mapned by Prof.A.C.Lawson in the San Francisco folio may be the upper member of the series. The type sention is at Point Bonite,where with the other members of the Franciscan series it appears to din rather steeply to the south and south east.This | member while in general strongly indurated seems locally to be more friable than other members.It appears fresh but its readiness to crumble mey be due to weathering and may be due in part to the lerge con- tent of shale. It is well exposed in a cliff on the east side of the point directly north of the tunnel.It is massive and gray in color,and contains an abundance of shale fragments.The mass of rock appears to be shot through with the shale in particles grading from one milli- meter up 40 2 half inch in diameter.They are all an- gular and exceptionally are very large.Fragments corresponding to Crabau's description of "clay gauls" are found.Cneof these rectangular blocks being twelve by seven by four inches (12"x7"x4") in di- mension.Associated with the shale are minute particles of carbon,casts of wood and small seams of crystallized hard coal.llany of these carbon inclusions are thin and elongated suggesting the structure of the original “Ow plant,but fracturinc and pressure have erased all evidence by which they might be determined.Cne piece of carbonized wood showing the structure and also the structure of a knot in the center was found.The specimen is four by one and a half inches (4"x1.5") and shows definitely the origin of the deformed flake of carbon found in place. ) These smaller features are associated with lenses, and seams of coal,none having any defini‘e plant remains and all showing evidences of considerable pressure.This same phenomenon is well represented in the sandstone of Humboldt and one readily recognizes an important field for future research work,for the abundance of carbon at least suggests the possibility of other fossil remains. Petrographical Desctiption of the Benita Sandstone. Two thin sections of the Bonita sandstone were ex- amined under the microscope and found to be very much alike,It is both a friable and an indurated sandstone but the mineral constituents are much the same. The sandstone is from fine to medium grain and varies from gray to greenish in color.It consists chiefly of quartz and feldspar but fragments of other minerals and rocks are prominent.The quartz is decidedly sharr and angular and stands out prominently from the other minerals,on account of their weathered appearance. The larger frarments of mineral are surrounded by a Q ryptocrystalline mass made up of the recrystallization 13. of the finer particles of arts and is sometimes slightly colored by chlorite.The feldspars are renpresented by orthoclase,microcline and acid plagioclase.All are censideradbly altered, presenting a cloudy surface and some are completely changed to kaolin,being gray and amornhous.The orthoclase is determined by the carlsbad twin and the index of refraction.lMicrocline is rare but the characteristic gridiron structure is seen in a few rlates.The plarioclase ape all acid and probably vary rom albite to olpgoclase but they could not be sep- arated by measurement. Another feature is the irregular bits of chlorite and serpentine.The serpentine can be readily seen in round and angular fragments in the hand specimen and show a fibrous structure in thin section.Some of these sernentine fragments are considerably larger than the grain of the rock,they are non pleochroic and have a low birefrigence.The chlorite is not so prominent as the serventine and seems to be more like a stain than a definite mineral.Its pleochroism and its similarity to flakes of biotite would indicate that it is simply altered flakes of biotite.They are dull Sreen in color, both pleochroic and non pleochroic and frequently show the anomolous interference color,a deep blue. Fragments of shale with metamorphosed and re- crystallized sediments ars very characteristic.They vary in color from brown to coal black and are either spotted or solid color.Many of the fragments show minute particles of secondary quartz disseminated through them. -14- They are both angular and rounded and are invariably larger than the associated rock fragments.Some of the fragments are probably pure carbon as is readily seen in hand specimens,but it impossible to differentiate between them in thin section. Hornblende is present in foliated cryssals showing good cleavage but being more or less twisted.It is dark green incolor,strongly pleochroic and has low birefringence.fpidote is more commonly seen than the hornblende in well defined angular and rounded crystals.It is a bright yellow green,shows the char- acteristic greenish to light yellow absorption and hich flecked interference colors.It is generally in small crystals and seems to be primary.Muscovite is more nrominent in the thin section hhan the brown mica,although brown mica is present in altered flakes and is probably represented by much of the chlorite. The muscovite is in elongated plates showing cleabage and is extremely foliated.It feathers out at the énds and shows high flecked intereference colors.A very characteristic component is a green volcanic rock in small and large,angular and subangular fragments, whizh are very prominent in both thin sections.They vary in shade of color,are somewhat altered and occas- ionally show well developed mintite lath feldspars.Some fragments show a perfect diabasic cleavage.Foliated and folded flakes of schist are seen in fine laminae, foliated and twisted. £3 The Marin Angular fragments of chert are common, Some are recrystallized and show a structure similar to a very fine quartrzite.Some are ~ryptocrystalline and opaque. Magnetite and zircon are present ,being not as prominent in these slides as in some others.The nag- netite is in rounded fragments;but the zircon is el- ongated or basal.They are sparsely disseminated through the slide and show the characteristic high index of refraction and birefringence. Cne fragment of titanite was found.It was light colored,slightly pleochroic,highly fractured and of very high inde" x. Sandstone. D Ll The middle or Marin sandstone is exposed on the Peninsula of Sausalito from Tennessee Cove to Lime Foint,one of the best exposures being on the cliff and stack just north of Lime Point.At San Francisco it can be followed through the Fresidio and from Fort Point west alcng the shore.Here it is associated with the serpentine and in some pnlaces,chert.At Sausalito it lies chiefly between the two horizons of chert,thouch is occasionally in contact with the basalt. After evamining several thin sections the only difference in the sandstone,from the different local- ities seems to be the size of grain and relative prom- inence of shale fragments.Coal is found along the “1G. coast west of Fort Point and is apparently as character- istic of this formation as of the Bonita.At all exposures the rock is greenish gray and variss from a very fine grabbed tc medium grained sandstone.The sandstone con- tains shale fragments in ahundance.In the very fine grained rock the shale is of the same grain as the grain of the other constituents,but in the coarser sandstone the shale fracments are the largest con- stituents,and stand out prominently both in thin section and hand specimen. This formaticn like all others of the Franciscan group consists essentially of angular fragments of quartz and feldsvar.The quartz is sharp,angular, well defined in crystal grain and is the dominant constituent of the sandstone.The feldspars are an- gular,and altered,presenting a rough,clouded surface in thin section.Some are altered to a secondary zone of sericite encircling the feldspar grains.They in- clude both an acid plagioclase and orthoclase. A third prominent component is the black shale. This shale varies in form from rounded fragments to prismatic and elongated fragments.They are subangular, having the corners worn off,They are coal black,gray and brown in color and probably contain considerable carbon. Some are recrystallized for they contain minute inclusions of secondary quartz. Biotite and muscovite are present in varying provortions in different thin sections.The flakes showing cleavage are foliated and feather out at the ends.The -17- ££) relatively large amount of chlorite and its similarity in form to biotite suggests that biotite was at one time the dominant mineral and has been altered to chlorite before or after transportaticn.Chlorite is abundant in various forms,pleochroic or non-pleochroic depending on the section,colorless or green and frequently shows the shomolous dark blue interference color. Augite is present in almost colorless fragments, somewhat altered,and of medium birefringence.Hornblende is more prominent in dark green crystals,some foliated, | all pleochroic and of medium birefringence.Fragments | of igneous rock are present,greenish,altered and in some cases opaqus.In some,minute feldspar 2rystals can be s=en. Magnetite is hard to differentiat from the shale, but is present.Pyrite is seen in a few plates.Under high power lenses the plates appear to be studded with minute cubes,showing dull yellow in reflected light. Zircon and titanite are both present.Both have a high index,are colorless and have high birefringence. Titanite is in larger crystals and is considerably fractures but the zircons are the most numberous.Per- fect hexagonal cpystals terminated on one end are frequently seen.Some granular fragments of rutile are seen,reddish brown in color,with very high index and birefringence.The shale fragments are by far the most prominent in size of the rock constituents,they stand out prominently in both thin section and in the hand specimen. y Cahil Sandstone. The lower or Cahil sandstone is exposed at Sausalito but is much more prominent south of San Francisco at the San Bruno Mountains and west of San Mateo running in a general way parallel to the San Andreas fault rift. An interesting feature of this formation is to be seen at Casey's Quarry about two miles southeast of San Mateo. The sandstone is massive and shows little evidence of bedding as a whole.At the surfacs it is covered. with a deep soil and has been weathered down about ten feet,fop through this distance it is highly fractured and is filled with secondary veining.About thirty feet from the surface,definite bedding can be seen in what at first appears to be very fine fracture lines.Cn close csxamination the sandstone for a few feet is banded with magnetite.In some places there is a band of magnetite a quarter of an inch wide,almost ‘pure magnetite but it is generally shown by a con- centration of magnetite in with the other constituents along certain definite lines. A thin section from San lMateo,Sausalito and of the banded section was examined.From San Mateo a greenish gray medium grained rock was examined.It aponeared to be essentially large fragments of quartz, feldspar and volcanic materiegl,with less abundant minerals imbedded in a matrix of finer fragments of feldspar or quartz and all cemented by an opaque re- crystallized paste of silica.Frgaments large and small ~10- p— | are angular and are surrounded by a considerable amount of this opaque cement. | Fragments of recrystallized chert are seen having the apnearance of a fine quartzite.The igneous material | is in greenish and brownish irregular fragments highly ltered and cannot be determined.Inclusions of per- fectly terminated and twinning crystals of apatite are common in the quartz grains,and are evidently primary minerals of the origingl igneous rock. Hornblende and biotite ane prominent in the slide both being somewhat altered andfoliated due to lateral stress.Chlorite is abundant,showing characteristic color,pleochrism and anomolous interference colors. It resembles biotite in form and optical properties and is probably directly altered from the brown mica. Small angular fragments of shale are prominent in thin section but are not plainly seen in hand specimen. Muscovite is present but not common.lMagnetite and vyrite are also rare.Titanite and zircon are rather prominent showing the high index and birefringence. The zircons are much smaller but are more abundant. ‘The titanite is considerably fractured. The lower sardstone from Sausalito is very similar. The constituents are characteristically angular,the | smaller fragments fillings in about the larger and all ‘strongly indurated.Quartz frequently shows good cry- stal form and apatite inclusions are common.The feld- spars are represented by acid plagioclase,orthoclase and microcline.Twining is common and the microcline exhibits the characteristic structure. The rock contains a remarkable amount of horn- blende and biotite,crushed and folded but only slightly 83tered and retaining their pleochréssm and moderate birefringence.Chlorite is abundant in the form of altered biotite and other minerals.Muscovite is rare but is seen in long cleavage s=ctions feathering out at the ends and showing flecked interference colors, Zircons are present but not abundant.Shale fragments are prominent and stand out in thin section if not in the hand svecimen.Fragments of igneous rock and chert are very common.Much of the chert is greenish and opcque,some is recrystallized.The igneous fragments are weathered,greenish in color and in some cases have no definite boundaries.In some,minute and el- ongated feldspmars can be seen. The sandstones are of medium grain but in thin section they have the apvearance of a few large grains set into a matrix of finer fragments and cement.Part of the igneous fragments have colored the cement and in fact one can hardly define the boundaries of these fragments.In some areas of the slide the large grains seem to be adjusted with a fine line of cement be- tween but in other parts this cement,of recrystallized finer materssl ,opaque and slightly stained,seems to be the bhief constituent with only an occasional fragment.All the tin sections present a cloudy ap- -S1~ (a | Special pearance due to the alteration of the feldspars and the dark opaque fragments of igneous material and the other colored constituents. features of the Cahil Sandstone. The special feature of this sandstone at San lateo is the magnetite banded sandstone of Casey's Quarry. This sandstone while similar in a few essentials to other "ranciscan sandstones is extraoddinary in its differences.It is a very heavy magnetite rock and contains many of the minerals which are associated with marnetite in heavy sands.Although the Franciscan sandstones 2s a whole are prominent in ferro-magnesian minerals this particular phase contains an extraordinary amount o® biotite,epidote,augite,hornblende and chlorite, while the rock is essentially quartz with feldspar. At first glance in thin section it appears to be one half made up of these dark and light green. minerals closely associated with the magnetite. The magnetite occurs in bands from one millimeter to one quarter of an inch in thickness. Microscopically the bands appear to be entirely magnetite and the grains thickly coat a knife scratched alon~ their surface.But microscopically the magnetite occurs in rounded and elongated grains,closely ass- ociated in bands and elongated in the genepal direction of bedding.And it is clesely associated with these bands of magnetite that we have the minerals zircon and titanite as well es a concentration of augite,chlorite and epidote.So that within the limits of this banding the quartz and feldspar are in as great a minority,in relative pronortions,as the grains of marnetite,titanite and zircon are through out. The sandstone is characteristically heavy bedded, well indurated,essentially quarts and feldspar,and the minerals mentioned above,all angular or subangular in form.It is rather fine grained and uniform,the quartz and feldspar being in small but decidedly angular fragments.The feldsvars are represented by acid plagio- clase and orthoclase all altered and presenting a rough,clouded surface.In some cases the alteration has gone over completely to sericite and zones of sericite can be seen about the fgéddspar plates.Biotite is present in thin basal and in pleochroic,faddded and altered cleavage sections.Both ths micas show considerable lateral stress and are severely folded and twisted.Fragments of chert are common in verying size.Epidote and augite are exceptionally abundant and in some cases hard to differentiate.They occur throughout the entire rock but are thicker and more prominent associated with the magnetite.The augite is high colored,non pleochroic and can most readily be distinguished from the epidote by the cheractcristic yellow-green absorption and peculiar flecked inter- ference colors of the latter. Chlorite is abundant throughout the rock, some plainly showing by its form an alteration of biotite. -33. Titanite and zircon are present,the titanite being grained rock and is well indurated.lLenses afid bowlders by far the most abundant.The titanite is reddish of conglomerate and shale are a prominent feature.At brown,very light in color,strongly pleochroic,in the upper end of Flk Valley,lfarin County,a large shade of color.and in some sections is highly bowlder of conglomerate was found,crntaining pebbles <