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Cornell University Library 
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 The geology of the Isles of Scilly 
 
 3 1924 004 542 365 
 
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MEMOIRS OF THE GEOLOGICAL SURVEY. 
 
 ENGLAND AND WALES. 
 EXPLANATION OF SHEETS 357 AND 360. 
 
 THE GEOLOGY 
 
 OF THE 
 
 ISLES OF SCILLY. 
 
 BY 
 
 GEORGE BAEEOW, F.G.S. 
 
 WITH PETROLOGICAL CONTRIBUTIONS BY J. S. FLETT, M.A., D.Sc. 
 
 PUBLISHED BY ORDER OF THE LORDS COMMISSIONERS OF HIS MAJESTY'S TREASURY. 
 
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Geology of Isles of Scilh/. 
 
 Plate I. — Frontispiece. 
 
MEMOIRS OF THE GEOLOGICAL SURVEY. 
 
 ENGLAND AND WALES. 
 EXPLANATION OF SHEETS 357 AND 360. 
 
 THE GEOLOGY 
 
 OF THE 
 
 ISLES OF SCILLY. 
 
 BY 
 
 GEORGE BAEKOW, F.G.S. 
 
 WITH PETROLOGICAL CONTRIBUTIONS BY J. S. FLETT, M.A., D.Sc. 
 
 PUBLISHED BY ORDER OP THE LORDS COMMISSIONERS OF HIS MAJESTY'S TREASURY. 
 
 LONDON. 
 
 PRINTED FOR HIS MAJESTY S STATIONERY OFFICE, 
 
 By WYMAN & SONS, Limited, Fetter Lane, E.C. 
 
 And to be purchased from 
 
 E. STANFORD, 12, 13, and 14, Long Acre, London ; 
 
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 HODGES, FIGGIS & Co., Grafton Street, Dublin; 
 
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CONTENTS. 
 
 Preface by the Dieectoe 
 
 Chaptee I.— Introduction 
 
 Chapter II.— Granite and Associated Rocks 
 
 Chapter III. — Drift 
 
 Chapter IV. — Kecent Movements 
 
 Page 
 iii 
 
 1 
 
 4 
 15 
 32 
 
 Appendix. — Bibliogeaphy. List of Principal Works on the Geology 
 
 of the District - - 35 
 
 Index 36 
 
 ILLUSTRATIONS. 
 
 Plate I. — Submarine Contours around Scilly 
 Plate II. — View taken near Old Town, St. Marys 
 
 Plate III. — Kettle and Pans, near Peninnis Head, St. 
 
 Marys 
 
 Plate IV.— West side of Peninnis Head 
 
 Plate V. —Bread and Cheese Cove, west of St. Martins 
 Head < 
 
 Plate VI— Gully at the south end of White Island, 
 St. Martins 
 
 Plate VII.— South-west end of St. Martins 
 
 Frontispiece. 
 
 At 
 end of 
 Book. 
 
( iii ) 
 
 PREFACE. 
 
 This Memoir deals with the area represented by the colour- 
 printed map of the Scilly Isles (Sheets 357 and 360). If we except 
 certain scattered references in De la Beche's Report on the Geology 
 of Cornwall, Devon and West Somerset, no description of these 
 islands has been published by the Geological Survey. The most 
 complete account hitherto available is contained in an important 
 paper by Mr. Joseph Carne, to which reference is made in the 
 following pages. 
 
 The detailed survey of the islands was entrusted to Mr. Barrow 
 under the superintendence of Mr. Reid, and the work was carried 
 out during the years 1903 and 1904. A few points of novelty 
 have been brought to light, the most important being the recogni- 
 tion of greisen-bands in the granite, the discovery of a narrow 
 strip of tourmalinized killas at the north end of White Island, 
 the occurrence of ice-scratched stones and the presence of a some- 
 what doubtful outlier of Eocene gravel. 
 
 The petrographical descriptions of the roches are new and have 
 been either contributed or revised by Dr. Flett. Mr. Reid has 
 acted as general editor. 
 
 J. J. H. TEALL, 
 
 Director. 
 
 Geological Survey Offices, 
 28, Jermyn Street. 
 
 21st February, 1906. 
 
 8675. 500.— Wt. $623. 8/06. Wy. & S. 3835?-. 
 
THE GEOLOGY 
 
 OF THE 
 
 ISLES OF SCILLY. 
 
 CHAPTER I. 
 INTRODUCTION. 
 
 The Scilly Isles, which lie some thirty miles south-west of the 
 Land's End, may be described as a series of flat-topped granite 
 masses varying greatly in size and in the height to which they 
 rise above the water. Altogether about 140 rocky islets and 
 rocks have been counted, but only five are now inhabited. The 
 largest, St. Marys, is, roughly, two miles long and 160 feet in height; 
 and from this there is every gradation to a mere speck of rock 
 visible at low-water. The larger islands are at the north-east 
 end of the gifoup, and the sea between them is quite shallow ; but 
 this enclosed, or, as it may be conveniently termed, " interior- 
 sea " steadily deepens towards the small isolated rocks at the 
 south-west end. A chart soon makes it clear that this deepening 
 marks the position of a submerged inland valley, and that the 
 present distribution of land and water results from the form of 
 ground left by inland denudation, before the area subsided to its 
 present level. If the submarine slopes be contoured to show the 
 level of 200 feet below the sea (see Plate I.), then it seems 
 further evident that the Islands result from the erosion and 
 partial submergence of a continuous oval mass of granite similar 
 to those of the mainland of Cornwall and Devon ; as was long 
 since suggested by De la Beche. * 
 
 * " Report on the Geology of Cornwall, Devon and West Somerset," 1839, 
 pp. 9, 156, etc. 
 
 8675. A 
 
2 GEOLOGY OF ISLES OF SCILLY. 
 
 The encircling ring of altered stratified rocks has been cut down 
 and is now submerged. In one place only has any of the altered 
 Killas been so far met with ; nevertheless, it must exist under water, 
 and on the north-east end of the group it cannot be far from the 
 present termination of the granite. At the southern end it is 
 probably much farther out to sea. 
 
 The material formed by the denudation of the granite is an im- 
 portant component of some of the islands. This granite waste 
 takes two forms, angular Head, and Blown Sand. The latter is 
 by far the more important so far as bulk is concerned, and has often 
 played the curious rdle of uniting separate granite masses to form 
 one island or continuous land-surface ; indeed, this phenomenon is 
 one of the features of the Scilly Isles. 
 
 The climate of the Scilly Isles is essentially mild, being cooler in 
 summer and warmer in winter than that of the adjacent mainland ; 
 even sub-tropical plants grow well in sheltered places, not so much 
 from the warmth of the climate as from the absence of cold in 
 winter. The finest examples of these exotic plants are seen in the 
 gardens of Tresco Abbey. 
 
 The rainfall is only some 32 inches, much less than on the main- 
 land of Cornwall ; but a considerable amount of moisture is slowly 
 deposited on the land at certain seasons owing to sea-fogs produced 
 by the action of cold air-currents passing over the warm surface of 
 the Gulf-stream. 
 
 The industries of the islands are chiefly horticulture and agricul- 
 ture ; much of the fishing being carried on by vessels coming from 
 a distance and benefiting the islands to only a small extent. The 
 most remunerative employment is that of growing early spring 
 flowers and vegetables ; and, later, tomatoes and grapes, both of 
 which can, be produced without the aid of artificial heat. 
 
 Agriculture (including dairy-farming), is carried on to a small 
 extent, but, except in the case of fresh milk and possibly potatoes, 
 the products are insufficient to supply the wants of the population 
 in summer, though this is never large. The resident population ' 
 in 1851 was 2,601 ; in 1901 it was 2,092. 
 
 Although the Islands are composed of granite, the rock is not 
 quarried to any great extent and is never worked for exportation. 
 The quarries that do occur are in comparatively soft material, and 
 there are few accessible places where any rock sound enough to 
 pay for working could be met with without first removing a 
 considerable amount of more or less decomposed granite. The 
 comparatively soft rock often forms a fairly good building stone, 
 it undoubtedly contains much soluble silica. When the rock is 
 placed in such a position that it can dry thoroughly, as in the walls 
 of buildings, it hardens as the silica becomes insoluble, and is prac- 
 tically impervious to water. 
 
 The water supply of the Islands is obtained in two ways, from 
 wells and directly from rain collected on the roofs and stored in 
 
INTRODUCTION". 3 
 
 underground tanks. On the whole, the larger quantity is obtained 
 in the latter manner. With the exception of a small runnel fed by 
 a spring at Holy Vale, on St. Marys, there are no streams of any 
 kind in the islands.* One strange point about the wells is that a 
 fair number are comparatively close to the sea and are sunk a little 
 below sea-level. The water in these rises and falls to some extent 
 with the tide, yet the water itself seems quite fresh. The sea simply 
 keeps in the fresh water that is working through the granite to- 
 wards sea-level. The quantity drawn at any one time from such 
 wells is of course comparatively small, or the salt water would find 
 its way in. On the small island of St. Helens, near the old 
 Quarantine House, there is an ancient well, the bottom of which is 
 some depth below sea-level ; and it is confidently reported that the 
 sea-water does enter in this case, but loses its salt in so doing ! 
 
 The earliest geological descriptions of the Isles, beyond brief 
 notes by R. Heath, were published in the middle of the eighteenth 
 century by the celebrated Cornish naturalist, William Borlase, 
 whose accounts, however, are mainly archaeological. Then, after 
 an interval of sixty years, the Tsles were again briefly alluded to 
 by Majendie, In 1839 Pe la Beche's " Report " contained a few 
 additional particulars ; but it was not till Joseph Came wrote, 
 in 1850, that the geology of the Isles was systematically described. 
 Carhe dealt with the character, structure, and jointing of the 
 granite,, with its disintegration, and with the products of its 
 destruction, . thus laying the foundations of our knowledge on a 
 firm base,, , 
 
 The rpcks represented in the Scilly Isles are as follows :— 
 
 t, r Blown Sand. 
 
 • Recent - j^^. 
 
 (Upper Head. 
 Iron-cement and Glacial-deposit. 
 Lower or Main Head. 
 Old Beach. 
 
 Eocene ? - Gravel of St. Martins. 
 
 Paleozoic Altered Killas. 
 
 T (Elvan or Quartz-porphyry. 
 
 Igneous I i nnerL g ramte (finer). 
 
 Kocks [Outer-granite (coarser). 
 
 * For particulars of some of the old *ells and springs see R. Heath, " A" 
 Natural and Historical Account of the Islands of Scilly," 1750, pp. 77-81. 
 
 8676. A a 
 
CHAPTER II. ; 
 
 GRANITE AND ASSOCIATED ROCKS. 
 
 In the Scilly Isles there are two common types of granite, one 
 coarse-grained with porphyritic crystals of felspar, the other finer 
 and non-porphyritic. The coarse-grained granite is the dominant 
 constituent of the islands, the finer rock being confined to the north 
 and west sides of St. Marys, the south part of Tresco and Bryher, 
 Samson, and the north-west side of Annet. It will be seen that 
 the fine-grained granite occupies the central part of the Scilly area 
 and is everywhere surrounded by the coarser ; probably between 
 the above-named islands a considerable extent of the fine rock is 
 covered by the sea. As a rule there is no sharp boundary between 
 the two varieties of granite, but a gradual passage from the one type 
 to the other. This is well shown on the west side of the north end 
 of St. Marys where, after repeated examination of the ground, it 
 was found impossible to fix any definite line of demarcation, and 
 consequently that shown on the map is more or less arbitrary. In 
 some places, however, the contrast between the two types is 
 strikingly evident, as on the coast to the north-east of Hugh 
 Town, and near New Grimsby in Tresco, but this is exceptional. 
 The fine-grained granite may pass into a coarser rock, while the 
 porphyritic coarse-grained granite, which retains its porphyritic 
 structure over all the southern islands, almost loses this character 
 towards the north end of St. Martins and Tresco, and often assumes 
 a slightly foliated aspect. 
 
 Veins of fine granite are not unfrequently to be observed inter- 
 secting the coarser porphyritic variety ; these will be described in 
 a subsequent part of this chapter, but as they resemble very 
 closely the fine granite of the interior mass, they render it probable 
 that as a whole this is intrusive into the coarser rock.* 
 
 Both granites contain muscovite and biotite, orthoclase and quartz- 
 Tourmaline is more or less sporadic in its distribution, being abundant in 
 some places while elsewhere it is scarce. The porphyritic felspars in the 
 coarse granite have an average length of an inch or more, but do not usually 
 show very perfect crystalline faces. They have generally a marked parallel 
 arrangement which gives the rock a deceptive appearance of bedding. Occa- 
 sionally all the long axes of the crystals have the same orientation ; this is 
 best exemplified in the eastern islands, especially the Great Ganilly. This 
 structure is apparently due to fluxion movements during the slow process of 
 injection, and very frequently the lines of fluxion are undulating, so that the 
 tabular felspars become vertical for a space and then resume their nearly 
 horizontal disposition. Most of the large felspars show Carlsbad twinning 
 clearly to the naked eye. The felspar is nearly always white and opaque, 
 though sometimes yellow or brownish from staining by iron-oxides. Often 
 the crystals, especially the phenocrysts, contain crystals of mica, and these 
 may be arranged in zones parallel to their surfaces. Plagioclase is not 
 
 * See Be la Beche, " Report on the Geology of Cornwall, &c," p. 161, 
 
GRANITE AND ASSOCIATED ROCKS. 5 
 
 abundant and in the typical coarse granite is not easily recognised in the 
 hand specimens, but when the rock is fairly fresh the oligoclase is often more 
 decomposed than the orthoclase and has a yellow or reddish-yellow colour. 
 The quartz varies considerably in amount and is less abundant than the 
 felspar. 
 
 A section of the typical coarse granite from Peninnis Head on the south 
 side of St. Marys consists essentially of orthoclase, plagioclase, quartz, 
 muscovite and biotite. The orthoclase is somewhat turbid with decomposi- 
 tion products, but in polarised light shows Carlsbad twinning and almost 
 invariably also a perthitic structure, as veinlets and minute irregular patches 
 of a plagioclase felspar are scattered abundantly through it. It contains 
 also scales of muscovite and biotite, idiomorphic plagioclase crystals and 
 rounded grains of quartz. Myrmekitic or vermicular micropegmatite often 
 borders the orthoclase and sometimes also the plagioclase. The idiomorphic 
 crystals of plagioclase are frequently zonal and are often much decomposed 
 near their centres. They belong to oligoclase and albite-oligoclase, as can be 
 easily proved by their refractive indices when they contain quartz enclosures. 
 The behaviour of these crystals in cleavage flakes shows that they are not all 
 of the same composition, as some of them are normal oligoclase while others 
 contain more of the albite molecule. The quartz is in every respect that 
 which is typical of granites and occurs as an interstitial material between the 
 felspars, and to a much less extent as rounded or irregular enclosures in the 
 perthitic orthoclase. It occasionally shows effects of crushing, by polarising 
 as a mosaic of interlocking grains which have slightly different positions of 
 extinction. Fluid enclosures are very numerous and give it a dusky aspect. 
 They lie in rude bands or strings and contain a mobile bubble and often also a 
 small cube or rod-shaped microlith. 
 
 The biotite and muscovite tend to occur in clusters and are frequently in 
 parallel growth, the muscovite being then the later of the two minerals. 
 The biotite is very strongly pleochroic with colours ranging from pale brown- 
 ish yellow to dark brown with a reddish tinge. Dark pleochroic halos are 
 very common, and the edges of the crystals have sometimes a deeper colour 
 than the centres. Both micas colour the blowpipe flame a faint red, and on 
 spectroscopic examination yield a strong lithia reaction ; the biotite is con- 
 sequently a lithionite. Its axial angle is always small ; the pale mica is a 
 lithia-bearing muscovite. Occasionally the crystals show idiomorphic six- 
 sided outlines, but generally they have irregular borders, which in the case of 
 the muscovite are often frayed out or feathery. Apatite occurs in rather 
 large grains ; zircon is frequent and small dark hairs of rutile may be present 
 in the quartz. 
 
 Taken as a whole the coarse Scilly granites show little tourmaline 
 whether in the hand specimens or under the microscope, but in 
 some places patches have been met with which are rich in schorl. 
 The true relations of these to the main mass are not always clear, 
 but they may be of the nature of segregations or perhaps of intrusive 
 origin. A good example of this modification occurs at the extreme 
 south end of St. Agnes, and a few other instances have also been 
 met with. 
 
 The granite from the above-mentioned locality carries phenocrysts of 
 orthoclase one and a half inches in length, Carlsbad twinned, lying in a matrix 
 of orthoclase, tourmaline, muscovite and quartz. The black prisms of schorl 
 and pearly crystals of white mica are very obvious in the hand specimen, the 
 mica being often more than a quarter of an inch in diameter. The felspar 
 is white or slightly yellow, and quartz is comparatively scarce. Under the 
 microscope the principal component is orthoclase, often rather coarsely 
 perthitic. Clear, unweathered microcline appears also in a few places, grown 
 upon the surface of the microperthitc and probably filling micrplitic cavities. 
 
6 GEOLOGY OF ISLES OF SCILLY. 
 
 The small amount of quartz visible in the section is mostly scattered through 
 the felspar where it occurs as rounded or angular enclosures. The muscovite 
 is partly idiomorphic, partly in clusters of irregular flakes. It contains small 
 crystals of zircon, surrounded by pale yellow pleochroic halos, but is elsewhere 
 colourless. Though yielding a strong spectroscopic reaction for lithia it is in 
 other respects a normal muscovite with an axial angle about 70 degrees. The 
 tourmaline occurs in grains which have often little trace of crystalline outline, 
 mostly mixed with muscovite and qup-rtz in the interspaces between the 
 felspars. Their colour is very varied, being dark brown, yellow, green and 
 blue, and they are often zonal. Some crystals have pale blue centres passing 
 into yellow and brown at the margins. Others are yellow in the interior but 
 have a dark brown border with sometimes another thin blue zone external to 
 this. Not unfrequently a growth of small acicular prisms surrounds a large 
 compact crystal. Zircon and apatite are both common, the latter in large 
 grains. The complete absence of biotite indicates that tourmaline under 
 some conditions replaces it. 
 
 As already stated there is in most cases a passage from the 
 coarser to the finer granite. A good specimen of the intermediate 
 type was obtained from the bottom of a well sunk on the north 
 side of St. Marys by the edge of the road under the old castle. 
 
 It is a medium grained or rather coarse granite without conspicuous ortho- 
 clase phenocrysts, and showing both muscovite and- biotite in the hand speci- 
 men. In the microscopic section the features which characterise the ground- 
 mass of the Peninnis Head rock (vide supra) are all repeated, but tourmaline 
 occurs in large irregular brownish crystals mostly embedded in quartz, and 
 spotted with green pleochroic. halos around small indeterminable enclosures. 
 The biotite is a dark reddish-brown lithia- mica. with intense pleochroic halos 
 and very marked pleochroism. It is frequently surrounded by muscovite 
 in parallel growth. Plagioclase, usually turbid in the centre, but transparent 
 at the margins, where it has the optical properties of albite (oligoclase-albite) 
 is rather abundant. The orthoclase is very frequently perthitic, but micro- 
 cline is absent. In the tourmaline and sometimes in the muscovite and 
 quartz, large numbers of little acicular microlites are found. Their nature 
 could not be certainly established. 
 
 Typical fine-grained Scilly granite is well seen at the crag at the 
 landing place at the south end of Tresco. The average diameter 
 of its crystals of quartz and felspar is about one-tenth of an inch. 
 Muscovite and biotite are both visible in it, but tourmaline is scarce 
 and only one small crystal of it is present in the microscopic slide. 
 Another specimen from the coast near Porthloo, east of Hugh Town, 
 is still finer grained and contains abundant tourmaline and but 
 little biotite. The finest material occurs at the same locality, on 
 the boundary of the inner granite as shown on the map. Its felspars 
 are white, yellow and pink, and many small prisms of dark schorl 
 are scattered through the rock. 
 
 There is no essential difference in microscopic character between 
 the finer non-porphyritic granites and the common coarse-grained 
 types. They contain more, tourmaline, Which appears to be an 
 original ingredient and takes the place of biotite, as any increase 
 of the former is attended by a diminution in the quantity of the 
 latter. 
 
 The tourmaline is brown or greenish-brown, rarely blue, and though some- 
 times in well-formed prisms is usually irregular in outline, and often pene- 
 trated by, or in graphic intergrowth with quartz. The biotite and muscovite 
 
GRANITE AND ASSOCIATED ROCKS. 7 
 
 are similar to "those of the other granites, the biotite having always the same 
 red-brown colour, intense pleochroism and rounded pleochroio halos. Ortho- 
 cJase, very commonly perthitic, oligoclase and oligoclase-albite are always 
 present, and their relative proportion varies only slightly. Apatite is common 
 in large grains and zircon is never wanting, but iron oxides are very scarce. 
 Topaz has not been observed in any of these rocks. 
 
 Inclusions and Intrusions in the Granite. 
 
 The best locality for obtaining a clue to the minor details of the 
 granite is the shore of Great Ganilly,, one of the eastern isles. 
 Here four separate phenomena may be noted. (1) The occurrence 
 of a number of very small oval patches, the origin of which cannot 
 be determined satisfactorily by field evidence alone. (2) A great 
 number of anastomosing fine granite veins, clearly intrusive. (3) 
 Fairly numerous small, rather lenticular pegmatitic or coarse 
 granite patches and veins, clearly intrusive, for they cut the granite 
 veins above mentioned. (4) A number of cracks or fissures, 
 trending east-north-east, often filled with greisen and schorl rock, 
 along the edges of which the granite has been more or less altered. 
 These lines cut 2 and 3. 
 
 (1) The little oval patches are always fine-grained and somewhat 
 greyer than the normal rock, and even in a hand specimen they can 
 be seen to contain a greater proportion of biotite, which occurs as 
 small crystals. So far as their mode of occurrence goes they might 
 be the taper ends of small veins, but they have a distinctly different 
 composition and contain more biotite and plagioclase and less 
 muscovite than the normal granite. They are distinctly less acid 
 and slightly older than the rock which surrounds them and the 
 microscopic section prepared from one of these inclusions was found 
 to contain no tourmaline. The junctions with the coarser rock 
 seem too sharply defined for these to be claimed as mere segrega- 
 tions. Similar oval patches have been met with on the northern 
 and western parts of St. Martins, Tresco and Tean, but they are less 
 common than on the Great Ganilly, On St. Marys and St. Agnes 
 they are distinctly rare, and none were seen on the south side of 
 either of these two islands. 
 
 (2) The anastomosing narrow veins of fine granite are specially 
 abundant along the east side of Great Ganilly. In the majority of 
 cases there seems to be a master- vein from which the smaller ones 
 proceed and at this particular locality the former generally has a 
 north-north-west trend, though this direction is not strictly main- 
 tained in the other islands. The junction of the veins with the 
 coarser granite, though fairly clear, can hardly be called sharp ; they 
 never have fine-grained margins like those of the elvans, and how- 
 ever small, they are always granitic in texture. In many cases 
 the thicker veins have a faintly pinkish tinge due to the dissemina- 
 tion of small crystals of greatly decomposed oligoclase. 
 
8 GEOLOGY OF ISLES OF SCILLY. 
 
 A specimen (4287)* from one of the master-veins on the east side of the 
 island shows this well. In it the orthoolase crystals are fairly large, while 
 the plagioclase occurs in much smaller idiomorphic crystals, the centres of 
 which are greatly decomposed, while the margins are almost glass-clear. 
 Brown and white mica are both fairly abundant and schorl is also present, 
 though in less quantity than in some of the smaller veins. It is yellow and 
 brown, with rounded green pleochroic halos. On the crest of the south-west 
 end of the island, one of these veins swells out locally to a breadth of at least 
 50 feet, and afterwards narrows, clearly throwing out processes as it does so. 
 Along certain portions of the margins of this broader vein patches rich in schorl 
 occur, but no clear evidence of any modificaton of the adjacent granite was 
 observed. 
 
 These veins are common also at the north-west end of St. Martins, 
 on Tean and to a smaller extent on Tresco. A similar material 
 is extraordinarily abundant over the whole southern portion 
 of the west side of Bryher, and it occurs in a rather unusual way. 
 Ramifying sills are especially common, best seen on the projecting 
 headland at the extreme south-west end of the Island. Further 
 north, patches of all sizes and strange shapes are met with, which 
 from their composition must be part of the same material, yet 
 in many instances they have the aspect of an original outer edge of 
 an intrusion. The probable explanation is that a considerable 
 mass of the finer granite lies close below the present surface ; and 
 this view is strengthened by the strong resemblance of this ramify- 
 ing material to the first outcrop of the finer granite, near Porthloo 
 on St. Marys, already referred to, and also by the fact that similar 
 granite veins are rare in the inner granite core, as must obviously 
 be the case if they are essentially the same material. 
 
 A typical example of the granite veins in the northern part of the islands, 
 taken from the north-west end of St. Martins (4286) is composed of ortho- 
 clase, a little plagioclase and small grains of quartz, set in a rather confused 
 matrix of quartz, felspar, white mica, and schorl. The latter occurs in small 
 idiomorphic prisms, highly zonal with yellow, brown, green, and blue colours, 
 and with green or bluish pleochroic halos. No brown mica is present, and 
 this mineral seems always rare or absent in the finer granite veins. In the 
 specially massive portions of the granite, these veins are rare, and the few that 
 do occur are usually of extremely small dimensions. An example of these 
 occurs on the top of the chfls to the east of Old Town, St. Marys. It is a pale- 
 grey to white fine granite, composed of felspar, quartz, a little white mica, and 
 schorl in numerous small acicular crystals ; thus closely resembling the speci- 
 men from St. Martins in composition. , 
 
 (3) As already stated small veins containing felspar, quartz, 
 and schorl, often in large crystals, are fairly common on the east 
 side of Great Granilly ; they are obviously a form of pegmatite 
 veins, though they have usually a straight course. In a few cases 
 they seem to have affected the adjacent granite to a small extent, 
 though there is usually no sharp junction between the two. That 
 they are intrusive is clear, for they cut the fine granite veins. 
 These pegmatites occur over the whole of the Scilly Isles, but are 
 more common on the north and rare to the south. 
 
 • These numbers refer to the Microscopic slides in the Survey collection, 
 
ORANITE AND ASSOCIATED ROCKS. 9 
 
 (4) In Great Ganilly, the largest of the eastern isles, all the 
 rocks previously mentioned are traversed by a series of nearly 
 vertical cracks along which the granite is modified or altered to 
 a varying extent. These cracks may be conveniently called 
 greisen-lines, and by their abundance and the phenomena asso- 
 ciated with them they closely ally the granite of the Scilly Isles 
 to the large masses of the south-west of England, for in no other 
 intrusions in Great Britain do these greisen-lines form such a 
 conspicuous feature. They tend often to occur in groups, and in 
 this case the alteration on their margin is most marked. They 
 have over most of the islands a definite trend, approximately 
 east-north-east and west-south-west, though each individual 
 crack may have a slight local curvature and the members of a 
 group often anastomose. The central crack may at times contain 
 nothing between the bounding walls of altered granite or greisen. 
 More commonly there is a small thickness of quartz and schorl, 
 which, when finely commingled, assumes the bluish aspect of the 
 "peach" of the mainland granites, and of the tin veins. In the Scilly 
 Isles this infilling peach rarely attains the breadth so often met 
 with in Cornwall ; a vein more than six inches wide is quite excep- 
 tional, and the majority of the veins do not exceed an inch, though 
 several of these may occur comparatively close together. These 
 greisen-lines are most numerous and can be best studied on the 
 eastern isles, especially Great Ganilly. They are easily eroded, 
 and the little narrow depressions to which they give rise can be 
 traced across the island. A good example of an anastomosing 
 group occurs in the hollow, covered at high water, between the north 
 end of Great' Ganilly and the little detached islet of Morrow. The 
 belt of granite affected by this group is rather more than four 
 feet wide, but the extent of alteration varies not only in the belt 
 as a whole, but also along the edge of each separate crack. In 
 one instance (4288) the granite is completely altered to a depth 
 of six inches and is a true greisen or quartz -muscovite rock. The 
 white mica is very abundant, and by the unaided eye only this 
 mineral and quartz can be seen, the biotite and tourmaline being 
 very inconspicuous, while the felspar has entirely disappeared. 
 
 In the microscopic section the principal constituent is quartz which con- 
 tains many fluid cavities mostly of irregular form, but a few are negative 
 crystals. White mica in clustered tablets is almost equally common. Much 
 of it is perfectly idiomorphic ; it is for the most part colourless, but is occasion- 
 ally slightly green, and shows small, pale greenish-yellow pleochroic halos 
 round enclosed zircons. Optically it is muscovite with axial angle of from 
 60° to 70° (2E), but it colours the blow-pipe flame very faintly red, and witn a 
 pocket spectroscope yields a strong lithium line. A curious variety of biotite 
 which differs entirely from that of the normal granite is also present. Its 
 colour is brownish-green with marked pleochroism ; it forms hexagonal plates 
 or irregular scales which are sometimes grouped in radiate clusters. Un- 
 doubtedly it is of new formation, and occasionally can be seen to decompose 
 into dark-green chlorite. The slide contains also a small quantity of tour- 
 maline, brown or blue in colour and usually zonal. Its crystals are irregular 
 and are mixed With quartz. Pale blue needles, which become almost colourless 
 
' 10 GEOLOGY OF ISLES OF SCILLY. 
 
 when the nicol is rotated, may also be seen lying in the quartz and mus- 
 covite. Zircon is abundant ; apatite occurs in large rounded grains, but 
 topaz was not found either in the section or in the crushed powder of the 
 . rock. A very few grains of iron oxide may also be detected. 
 
 Along the edge of another crack of the same group, the alteration rarely 
 extends to a depth of more than two inches and the outline of the felspar 
 remains distinct right up to the edge, but the biotite is more difficult to recog- 
 nise. . Much schorl is present, which on being powdered up was mostly brown. 
 No metallic ore was seen. The infilling material varies from quartz with 
 easily recognised schorl needles to compact " peach." 
 
 In the more isolated lines the crack is generally filled with a similar ad- 
 mixture of quartz and schorl ; in some cases quartz alone occurs, in others 
 quartz and schorl mixed together, but each easily recognisable, but the most 
 common is a thin seam of fine "peach." The cheek of the crack is often merely 
 changed to a broan colour, the original minerals remaining recognisable except 
 the biotite, and even of this the outline can generally be traced. Thus 
 though the phenomenon of greisen- lines is just as characteristic as in the main- 
 land, the thickness of the quartz-schorl veinstone is usually far less and the 
 true greisen alteration is not so common. 
 
 Similar groups may be noted in the otter islands, though single 
 lines are far more frequent. A good example of the latter can he 
 seen at the steps of the lifeboat-house, on St. Marys ; while a 
 typical anastomosing group occurs on the north end of the west 
 side of the same island. 
 
 Small veinlets of typical peach and schorl-rock are abundant 
 at the north end of Tresco, about Cromwell's Castle ; and it would 
 appear that some of these must have contained a little tin, for 
 there are many narrow oblong pits on the ground above, clearly, 
 from their trend, dug along these veins. Borlase mentions that 
 in this neighbourhood " we found a row of shallow Tin-pits, none 
 appearing to be more than four fathom deep, most of them no 
 deeper than what the Tinners call Costean Shafts, which are only 
 six or eight feet perpendicular. . . . This course of Tin bears 
 East and West nearly, as our Loads, or Tin Veins, do in Cornwall. 
 These are the only Tin Pits which we saw, or are anywhere to 
 be seen, as we were informed, in these Islands."* The infilling 
 material and its bounding walls have been dug away and the 
 base of the hollows is covered with loose earth, so that the exact 
 nature of the original material cannot now be seen. A little to 
 the south of these hollows on the curious flat-topped tableland 
 small fragments of peach are extremely abundant and there is 
 good reason to suspect that the presence of this material, as well 
 as the occurrence of a small quantity of metallic ore in this locality 
 is in some way connected with the outer margin of the granite ; 
 in fact we may readily believe that the junction of the granite 
 with the altered killas is comparatively close to the extreme 
 northern end of Tresco. 
 
 On the south side of St. Marys and of St. Agnes, where the 
 granite is exceptionally solid, none of these nearly vertical greisen 
 
 * "Observations on the Ancient and Present State of the 1 1slands of 
 Scilly," 1756, pp. 45, 72. 
 
GRANITE AND ASSOCIATED ROCKS. 1 11 
 
 lines have been noted. But they are apparently represented by 
 curious nearly horizontal cracks filled with a thin film of quartz - 
 schorl rock. The alteration along these low-hade planes is usually 
 quite small and no true greisen has been observed. Excellent 
 examples occur on the coast about a quarter of a mile south-east 
 of Old Town, St. Marys, where they are numerous and can 
 be traced for considerable distances in an easterly direction. A. 
 Majendie, writing about the year 1814, mentioned that " A small 
 quantity of tin was raised in St. Marys about twenty years ago."* 
 
 On the south side of St. Agnes one of these planes is coated 
 with schorl needles uniformly orientated over rather large surfaces. 
 There is a good deal of schorl in the adjacent granite here, but 
 the presence of this mineral is not accompanied by any con- 
 siderable development of additional white mica. Unfortunately 
 the rock surface is so obscured by huge boulders that it is difficult 
 . to be sure that this is a modification of the normal granite and 
 not a thin sheet-like local intrusion. In some places the un- 
 doubted normal granite is in contact with this sheet of schorl 
 needles, but the amount of alteration is still singularly small 
 though the granite itself is at times slightly foliated to a depth 
 of several inches. In this vicinity some of these low-hade cracks 
 are filled with compact fine-grained dark-blue peach. 
 
 A specimen of this was sliced and proved to consist mainly of acicular or 
 granular blue tourmaline and quartz. The rock is a breccia in which angular 
 fragments of quartz, pieces of felted blue tourmaline and of a mixture of 
 quartz and tourmaline are cemented together by a much finer grained matrix 
 of the same composition. A vein of quartz, blue tourmaline, and white mica 
 traverses the centre of the specimen, and evidently represents the latest in- 
 filling material. Many very small grains of a brown mineral, which has the 
 appearance of cassiterite, are scattered through the section. Their average 
 diameter is about one-five-hundredth of an inch. A sample of this rock, 
 crushed and tested by Dr. W. Pollard, proved to contain tin. 
 
 North and South Cracks. 
 Clearly later than the lines of greisen-action are a series of cracks 
 or fissure lines trending slightly west of north. They are best seen 
 on the northern shores of the islands where the Atlantic waves are 
 better able to erode the crushed granite on their margins. So far 
 as is known, no intrusions occur along these lines and there is 
 no sign of greisen action along them, from which it may be fairly in- 
 ferred that they are the newest movement lines affecting the granite. 
 They have an important bearing on the configuration of the islands, 
 and on the production of the water-ways into the interior sea. 
 They have an additional importance, however, as, like the greisen- 
 lines they also occur on the mainland and still further serve to link 
 the granite of the Scilly Islands with those of Cornwall. In the 
 latter area, these lines, where they cut the killas, frequently 
 contain veins of lead or iron ore. Unfortunately no such ore has 
 been met with in the granite of the Scilly Islands. The features 
 due to these cracks are illustrated in Plates II. and VI. 
 
 ' 1 ; : : 
 
 * Trans. Royal Qeol. Soc. Cornwatt, vol. i., 1818, p. 31. 
 
12 geology of isles of scillt. 
 
 Weathering of the Granite. 
 
 Attention has been directed by most writers on the Scilly Islands 
 to the effects of weathering on the granite. 
 
 De la Beche, who gave figures of some of the weathered rocks, 
 thus refers to them : — * 
 
 " The rock known as the Giant's Punch Bowl, in St. Agnes, one of 
 the Scilly Islands, will probably exhibit the irregular decomposition 
 of the two blocks of granite piled upon each other as well as any we 
 could select. The upper block has externally a somewhat rounded 
 character, such as has been supposed to arise from the decomposition 
 of an original and concretional structure of the component parts, 
 while the lower exhibits a form as irregular as can be well desired, 
 being decomposed in a notch on one side, and projecting out in 
 an angular manner on the other. The highest block is decomposed 
 on the upper surface, so that one of those cavities, commonly termed 
 rock basins, and supposed formerly to have been the work of the 
 Druids, is produced, extending even through the upper block to the 
 division between it and the lower. There seems little reason to 
 doubt, however isolated these blocks may now be, that they are the 
 remaining portions of granite, similar to that on which they stand, 
 that have resisted decomposition better than those which sur- 
 rounded them, the latter having, by degrees, been disintegrated 
 and removed." 
 
 " Rock basins are so common in all the chief granite districts, 
 that it is difficult to find any number of tors or earns on which some 
 traces of the kind of decomposition thus termed may not be found. 
 The largest cavities thus formed in any number near each other 
 which we have seen are those represented in the Kettle and Pans, 
 on St. Marys, Scilly.f Though we may regard these basins as 
 natural productions, due to atmospheric influences, and therefore 
 not the work of the Druids; it does not follow that the latter may 
 not have made use of a few which may have been properly situated 
 for their purposes, even giving a more artificial form to some, 
 should they have required similar basins at all." 
 
 "As we have often observed these fragments in motion during 
 high winds, both when the basins were dry, or a small quantity of 
 water in them, we are inclined to believe that this may be the case." 
 
 Elvan or Quartz Porphyry. 
 
 Only two occurrences of elvan have been noted in the Scilly Isles, 
 and both are on St. Marys. Indeed it is probable that they belong 
 to one and the same dyke though the outcrops are not continuous at 
 
 * "Report on the Geology of Cornwall, etc.," pp. 451, 452: see also Carne, 
 Trans. 'Royal Oeol. Soc. Cornwall, vol. vii., p. 143 ; and F. F. Statham, Oeolo- 
 gist, vol. ii., p. 26. (1859.) 
 
 t See Plates III. and IV. Dr. MoCullooh has suggested that the friction 
 of the quartz and felspar fragments, not unfrequently found in rock basins, 
 may have contributed to deepen them. 
 
GRANITE AND ASSOCIATED ROCKS. 13 
 
 the surface.* The first is seen on the coast at Porthloo, a little 
 north-east of Hugh Town. 
 
 It is a grey or pinkish-grey porphyritic rock with large phenoorysts of white 
 or pink felspar (some of which are an inch in length) and smaller grains of 
 quartz. Dark green pinite pseudomorphs, about one-eighth of an inch in 
 diameter, can be detected in many specimens, and are locally abundant. The 
 matrix is fine-grained and grey or reddish in colour. It is most abundant 
 near the margin of the dyke, where it may show a fluxion banding when 
 weathered. The large felspars are highly idiomorphic ; they are somewhat 
 turbid with decomposition products, but most are perthitie orthoclase, a 
 lesser number being acid oligoclase. They frequently enclose idiomorphic 
 crystals of biotite and rounded blebs of quartz, and the orthoclase felspar 
 may surround the plagioclase. The quartz is in most cases slightly rounded 
 by corrosion and contains enclosures of the ground-mass, small irregular 
 fluid cavities (mostly negative crystals) and occasional scales of biotite. The 
 brown mica is idiomorphic, and is fresh only when surrounded by quartz or 
 felspar, elsewhere it is entirely altered into chlorite, rutile, etc. It is dark- 
 brown with many pleochroic halos. The ground-mass is a fine micro-granitic 
 aggregate of quartz, orthoclase, and scaly muscovite. 
 
 The pinite is seen in the hand specimen to form six-sided prisms not much 
 longer than broad and terminated by flat ends. Under the microscope it 
 consists of fine tufted sub-radiate muscovite, mixed with a considerable amount 
 of pale green feebly pleochroic chlorite. Apatite, zircon, and iron oxides are 
 the other accessory minerals. Towards the margin of the dyke the pheno- 
 crysts become smaller and less numerous and the matrix finer grained and 
 more abundant. A miero-poikilitic structure replaces the micro-granitic 
 structure of the ground-mass, and there are narrow graphic halos round the 
 quartz phenocrysts. These latter are small, and have suffered little corrosion, 
 as they mostly retain their sharp crystalline outlines. Minute scales of secon- 
 dary muscovite are very abundant, and there is often also much diffused 
 chlorite or haematite. Small pinite pseudomorphs (consisting of sub-radiate 
 chlorite and muscovite) occur also in the fine-grained margin of the dykes. 
 
 Altered Killas. 
 
 Only one small outcrop of Killas has been noted in the whole of 
 the Scilly Isles, and this oecurs at the north end of White Island, 
 to the north-west of St. Martins. It consists of a narrow dyke-like 
 patch, some 200 yards in length, but rarely exceeding three feet 
 in breadth. It distinctly tapers downwards, indeed, at the 
 west end of the granite spur, the patch of Killas comes to an apex or 
 point, and one can clearly see the granite underneath it. This 
 mode of occurrence at once suggests that it represents a local down- 
 ward extension of the original roof of the granite, and this idea is 
 strengthened by the fact that the latter becomes distinctly finer in 
 grain as the Killas is approached. As would be expected Erom its 
 position, this patch of sedimentary material is greatly altered, it is 
 thoroughly schistose and much tourmalinised. From the minute 
 puckered folding still visible on the weathered surface it was origi- 
 nally a finely banded shale . The microscopic sections show that the 
 
 * See De la Beche, "Report on the Geology of Cornwall, etc.," p. 174. The 
 occurrence of " porphyry " was noticed by Majendie. 
 
14 ."GEOLOGY OF ISLES OP SCILLY. 
 
 rock is a thoroughly crystalline schist mainly composed of quartz, 
 white mica and abundant schorl, the latter" occurring either as 
 fairly large crystals, or aggregated groups of small ones. The 
 tourmaline is usually blue at the centres of the crystals and brown 
 at the exterior. A little biotite is also present and grains of colour- 
 less 1 apatite. This rock shows essentially the greisen type of alter- 
 ation and a similar alteration of the Killas may sometimes be found 
 in rocks taken close to the margin of the granites in Cornwall. At 
 the junction with this altered Killas the granite is distinctly foliated, 
 and here and there shows a certain amount of greisen alteration as 
 well. 
 
15 
 
 CHAPTER III. 
 
 DRIFT. 
 
 With the exception of a small patch of reconstructed gravel, 
 perhaps of Eocene date, no trace of secondary or tertiary deposits 
 has yet been discovered on Scilly, and little can be said about the 
 geological history of the Isles between Carboniferous andPleistocene 
 times. 
 
 An outlier of subangular gravel, largely composed of Chalk-flints 
 and Greensand-chert, which caps the highest part of St. Martins, is 
 so likelthe Eocene river-gravels of Devon and Dorset, and so closely 
 resembles the reconstructed Eocene gravels of Mount's Bay, that 
 in all probability it also is a remnant of an old valley-gravel of 
 Eocene date. If such is the case the flat-topped Isles (see Plate II.) 
 must be a last relic of an old table-land over which Eocene rivers, 
 probably- radiating from Dartmoor, flowed outwards, across what 
 is now part of the Atlantic. An examination of the gravel shows 
 that much of the Cretaceous material consists of the peculiar flint 
 and chert so abundant at Haldon, on the east side of Dartmoor. 
 
 If we may judge by what happened in the Land's End district, 
 the Isles must have been entirely submerged in Pliocene times, for 
 on the mainland a, wide shelf or plane of marine denudation of 
 Pliocene date slopes gradually upward to about 430 feet above the 
 sea, above which level the hills begin. No trace of Pliocene deposits, 
 however, has been found in Scilly ; 'though this is not surprising, 
 for the whole of West Cornwall only shows three small outliers, but 
 one of which, that of St. Erth, is f (fesiliferous. 
 
 The Raised Beach. 
 
 In many places along the inner coast-line of the Scilly Isles, clear 
 traces of terrace-features may be observed ; but, on examining 
 the material of which these are formed, it is found to be the well- 
 known Head, which is certainly not of marine origin, and as the 
 terraces cannot be of fresh-water origin, the aspect of the ground is 
 at first difficult to account for. Further, in a good number of cases, 
 small patches of firmly cemented conglomerate, formed exclusively 
 of small granite boulders, were met with, and, owing to the sloping 
 surface on which it often rests, the true nature of this deposit and. 
 itsgeological significance are not at first sight easily grasped. The 
 sections seen on the Cornish coast, however, at once make clear the 
 meaning of the patches of conglomerate and terrace features. The 
 conglomerate is the remnant of an old beach, only very small 
 patches of which are now left ; but while the beach deposit has, 
 in the main, been eroded away, the old platform on which it rested 
 has been left in many places, with the result that the Head which 
 
16 GEoLO&Y OP ISLES OF SClLLY. 
 
 practically occupies the old position of the beach-material has a 
 terrace-like contour imparted to it. As already stated, the old 
 beach is made up exclusively of small boulders of granite and their 
 size is, on the whole, singularly uniform, roughly some three or 
 four inches in diameter. The beach is always firmly cemented 
 together and is, in fact, a conglomerate. The largest patches occur 
 on Annet ; where they rest on a somewhat uneven surface, and 
 possibly may be more continuous than the exposures on the shore 
 suggest. The beach must clearly have once extended over all but 
 the little rising patch of ground at the north end of the island, but 
 the surface is somewhat obscured by fine sandy material that has 
 been curiously honeycombed by the puffins in order to build their 
 nests. 
 
 Very small patches of similar conglomerate occur at the north end 
 of the adjacent island. Its true position is accurately fixed on 
 White Island* where it rests on the bare granite and underlies the 
 main Head, a glacial deposit in turn reposing on the latter. This 
 section brings out, in the clearest manner, the sloping nature of the 
 surface on which the beach rested at its highest point and it is 
 equally clear that, as in the case of Annet, the old platform must 
 extend over a considerable portion of White Island, a good deal of 
 the conglomerate being possibly preserved as well, but the fact is 
 again obscured by the newer deposits here consisting of Head and 
 blown sand. A minute patch of the old beach still exists at the 
 head of a little recess in the north-west coast of Bryher (Hell Bay). 
 It here lies well above high water mark and is again capped by Head, 
 with a thin skin of glacial-deposit above. This section shows ex- 
 ceptionally well the sharp slope of the upper portion of the old 
 platform, and is specially worth recording as it is being rapidly 
 denuded away. 
 
 The small size and obscure nature of the outcrops make this 
 deposit at first very difficult to recognise and, no doubt, several 
 minute patches have been overlooked, for much of the Islands had 
 been surveyed before the clear outcrops mentioned above had been 
 seen. The most accessible of these small remnants occurs at the 
 foot of the low cliff north-east of Hugh Town, St. Marys, just 
 before reaching Porthloo. The Head here forms alow cliff, and 
 beneath the former is a very small but unmistakable patch of the 
 typical conglomerate. This outcrop is specially valuable, as after 
 the nature of the material has once been realised, there is no diffi- 
 culty in recognising similar minute patches in other localities. 
 Another point of interest in the exposure at Porthloo is the occur- 
 rence in the overlying Head of a few isolated rounded boulders of 
 beach material, pointed out by my colleague, Mr. Reid. They 
 could only have got into this position from a slightly higher level 
 originally, and they could not have been at that higher level unless 
 the beach had a rather steep upward slope. These little boulders 
 
 * See p. £2, where this important section is fully described. 
 
DRIFT. 17 
 
 serve, moreover, to show that the material of the Head in which 
 they lie must have travelled seaward or away from the steep 
 granite slope behind it for a good many yards. 
 
 The terraced aspect of the upper surface of the Head at this 
 locality is now clearly seen to be due to its reposing on the old plat- 
 form ; and as similar terraced-looking patches of Head are extremely 
 common along the shores of the inland sea, the old beach must have 
 been at one time continuous along the whole length of these shores, 
 except where the granite forms projecting headlands ; indeed, in all 
 probability, it formed a more or less continuous fringe along the 
 outer coast-line too, but recent earth movements having brought it 
 within reach of the powerful Atlantic waves, all but the merest 
 trace of it has been destroyed. 
 
 Standing at the head of the pier at Hugh Town, on a calm day 
 ' when the tide is out, remnants of the old platform may be recognised 
 on many of the small patches of bare rock projecting above water. 
 These rocks have what may be conveniently termed a head and 
 shoulders ; the shoulders are the old platform at exactly the same 
 level on each side of the small central projecting portion or head. 
 All the shoulders, on all the small rocks, are at about the same 
 height, and it is now seen that, though there was a sharp slope to 
 the landward edge of the beach it was at a singularly uniform level 
 at a short distance seaward from the old cliff. Thus in the Scilly 
 Isles, as on the Cornish coast, there is an old Beach underlying the 
 Head ; but far more of it is preserved in the mainland, only very 
 small patches being now left on the islands. 
 
 A special significance, however, attaches to this old Beach of the 
 Scilly Isles, for not only does the main Head overlie it, but also the 
 glacial deposit described on p. 21. The occurrence of the latter in 
 this position identifies it at once with the old beaches described in 
 South Wales and along the south coast of Ireland.* It thus serves 
 to link these old beaches with that of the mainland of Cornwall. As 
 the main platform now occurs only a few feet above low water mark, 
 it becomes clear that considerable upward and downward move- 
 ments have taken place in this part of England in quite recent 
 geological times ; but the evidence for this can be more con - 
 veniently discussed after describing the blown sand and the 
 curious reason for the existence of such a large quantity of it on 
 these small islands. 
 
 Head. 
 
 A conspicuous feature at the foot of the inner cliffs of the Scilly 
 Isles is the accumulation of angular or subangular fragments of 
 granite in an advanced stage of decomposition. To this the term 
 Head has been applied. The same material also occurs at the foot 
 
 *See, W. B. Wright and H. B. Muff, " On a Pre-glacial Raised Beach 
 on the south coast of Irsland."- Proc. Royal Dublin Soc, vol. vi. (N.S-, 
 Part II., No. 25.) 
 
 8675. B 
 
18 GEOLOGY OF ISLES OP SCILLY. 
 
 of the outer cliffs, or those facing the Atlantic, but only in very 
 small quantity and in receding hollows or small bays. There was 
 obviously, at one time as much Head in the outer cliff foot as on 
 the inner, but so much greater is the force of the waves of the open 
 Atlantic that almost all of it has now been removed. It becomes 
 at once obvious that this material is now being removed far faster 
 than it accumulates, and that conditions must formerly have 
 prevailed which both favoured its formation and possibly pro- 
 tected it from denudation. 
 
 The Head usually shows little trace of bedding, and a feature 
 of the larger granite fragments is their curious form, one face being 
 rather flat, the other rounded. Its mode of formation and the 
 method by which it reaches its present position are not altogether 
 clear, but an examination of the shores of these Islands shows that 
 it often grows from both the top and base at the same time. In 
 many of the lower cliffs, where there is a small shelf of rock, just 
 about or above high water mark, the mode of growth can be seen 
 to be as follows. A master crack proceeds vertically downwards 
 into the granite and from this minor curving cracks proceed. In 
 extreme cases, the two cracks meet and a block of the granite is 
 detached from the parent rock altogether ; but in many instances 
 complete severance from the main mass has not taken place, and 
 then the exact process of formation of the curious shaped fragments, 
 flat on one face (that of the vertical crack) and curved on the other 
 can be clearly made out. It follows from these facts that the exact 
 base of the Head is difficult to define. 
 
 It clearly grows upwards from the top, owing to the rolling or 
 working down from the adjacent steep bank or old cliff of fragments 
 detached by the process just described, and as might be expected, 
 the coarsest material always occurs close to the foot of the steeper 
 slopes. 
 
 The Head, in special localities, can be divided into two parts, an 
 upper and a lower. This fact is well shown in the bay at the west 
 foot of the Beacon Hill of St. Martins, where the two portions are 
 separated by a curious glacial deposit to be described later. This 
 section also brings out clearly how small is the amount of the more 
 recent or Upper Head accumulated since the glacial deposit was 
 formed ; for here it extends quite a short distance from the very 
 steep bank or inland cliff. Like the Lower Head, it is composed of 
 coarse material only when close to this steep bank ; the fragments 
 becoming rapidly smaller as the lower slope is crossed. Indeed it 
 is only when quite close to a steep bank that any appreciable quan- 
 tity of Upper Head occurs and, further, the two divisions are recog- 
 nisable when the glacial deposit, or its equivalent, can be traced 
 beyond their junction in the direction of the steep slope. The total 
 quantity of it in all the Islands cannot be large, although it has been 
 accumulating in favourable localities ever since the close of the local 
 glaciation referred to later on. 
 
DRIFT. 19 
 
 Local Details. 
 
 A good and easily accessible exposure of the Head occurs on the 
 coast of St. Mary's in the low cliffs about Porthloo and east of Hugh 
 Town. It here attains a considerable thickness, but varies much 
 incoarseness of texture from point to point. This variation is clearly 
 due to the fact that the steep rock- face behind the Head alternately 
 recedes from and approaches the present low cliff -face ; in the 
 former case, the Head is finer, in the latter coarser. That there 
 has been little movement of the component fragments parallel to 
 uhe shore line is clearly shown by the Porthloo elvan or quartz- 
 porphyry. Fragments of this occur in the immediately overlying 
 Head, but certainly do not extend more than a few feet on either 
 s'.de of the outcrop of this dyke. On the other hand, that there has 
 been a considerable forward and downward movement is shown by 
 the fact (already noted) that in a few cases the well-rounded small 
 boulders derived from the old beach can be detected some feet up 
 in the Head ; these small boulders must obviously have moved 
 some yards seaward from the steep granite slope at the foot of which 
 the beach was formed. 
 
 On the north face of St. Marys, where the interior cliffs escape 
 the more powerful waves of the Atlantic, long and continuous slopes 
 of Head still remain, and these, at times, are of considerable thick- 
 ness. On the more exposed eastern and southern faces of the 
 Island, only small patches are now preserved, and these lie in the 
 more sheltered hollows, such as the Bay of Old Town and the larger 
 and deeper bay on the south side of Hugh Town. In the latter the 
 Head frequently assumes a terrace-like aspect and reproduces on 
 its upper surface the contour of the old sloping platform on which it 
 rests. The height of the upper part of these terrace -features 
 brings out clearly the steepness of the slope of the shore-end of that 
 platform. A precisely similar mode of occurrence and distribution 
 of the Head has been noted along the shores of St. Martins, Tresco 
 and Bryher, as well as along the margins of some of the smaller 
 Islands ; the upper surface of this material frequently assuming a 
 terraced aspect. 
 
 Most of the islands are too small to have hollows in the interior 
 of sufficient size to contain much Head ; even where this material 
 h clearly present, it is usually difficult to trace its boundaries. In 
 the larger Island of St. Marys, however, and in St. Agnes some 
 large patches have been mapped. 
 
 Iron -cement. 
 
 On examining the Head along the coast, especially where it 
 has a rather broad outcrop, the highest portion is generally found 
 to consist of very fine material. This deposit, though essentially 
 of a sandy nature, has been called " clay " by the inhabitants of 
 the islands, owing to its property of drying hard, in fact, of setting 
 
 8675, b 2 
 
20 GEOLOGY OF ISLES OF SCILLY. 
 
 like a cement, and it has been used in many old buildings to cement 
 the walls in place of mortar.* This curious property of setting 
 firmly when dry, raised the suspicion that it contained soluble silica 
 which becomes insoluble when dried ; in consequence, a specimen 
 from a small excavation recently made on St. Agnes was tested by 
 Mr. E. G. Radley, who found it to contain 3 - 8 per cent, of this 
 material. On boiling in weak hydrochloric acid and precipitating 
 with ammonia, a large amount of iron-oxide was seen to be present. 
 Both the silica and oxide of iron evidently assist in causing this 
 material to " set " when dry and bind it together with extraordin- 
 ary tenacity. In consequence of this tenacity, the sea along 
 the inner coasts will often eat away the underlying loose normal 
 material of the Head, leaving a small projecting shelf of this " iron- 
 cement." One notes with astonishment how small a thickness of 
 this projecting material will support the weight of a man in perfect 
 safety. This peculiarity of the fine, sandy material or Iron-cement 
 that caps the normal Head is specially well shown on the south side 
 of St. Martins, just about and above high-water mark ; it is, how- 
 ever, fairly well seen in most occurrences along the inner coast. 
 
 In the interior of the islands, a thin cover of it occurs in many 
 of the hollows intervening between the more or less bare patches 
 of granite, but it is too thin and irregularly distributed to be mapped 
 out. In St. Marys, a considerable amount of this fine material 
 occurs in the old clay-pit (so-called on map) by the road-side about 
 half a mile east-north-east of Hugh Town. The crest of the low 
 hill close by is composed of bare granite, and the normal Head 
 gradually sets in as the slope is descended ; toward the foot of the 
 slope, the Head is, in turn, capped with the fine, sandy deposit, 
 or Iron- cement. This material is obviously a fine, somewhat 
 micaceous, sand, and its junction with the normal Head is in places 
 distinctly sharp, suggesting a somewhat different origin from the 
 latter. The markedly sandy aspect and loose texture is possibly 
 due to the cut face having been washed by rain, whereby the bulk 
 of the soluble silicia and iron-oxide were extracted. In St. Agnes, 
 where the alternation of patches of bare granite and intervening 
 hollows is specially well shown, this fine material tends to accu- 
 mulate in the lower portion of these hollows, and a small pit was 
 recently opened in it a little above high-water mark, on Castella 
 Down in the south west corner of the island. It was from this 
 opening that the specimen was obtained which proved to contain 
 the soluble silica and iron-oxide. A quarry, some 600 yards south- 
 east of the lighthouse, has been opened in the Head, giving a 
 complete section of the softer materials resting on the granite. 
 At the top is some two feet of coarse granite sand, evidently 
 
 * This is probably the material mentioned by R. Heath, in his " Natural 
 and Historical Account of the Islands of Scilly," 1750, p. 67. He says, " Their 
 mortar is tempered with a sifted Earth, which they call Ratii, and is said to 
 make the strongest Cement for binding the Rock-stone together." 
 
DRIFT. 21 
 
 representing an Upper or Newer Head ; this rests on a fairly even 
 surface of the Iron-cement, which, in turn, rests on the normal 
 Head, but with a very uneven base. 
 
 An exceptionally large area of this fine material fringes bhe fresh- 
 water lakes of Tresco, and probably forms their bed. At first sight 
 the shape of the ground suggests that the flat area should be classed 
 as alluvium, but the component material is really the iron cement. 
 This is shown by a large excavation made in it nearly 300 yards 
 north-west of Tresco Abbey, on the south side of the Great Lake. 
 Another but smaller opening in the same material may be seen on 
 the south side of the fresh- water pool between the two bays on the 
 west side of Bryher. 
 
 The origin of this curious deposit is by no means clear ; it is 
 distinctly micaceous and far finer in texture than the normal blown 
 sand, as well as of a totally different colour and composition. It 
 may have an eolian origin, however, and possibly represents the 
 very fine material set free by the action of frost and blown directly 
 from the bare granite surfaces ; i.e., without being first washed 
 by the sea as the blown sand clearly has been. The idea of invok 
 ing the aid of frost in the production of this material is based on 
 its substantial identity both in composition, mode of occurrence, 
 and geological age, with the matrix of an undoubted glacial deposit, 
 co be described next. 
 
 Glacial Deposit.* 
 
 The occurrence of Chalk-flints in the Scilly Isles has been noticed 
 by several observers, among whom A. Smith f may be mentioned 
 as giving the fullest account of them. N. Whitley also in a 
 paoer on " The Evidence of Glacial Action in Cornwall and Devon " 
 refers to the " drift " in western Cornwall, remarking that, " These 
 gravel beds, must however, have formerly extended far beyond 
 the present coast line, as we find rocks of a distant origin and chalk- 
 flints in the trail of drift which swept over the Scilly Isles." J 
 
 Till recently, however, their exact mode of occurrence and the 
 manner in which the chalk -flints reached their present position 
 was not understood. The tracing of their distribution soon made 
 rs clear that these stones really form part of a curious hard fer- 
 ruginous sandy deposit closely resembling the Iron-cement already 
 described as forming the top of the Head. Within this ferruginous 
 material the flints associated with other stones occur in lenticular 
 patches. The geological age of this curious deposit was first 
 clearly defined in White Island, at the north-west end of St. Martins, 
 and consequently it is advisable to give full details of the section 
 seen there and then to refer the other exposures to this as astandard. 
 
 * This deposit was investigated in company with Mr. Reid. 
 ' f Trans. Royal Oeol. Soe. Cornwall, vol. vii., p. 343. 
 j Ibid., vol. x., pp. 133, 134. 
 
22 GEOLOGY OF ISLES OF SCILLY. 
 
 White Island clearly once consisted of a small, curving ridge • 
 of granite flanked on both sides by softer, superficial deposit*. 
 But so rapidly have the Atlantic waves cut away the exterior 
 portion of this ridge that all the superficial material and most of 
 the granite slope have disappeared from it, so that we now see only 
 an abrupt low cliff, above which the surface of the downward slope 
 of the other side of the ridge at once commences. As a consequence 
 of this shape, lines of weakness traversing the granite enable the 
 sea to cut the rock open for an unusual distance, since it has to 
 encounter a steadily diminishing thickness of resisting material. 
 Along the most southerly of these lines of weakness in White Island, 
 the Atlantic waves have cut a trench exposing the interior- slope ot 
 the granite and the softer material lying on it. (See plate VI.) 
 
 Starting from the seaward end of the gully, the granite slope is 
 first flanked by a small thickness of Head, which, though extending 
 only a small distance, continues far enough just to overlap the 
 taper edge of the Iron-cement bed containing the foreign stones. 
 This bed steadily thickens along the side of the gully till it attains 
 a maximum thickness of about three feet and is seen clearly reposing 
 on the Main or Lower Head, toward the west or shore end of the 
 section. The Head, in turn, overlaps the taper edge of a fragment 
 of the old beach, and thus the succession of these superficial deposits 
 is proved to be as follows in descending order : — 
 
 1. Upper or Recent Head. 
 
 2. Iron-cement bed or Glacial -deposit. 
 
 3. Main or Lower Head. 
 
 4. Old Beach. 
 
 5. Sloping surface of the Granite. 
 
 As already stated the Iron-cement bears the closest resemblance 
 to the hard, sandy ferruginous material already described as occur- 
 ring at the top of the main Head, but containing no foreign stones. 
 When these stones are present, this material is rather more ferrugi- 
 nous, and probably even more coherent. The peculiar mode of 
 occurrence of the stones is well shown in this section on White 
 Island ; they tend to occur close together in lenticular patches. 
 
 At this locality the stones are not so numerous, or, perhaps more 
 strictly speaking, show less diversity of origin than those met with 
 at the eastern end of St. Martins ; still, they show the typical 
 features well enough. The most common pebbles are flint and a 
 comparatively soft reddish-brown sandstone ; the latter are often 
 flat on one face, suggesting their having been split open by frost ; 
 in addition, many show faint striae and in a fair number of cases 
 these striae are well marked. Several of these stones have been 
 preserved in the boulder-collection at the Museum of Practical 
 Geology. 
 
 The details of the former structure and shape of White Island 
 have an important bearing on the extent of the glacial deposit, 
 for on crossing the low bar or beach that connects White Island 
 with St. Martins, these stones are met with in great abundance. 
 
DRIFT. 23 
 
 Their occurrence, in such a position, clearly shows that they are 
 the remnant of the softer materials once flanking the eastern, or 
 outer, side of the granite ridge of White Island in a precisely 
 similar manner to that still left on the west. The finer portion 
 of the matrix has been carried away entirely, but a large number 
 of the harder stones have been thrown up by the waves, and now 
 attest the former, far greater, extension of the glacial deposit. 
 Proceeding eastward along the north shore of St. Martins it is 
 impossible to tell if this deposit is present or not owing to the 
 great amount of blown sand. But in St. Martin's Bay the Head 
 emerges once more and is thirty feet thick, although its outcrop 
 is so narrow that it cannot be shown on the map. No foreign 
 pebbles were detected here, and it is not till the east side of the 
 bay, north-east of the Higher Town, is reached, that they are 
 again encountered. Here they are not numerous, and are lying 
 loose and not embedded in the typical cement. Crossing the 
 projecting headland into the next bay, however, two good sections 
 of the glacial deposit are seen, one on each side of the bay. 
 
 In the more westerly section the glacial deposit rests on 
 the Head and is less interesting than in that further east. Here 
 the newer Head again caps the glacial deposit at the foot 
 of the steep headland. (See Plate V.) In one place the 
 former is eight feet thick and entirely composed of compara- 
 tively fine material, markedly ferruginous at its base. Below 
 this is the glacial deposit, which here alone consists partly of tough 
 clay. Indeed, at the centre of the outcrop, the Iron-cement and 
 associated pebbles are embedded in the clay, the latter containing 
 only a few isolated pebbles. One of these, however, was six inches 
 in diameter, composed of pale basaltic material. 
 
 The western part of this small outcrop contains less clay, and 
 the deposit almost resumes its normal aspect. It is soon seen, 
 however, that the greatest variety of stones occurs here, and a 
 number were collected, the more important features of which 
 may be summarised as follows : — 
 
 The general aspect of the stones is such that a considerable 
 portion of them must have been derived from an older deposit, as 
 many of them are too well rounded to leave any doubt that they 
 were derived from some gravel and not directly from the parent 
 rock. They are divisible into the following groups : — 
 
 1. Chalk -flints. 
 
 1 a . Greensand- chert . 
 
 2. Soft red-brown Sandstone. 
 
 3. Rocks from within an aureole of metamorphism sur- 
 rounding one of the granites of the south-west of England. 
 
 4. Fragments of more or less hardened Killas. 
 
 5. Well-rounded pebbles of harder Sandstone. 
 
 6. Pale greenish sheared igneous material, suggesting 
 fragments of dykes, but almost certainly not connected 
 with the post-carboniferous granites. 
 
24 GEOLOGY OF ISLES OF SCILLY. 
 
 1. The Flints, which are the most abundant of all the stones, are of two 
 kinds : one unmistakable Chalk-flints, the other more difficult to determine, 
 but many, when examined by Mr. Keid, proved to be Greensand chert. 
 In many cases, however, the true nature of these flints could not be 
 ascertained without the aid of a microscopic section. Both flint and chert 
 are of somewhat peculiar character ; but they agree closely with, and are in 
 the same state of preservation as those in, the subangular Eocene river- 
 gravels of Haldon, in Devonshire. Their occurrence suggests an easterly 
 origin for these pebbles. 
 
 2. The soft red-brown Sandstone. The softness of this rock enables it to 
 be easily scratched, while it is sufficiently hard to retain the scratches. 
 This sandstone seems peculiarly liable-to be split open along the old bedding 
 planes when exposed to frost. The flat faces so often shown by the pebbles 
 must have been produced after extraction from the tertiary deposit, and 
 during their transport to their present position. A considerable variation 
 in tint and size occurs in these pebbles, but they possess sufficient characters 
 in common to enable them to be easily distinguished from the well-rounded 
 fragments of the far harder sandstones which rarely, if ever, have flat faces. 
 
 Microscopic sections (4473-4474) show that the composition of the sand- 
 stone is so peculiar that it will be easily identified when a complete series 
 of sections has been made of the rocks of the mainland. Fragments of 
 quartz and felspar, clearly derived from a granite are common and the latter 
 contains the parallel-arranged micas and zoisite so common in the very old 
 granites. Sedimentary material is represented by chips of sandstone and 
 shale showing a low phase of hornfels type of alteration, as well as by 
 fragments of graphitic (?) phyllite and quartz-schist. Smaller igneous 
 masses are represented by a rock having a felspar-lath groundmass. Theri; 
 is also present a clear almost glassy material, often full of brown dust, 
 enveloping crystals of felspar. With such a composition, these pebbles 
 will be identified in the first section made of the parent rock. 
 
 3. The Metamorphic Bocks. The pebbles of this group possess the 
 peculiarity that, so far as structure and composition is concerned, they 
 might have been derived from the aureole of metamorphism that must 
 have existed to the north of the present Scilly Isles (and still exists under 
 water), or from the margin of any of the other of the Cornish granite 
 masses. The largest and most abundant of the metamorphic rocks 
 consist of epidiorite, as might have been expected from the well-known 
 hardness and durability of this material. Four sections have been made 
 (4476-4479). The largest pebble is 4 inches in diameter and well foliated, 
 being built up of yellowish and dark lenticles. The yellowish portion is 
 composed of minute grains of sphene and felspar, the dominant constituent 
 of the dark material being actinolite. Another specimen (4478) shows no 
 foliation and is a typical example of paramorphic heat alteration, the 
 original ophitic structure being well preserved. In 4479, the original 
 structure was sub-ophitic and is partly preserved, the heat alteration here 
 being less pronounced, while 4476 is a more schistose phase. All these 
 rocks can be closely matched on the mainland in the aureoles of alteration 
 surrounding the granite. Fragments of the dark mudstones showing the 
 hornfels phase of metamorphism are not uncommon, while some fragments 
 could be matched exactly at Cape Cornwall and Mousehole, on the main- 
 land. Some of the hornfelsed rocks can be seen from their weathering to 
 have been originally calcareous mudstones, and these, on a fractured face, 
 have a somewhat cherty aspect. Identical rocks occur near the margin of 
 the Bodmin Moor granite, and doubtless of the other masses. A curious 
 small pebble with a somewhat cherty look proved on fracture to be a fine 
 pale grit, the matrix of which has been rendered almost vitreous ; its 
 origin is not yet known. 
 
 4. The fragments of Killas are often sufficiently hard to suggest a certain 
 amuunt of thermometamorphism ; but a section examined by Dr. Flett 
 id not show any clear evidence of this type of alteration. They vary 
 considerably in composition ; some having been pure shales originally, 
 others sandy shales. The latter show well the intense folding to which 
 much of the Killas has been subjected. 
 
DRIFT. 25 
 
 The pebbles of harder sandstone vary in tint from green to grey. They 
 are always well-rounded and apparently never split by frost. Some of the 
 lighter coloured varieties may be fairly called quartzite. 
 
 5. Pebbles of a pale green igneous rock are fairly common at this locality, 
 though comparativly rare elsewhere. From their fine-grained character 
 they have probably been derived from dykes or small igneous mas3e3, though 
 probably not in any way_ connected with the Cornish granites. 
 
 The most easily identified of these is composed of a felspar-lath matrix 
 in which are set abundant flakes of biotite and some very pale hornblende. 
 The biotite is quite unlike that associated with the Cornish, granites. 
 
 Four other specimens are all augite and felspar-lath rocks and mostly 
 much decomposed. They vary in basicity and in minor points of structure, 
 but one is characterised by the abundance of apatite needles. The rocks 
 of this group will certainly be identified as the survey of the mainland 
 progresses. 
 
 St. Martin's Head. — Ascending the steep slope towards St. 
 Martin's Head, these stones are very rare ; only a few irregular 
 shaped flints were found after a careful search. The gradient 
 is obviously too steep for well-rounded pebbles to rest on it ; but on 
 the flat top of the Beacon Hill these stones are extremely numerous, 
 so much so as to raise a suspicion that they represent a remnant 
 of an old tertiary deposit. This view is considerably strengthened 
 by the fact that no ice scratchings were observed on the soft brown 
 sandstone fragments, though the stones, as a whole, have the 
 same composition and occur in much the same relative proportion 
 as regard number. These foreign stones continue abundant for 
 a distance of some 200 yards down the low slope south of the 
 Beacon Hill. Still further south, they slowly decrease and at the 
 end of another hundred yards they are either absent or very rare. 
 They have not been met with on any other part of the higher ground 
 of St. Martins, nor anywhere along the southern shore of the 
 island. 
 
 West and South-West of White Island. — On the projecting north- 
 west spur of St. Martins the deposit is represented simply by 
 the foreign stones which rest on bare granite ; while at the north- 
 west point of Tean a small quantity of a brown cement is present 
 as well. On the west and north shore of St. Helens, however, 
 patches of the typical brown ferruginous cement are common 
 and in this the foreign stones occur aggregated together to form 
 the usual lenticular patches. In these fragments of altered Killas 
 are rather more common than usual. 
 
 Tresco. — The distribution of these stones in Tresco is peculiarly 
 interesting, as it was at the north end of this island that the first 
 clue was obtained to their true mode of occurrence. In the bay 
 (Gimble Porth) on the east side of the north end of the island, 
 there is a long narrow strip of Head just about high-water mark. 
 No foreign pebbles were found in any part of this deposit till quite 
 the north end of the bay was reached, when they appeared quite 
 suddenly. At first flints only were seen, the majority of which 
 are small broken chips, but just before the end of the outcrop is 
 reachad, the normal assemblage of stones can be found. For 
 a short distance further north the granite is bare of superficial 
 
26 GEOLOGY OF ISLES OF SCILLY. 
 
 deposits ; after which another outcrop of Head is reached, 
 again this curious deposit with foreign stones rests on it. This 
 patch of Head is small, and the granite further north has been 
 washed practically clean by the powerful waves of the Atlantic. 
 As so often occurs, the small patch of Head lies in a little hollow, 
 and this hollow is continued up the hill inland with a comparatively 
 gentle slope. Up the whole of this slope the pebbles in considerable 
 numbers can be traced to the crest of the ridge (105 feet above 
 sea-level) and down the corresponding slope on the opposite side 
 of the headland. Moreover on the crest of the ridge they may 
 be traced to some little distance to the south. On the opposite, 
 or western, side, owing to the exposed nature of the coastline, prac- 
 tically no Head is met with north of Cromwell's Castle, but im- 
 mediately south of this is a small and more protected bay in which 
 another patch of Head has escaped denudation. On top of this 
 patch the foreign stones are abundant in the northern portion 
 only of the outcrop ; further south they cease just as suddenly 
 as they did on the opposite side of the island. If now the two 
 points of cessation of this deposit on the two shore-lines be joined 
 up their line of junction is seen to pass approximately along the 
 southern limit of the stones on the crest of the hill. 
 
 Their distribution at the north end of Bryher is on exactly 
 similar lines. A belt strewed with these stones crosses the northern 
 end of the island from side to side, reaching a height at the summit 
 of over 100 feet ; but, as before, they cease' somewhat abruptly 
 to the south or rather, in this case, to the south-east, for the line 
 marking their cessation is now turning round to the south. 
 
 On the Island of Samson erratics occur only at the south end, 
 where they are associated with a small quantity of the ferruginous 
 cement, but there has probably been much more of this deposit than 
 is seen at present. 
 
 On Annet it has- clearly fringed if not covered the whole island, 
 excepting, of course, the small rising patch of bare granite at 
 the north end. The present surface of this island is a mixture 
 of fine soft material which has been completely dug over by puffins 
 in order to make their nests, so that it is impossible to say what the 
 first solid material is. The typical mode of occurrence of the 
 stones is well shown along the shore line, and a distinct change 
 in their nature can now be noted. Greensand-chert is specially 
 abundant, while the characteristic soft red-brown sandstone is 
 far less common. A white sandstone, hard, but not close enough 
 in texture to be claimed as a quartzite, is often found. 
 
 The last place at which the pebbles occur in considerable numbers 
 is on the north side of the first little bay south of the extreme west 
 end of St. Agnes. Here, only traces of the cement are seen, but 
 Greensand-Chert, Chalk-flints, and the white sandstone just re- 
 ferred to are fairly common. On the extreme north-west point 
 of the island a few loose flints were noted, but unaccompanied 
 
DRIFT. 27 
 
 by any of the typical cement or any ol the other stones. On no 
 other part of St. Agnes has this deposit been met with and no 
 trace was seen of it on St. Marys, though several localities where 
 it was likely to occur were revisited after all the above recorded 
 outcrops had been examined.* 
 
 That these stones have been brought into their present position 
 by ice admits of little doubt. In the first place, the striations 
 so often noted on the soft sandstone are almost certainly due 
 to their being embedded in ice and scrubbed against some specially 
 hard material, probably the quartz-grains of the granite of the 
 Scilly Isles. Secondly, their curious distribution is such as to be 
 unintelligible except by invoking some other means of transport 
 than water. It is, however, the strange way in which these foreign 
 stones sweep across the projecting headlands, rising more than 
 100 feet above sea-level, and yet ceasing abruptly only a few yards 
 further south actually at sea-level, that seems to indicate the 
 mode of transport. It is quite clear that they must have been 
 carried by floe-ice, which at the projecting headlands broke to 
 pieces, packed and was heaped up till a trail of it completely crossed 
 the headlands. The fact that a deposit of it is found on the north 
 and west sides of the group of islands as a whole suggests that 
 either the wind, or powerful currents, from that quarter swept the 
 ice against the land. It is most probable that the ice gathered 
 along the shores in winter and was packed or heaped up by winds 
 or tide. There is one peculiarly striking piece of evidence bearing 
 on the mode of occurrence of the stones ; they often occur in 
 lenticular patches. The study of rocks in areas of great dynamic 
 alteration shows that these solid bodies invariably break up into 
 lenticles, or, more strictly, phacoids. In the lenticular mode of 
 occurrence then of these foreign pebbles we have an actual record 
 of the breaking up of the floe-ice, and of the heaping up on itself of 
 the particular band of the latter in which the stones were specially 
 abundant. 
 
 Since the above account of the Iron-cement and the Glacial- 
 deposit was written, an opportunity has occurred of examining 
 the " Limon " on the Brittany coast, north of Morlaix, St. Brieux, 
 etc. This material was at once recognised as identical with the 
 stoneless phase of the Iron-cement, and it has the same curious 
 properties. The sides of railway cuttings made in it remain prac- 
 tically vertical, owing to its curious property of " setting " when 
 dry. For the same reason it was formerly used to some extent in 
 place of mortar in the walls. 
 
 The position of the Limon is the same as that of the Iron-cement. 
 Where the succession is complete we have at the base patches of 
 the old beach, above which comes the Head ; while this in turn 
 is succeeded by the Limon. As in the Scilly Isles, a small local 
 
 * See, however, A. Smith, loc. cit., p. 344; Statham, Geologist, vol. ii., 
 p. 14. (1859.) 
 
28 GEOLOGY OF ISLES OF SCILLY. 
 
 deposit of Upper-Head rests on the Limon, only where the whole 
 rests at the foot of a steep and rather high bank. So small and 
 local is this latter material, that Dr. Barrois, who kindly accom- 
 panied me, prefers to call it " Wash," to distinguish it from the 
 more extensive true Head, which possibly was accumulated under 
 somewhat different conditions. It thus appears that over much 
 of the Scilly Isles we have a thin equivalent of the stoneless Limon 
 of Brittany ; but on the northern and western parts of the islands, 
 this deposit contains numerous well scratched stones and is essen- 
 tially a glacial deposit. 
 
 As the Limon is generally held to be the equivalent of the Loess, 
 the latter can now be gradually traced westward and northwards 
 to the Scilly Isles, over part of which it is still Limon or Loess, while 
 elsewhere it is a true Glacial deposit. 
 
 Alluvium. 
 
 Two patches only of Alluvium have been met with in the Scilly 
 Isles, both of which occur on St. Marys and are shown on the map. 
 Both appear to occupy the same position and have a similar origin, 
 the nature of which is best shown by the larger patch a little east of 
 Hugh Town. As already stated, bars of blown sand keep the 
 spring-tide water out of this low-lying area, but before these were 
 built up, the sea passed through this hollow and the greater part 
 of it was filled with storm-beach boulders, before the fine material 
 now seen at the surface was deposited. The latter rarely exceeds 
 two feet in thickness and consists of a pale -grey or fine white clay, 
 not unlike china clay in its tenacity. This fine clay was partly washed 
 down from the marginal slopes and partly brought down by a small 
 stream flowing through the valley to the north-east, during heavy 
 rain, for there is no stream there during normal conditions at the 
 present day. What underlies the storm-beach is not known ; most 
 probably bare granite, for the sea passing to and fro through this 
 hollow would probably scour out any softer deposits ; it would, 
 however, be interesting to ascertain exactly what does occur here. 
 
 The somewhat smaller patch of Alluvium to the east has the 
 same white clay over much of its surface as that already referred 
 to ; but what-underlies much of this is not known. A very small 
 stream — the only one known in the Scilly Isles — flows through the 
 northern part of this flat ground and enters into a brackish or 
 almost salt water pond, which communicates with the sea by 
 means of an artificial drain. At high tide the water flows up this, 
 and small- fish pass through it to the little lake. 
 
 Blown Sand. 
 Considering the small size of the area above water, the amount 
 of blown sand in the Scilly Isles is remarkable. It is exceptionally 
 abundant on the Island of Tresco, where the characteristic features 
 of its mode of occurrence and the phenomena to which it has given 
 rise can be best studied. Along the whole of the southern and 
 
DRIFT. 29 
 
 most of the eastern shore of the island it forms a practically con- 
 tinuous ridge or bar. This bar commences about high-water mark 
 and rises in some cases to about 35 feet, forming an admirable bank 
 which keeps out the sea. Always steepest on the seaward face, 
 it slopes more gradually on the reverse or inland side. In a few 
 cases, the bar breaks up into two parts, an outer and a higher, and 
 an inner and lower bar ; a small flat-bottomed hollow lies between 
 the two, and in some cases is as low as high-water mark if not 
 even slightly lower. Within the main bar on the southern part 
 of Tresco is a depressed area containing a large fresh-water lake 
 (the Great Pool), the surface of which is distinctly below high-water 
 mark ; yet the sea water does not seem able to filter through, in 
 spite of the porous nature of the protecting ridge of blown sand. 
 
 Little crags of bare granite here and there interrupt the con- 
 tinuity of the sand in this barrier, and there seems good reason to 
 believe that these small projecting bosses first arrested the moving 
 sand, and formed the original gathering points, from which the 
 building up of the sand bars appears to have often started. 
 
 A more local, but high, ridge of blown sand occurs on the west 
 side of the island, at New Grimsby, again acting as a bank and 
 keeping out the sea from the north-west end of the Great Pool. It 
 now becomes clear that, but for the blown sand ridges, the southern 
 portion of the Island of Tresco would be separated at high-water 
 from the northern ; in other words, the blown sand has had the 
 effect of joining different granite patches or small islands together 
 so as to form a continuous land-surface or one island. This latter 
 effect is one of the characteristic features of the Scilly Isles, and the 
 following examples of it may be quoted : — 
 
 Hugh Town, the port of St. Marys, is largely built on a broad 
 bar of blown sand, which serves to unite the western granite mass 
 with the long central one ; were the bar removed, these would be 
 separate islands, at least at high water. A special interest attaches 
 to this bar, as it is referred to in Borlase's account of the Scilly Isles, 
 published in 1756. He notes that Hugh Town is liable to inunda- 
 tions from the sea ; but since that date the amount of blown sand 
 has so increased that any serious inundation is now impossible. 
 
 In like manner the sea is prevented from inundating the alluvial 
 area shown in the central part of St. Marys by one bar on the south 
 side (at Old Town) and three on the north ; were these all removed 
 as well as the one at Hugh Town, St. Marys Isle would be broken 
 up into three moderate -sized islands and two or three small islets. 
 
 Samson again consists of two separate granite masses united 
 by an unusually large mass of blown sand in the centre of the 
 present island. Here, also, there is evidence of the large accumula- 
 tion of this material in recent times, but far anterior to Borlase's 
 visit. On the south side of the mass of blown sand about the 
 centre of the island is an exceptionally large heap of limpet shells, 
 clearly an old kitchen-midden. Large as it is, it has been almost 
 buried under blown sand. 
 
30 GEOLOGY OF ISLES OF SCILLY. 
 
 In St. Agnes also, the eastern part of the island is joined to the 
 west by a sand bar, while the latter is united to a patch of granite 
 in the north-west in a similar manner. 
 
 On the west side of Bryher detached masses of granite are joined 
 to the main island by these bars, and similar phenomena may be 
 seen on St. Martins, Tean and the Eastern Isles. 
 
 Specially fine examples of blown sand bars acting as walls to 
 keep out the sea from low-lying land behind them, occur on the 
 south side of St. Martins. 
 
 High Level Blown Sand. 
 
 In addition to the modes of occurrence already described, this 
 blown sand occurs in three places at specially high levels. The 
 first of these deposits is on the north-east side of Tresco, extending 
 from Old Grimsby harbour to Grimble Porth, almost burying a 
 low ridge of granite between these two small bays. 
 
 The second and third occurrences are, however, the most interest- 
 ing, and these occur in St. Martins and St. Marys. That on St. 
 Martins practically crosses the island at right-angles to the coast- 
 line and about Middle Town forms a number of ridges or dunes 
 trending roughly north and south. The sand forms a thin sheet at 
 a height of 100 feet above sea-level on the flat top to the east of the 
 houses. From this top it descends to the shore, thickening as it 
 does so. 
 
 On St. Marys, a corresponding high mass of sand occurs on the 
 most northerly point of the island. Just off this point is the Crow 
 Bar, a great impediment to navigation at low water. This bar, 
 formed of white sand identical with the blown sand, really repre- 
 sents an old blown sand ridge, now submerged, of larger dimen- 
 sions than any at present existing, and the high level masses just 
 referred to are the two ends of the bar. 
 
 It is this submerged bar that gives the clue to the large amount of 
 blown sand present in the modern islands. * 
 
 It was clearly formed when the ground stood at a higher level 
 when, in fact, the sea was more than 25 feet lower than at present. 
 Now if the area lying between low -water mark and 25 feet below 
 sea level were raised, it is at once apparent that the land area 
 would be doubled. If to this be added the area between the 25 
 and 50 feet submarine contours the further increase in dry land is 
 comparatively small, except in the ground about Samson and 
 St. Agnes. 
 
 From this it follows that the old bar was formed when there 
 was a larger area above water to supply the component material. 
 The sand of this old bar, like the present blown sand, is essentially 
 composed of finely comminuted granite, the dominant consti- 
 tuents now being minute grains of quartz, and white felspar ; 
 the latter imparting the snow-white colour. The bulk of the 
 
 * See Statham, Geologist, vol. ii., p. 21. 
 
DRIFT. 31 
 
 blown sand was not, however, derived directly from the granite, 
 but rather from the iarge sheets of Head, the former greater 
 extent of which has already been pointed out. This granitic 
 material is extremely soft and incoherent, and would easily 
 be pounded to pieces and bleached by the action of the sea-waves. 
 Thus, not only was the land higher when the Crow Bar was formed, 
 but there was a far larger mass of soft material at hand to form the 
 sand for building it. 
 
 As the land sank, the bar was partly spread out over the shallow 
 sea within, and all the Head inland of it, which must have been of 
 great extent, would equally come under the pulverising influence of 
 the waves. This has finally resulted in the whole of the ground, 
 except little projecting bosses of granite, above the submarine 25 
 foot contour being buried under a deposit of snow-white sea-sand. 
 So shallow is this sea that, in stormy weather, the waves stir it up 
 and it is carried to the shores at high water and left at low -tide. 
 The winds blowing over the shore heap it up above high -water mark, 
 and in this way the bars of blown sand have been formed. Their 
 great size in proportion to the small area at present above sea-level 
 is thus clearly due to the vast amount of this fine white granite sand 
 only a few feet below the surface of the water at low-tide. 
 
 Not only do the waves throw up this material on the shore during 
 storms, but, aided by powerful currents, they carry it out to sea 
 through the deeper channels communicating with the open ocean. 
 Approximately opposite the deeper channels, which communi- 
 cate with the shallow area, are huge fans of the finest sand, so 
 carried out, and their position is admirably shown by the trend of 
 the 200 foot submarine contour, as the small accompanying map 
 shows. (See Frontispiece, Plate I.) 
 
 Borlase referred to the several varieties of sand, much of it derived from the 
 Moorstone (granite). " The finest sand, much coveted by the Cornish and 
 others, for scouring brass, pewter, etc., and for drying up Writing-ink, is 
 found only in Porthmellyn Cove on St. Mary's." 
 
 " In one part only of St. Mary's they have a shelly Sand, and those who 
 carry on the best husbandry use this, and find their account in it." 
 
 He remarks that the soil generally is " what we call Growan ; that is mixed 
 with rough gravel (as the Cornish word implies), and therefore not apt to 
 bake or grow stiff." * 
 
 The fertility of the growan soils is much greater in the low-lying situations 
 of the Scilly Isles, than in the upland regions of Cornwall and Devon, t 
 
 * " Observations on the Ancient and Present State of the Islands of Scilly,'' 
 1756, pp. 67, 68 ; see also Came, Trans. Ceol. Soe. Cornwall, vol. vii., p. 153. 
 
 f De la Beche, "Report on the Geology of Cornwall, etc.," pp.475, 476 ; 
 Statham, Geologist, vol. ii., p. 18. 
 
32 
 
 CHAPTER IV. 
 
 RECENT MOVEMENTS IN THE SCILLY ISLES. 
 
 Attention has already been drawn to the existence between St. 
 Marys and St. Martins of a submerged and denuded ridge of blown 
 sand (the Crow Bar), which could only have been formed when the 
 land stood at a distinctly higher level. Information has, however, 
 gradually accumulated to show that in quite recent geological 
 times even greater movements have occurred in the Scilly Isles. 
 
 In Borlase's account of these islands, published in 1756, evidence 
 is given suggestive of recent subsidence, a special appeal being 
 made to the walls well below high -water mark on the sands between 
 Tresco and Bryher and Samson. This evidence, however, is not 
 conclusive, as the walls would obviously be carried out some dis- 
 tance below high-water to prevent cattle creeping round them at 
 low -tide. The fact, however, remains that it is difficult to recognise 
 any trace of the3e walls in the position indicated by Borlase ; they 
 have either been thrown down and been buried under sand, or 
 possibly a slight further subsidence has taken place. Clearly, this 
 line of evidence is not now satisfactory, in the absence of a map 
 showing how far seaward in his day the walls could be traced. In 
 the same work, attention is also called to the paved causeway at 
 Old Town, St. Marys, which he asserts could never have been 
 made in such a position unless the ground were slightly higher above 
 sea-level than at the time he wrote. The view here suggested is 
 even more true at the present day, but even in this case it is quite 
 possible that the blown sand bar which now renders the assist- 
 ance of a causeway unnecessary did not then exist, and the latter 
 was really built on the highest ground then existing. As already 
 shown, the height of the soft material close to the sea -shore has not 
 diminished, but in most cases considerably increased, owing to the 
 piling up of the blown sand. In the north-west corner of St. 
 Agnes is a small round pond, which formerly covered a larger area. 
 In order to reclaim as much as possible of this, a drain was cut to 
 carry off the water at low-tide; and at a depth of six feet, ferns were 
 found which occurred in such a way as to suggest an old soil or 
 land-surface. Unfortunately, the information obtained on this 
 point, beyond the actual occurrence of the ferns, was too vague to 
 be considered conclusive ; though the impression that it was the 
 old land -surface seems strong among those who actually saw the 
 drain made. * 
 
 Whan, however, we turn to geological evidence on broader lines, 
 more satisfactory conclusions can be arrived at. As already stated, 
 there is found above the old beach in the Scilly Isles not merely 
 
 * See also Statha,m, Geologist, vol. ii., p. 13. (1859.) 
 
RECENT MOVEMENTS IN THE SCILLY ISLES. 33 
 
 the Head, as is the case on the adjacent Cornish coast, but in 
 addition we have a true glacial deposit. The occurrence of this 
 is of the utmost importance, for not only can the old beach be now 
 seen to be identical with that on the Cornish coast, but it is obviously- 
 contemporaneous with that described by Messrs. Wright and Muff 
 occurring on the south coast of Ireland. It is also identical with 
 that occurring in the South Wales area, for in both instances the 
 Head overlying the old beach is capped by a glacial deposit. 
 Thus, then, the old beaches in the Scilly Isles, in Cornwall, in South 
 Wales and in the South of Ireland are not only contemporaneous 
 but, in addition, are older than part of the glacial deposits. 
 
 This beach now stands slightly above or slightly below modern 
 sea -level, according to its position and the area in which it occurs ; 
 but it must have recently sunk at least 40 or 50 feet, for the evidence 
 that there has recently been such a subsidence both in Cornwall and 
 South Wales is conclusive. 
 
 In the case of the Cornish area, papers have been published, 
 from time to time, in the Transactions of the Eoyal Geological 
 Society of Cornwall, by Colenso, Came, Henwood and others. 
 All concur in showing that in following the stream-tin deposits 
 under the estuaries, old land surfaces, apparently of Neolithic age, 
 were met with at a depth of nearly 40 feet below the present 
 sea-level. At a somewhat smaller depth human skulls have at 
 times been found, clearly demonstrating the comparatively recent 
 age of the deposit. 
 
 More recently, a paper was published by my colleague, Mr. 
 Strahan, in which he described the occurrence at a depth of 30 feet 
 of an old land-surface at the base of the new Barry Dock. Roots 
 of trees were here found embedded in Boulder clay, clearly showing 
 the downward movement of at least 30 feet in post glacial times.* 
 A broken polished flint implement of human workmanship, was 
 found in one of the upper peat beds. 
 
 It is thus apparent that the following stages in the movements 
 that have affected the Scilly Isles, can be clearly established : — 
 
 1. The formation of the Old Beach, at sea-level. 
 
 2. The Old Beach was raised at least 40 feet. 
 
 3. A comparatively recent subsidence has depressed the 
 whole area of the South of Ireland, South Wales, Cornwall 
 and the Scilly Isles at least 40 feet, for it has brought the 
 Old Beach back again to sea-level ; indeed in the case of 
 the Scilly Isles, almost to low -water mark. 
 
 The comparatively modern date of the downward movement 
 along these coasts is clear enough ; but the length of the preceding 
 period that elapsed between the formation of the old beach and its 
 
 *]?or further details of these movements, see Trans. Royal Geol. Soc. Corn- 
 wall, vol. iv. (1832), in which several papers occur bearing on this point. Also 
 A. Strahan, Quart. Journ. Geol. Soc, vol. lii. p. 474 (1896), (Account of the 
 Barry Docks). 
 
 8675. C 
 
34 GEOLOGY OF ISLES OF SCILLY. 
 
 maximum elevation is more difficult to decide. Making all allow- 
 ances for the more rapid formation of the Head anterior to the 
 deposition of the Iron-cement, it is still clear that this so enor- 
 mously exceeds in bulk the Head formed in post-glacial times, that a 
 much longer period must have apparently been required for the 
 formation of the older portion of this material. 
 
 In view of the clear evidence of movement given above, the 
 question naturally arises : are the coasts of this portion of the 
 country still sinking ? . There is no clear evidence of this ; the 
 only fact suggestive of it being the occurrence of the fine Iron 
 cement around the margins of the Great Pool, on Tresco. The 
 colour of this material is conclusive that it cannot have been under 
 the sea, for in the latter case it would have inevitably been bleached, 
 like the almost snow-white granite sand. The edges of the pool 
 are below high -water mark and obviously the ground has here sunk 
 since the blown sand bars were built up, which so effectually keep 
 out the sea-water. Unfortunately the exact date of this sinking 
 cannot, with our present knowledge, be determined ; and to prove 
 a very small sinking of the sea, in view of the constantly disturbed 
 nature of its surface, is by no means easy. 
 
 One point has been established in the clearest manner : the 
 area above water in the Scilly Isles has not diminished in recent 
 times, but has distinctly increased ; and this increase is due to 
 the constant washing up of the fine sand from the shallow sea -floor 
 to the foreshore. From this position, in dry weather, it is blown 
 further inland and so constantly continues the process of connect- 
 ing one isolated island or islet with another and sheltering the 
 low-lyins; ground behind from the inroads of the sea. (See 
 Plate VII.) 
 
53 
 
 APPENDIX. 
 
 Bibliography. 
 
 I ist of Principal Works on the Geology of the District. 
 
 1750. 
 Heath, R. A Natural and Historical Account of the Islands of Scilly, and a 
 General Account of Cornwall. 8vo. London. 
 
 1754. 
 
 Borlase, Rev. W. Ad Account of the great Alterations which the Islands 
 of Sylley have undergone since the Time of the Ancients, who mention 
 them, as to their Number, Extent, and Position. Phil. Trans., vol. xlvii., 
 p. 55. 
 
 1756. 
 
 Observations on the Ancient and Present State of Islands of Scilly- 
 
 4to. Oxon. 
 
 1758, 
 The Natural History of Cornwall. Pol. Oxon. 
 
 1818. 
 Majendie, A. Not3S on the Coast West of Penzance, and on the Structure 
 of the Scilly Islands. Tran >\ Roy. Oeol. Soc. Cornwall, vol. i., p. 27. 
 
 18.", 9. 
 
 De la Beche [Sir] H. T. Report on the Geology of Cornwall, Devon and 
 West Somerset. (Oeol. Survey.) 8vo. London. 
 
 1850. 
 
 Carne, Joseph. On the Geology of the Islands of Scilly. Trans. Boy. Oeol. 
 Soc. Cornwall, vol.vii., pp. 140-154. 
 
 1858. 
 
 Smith, A. On the Chalk Flints and Green sand Fragments, found on the 
 Castle Down of Tresco, on© of the Islands of Scilly. Trans. Roy. Oeol. 
 Soc. Cornwall, vol. vii., p. 343. 
 
 1859. 
 Statham, Rev. P. T. On the Geology of the Scilly Isles. (Brit. Assoc.) 
 Geologist, vol. ii., pp. 12-27. 
 
 1870. 
 Richardson, C. T. On a Visit to the Scilly Isles. Proc.Geol. Assoc, vol. ii., 
 pp. 36, 37. 1879. 
 
 Scott, L. and H. Rivington. The Agriculture of the Scilly Isles. Journ, 
 Roy. Agric. Soc, ser. 2, vol. vi., pp. 374-392. 
 
 Ussher, W. A. E. Historical Geology of Cornwall. Oeol. Mag., pp. 27-36. 
 
 1884. 
 Hunt, Robert. British Mining. 8vo. London, [Scilly Isles, pp. 9-15 
 
 8675. c 2 
 
36 
 
 INDEX. 
 
 Agriculture, 2. 
 
 Albite-oligoolase, 5 
 
 Alluvium, 28. 
 
 Annet, two types of granite in, 4 ; 
 
 glacial deposit, 26 ; raised 
 
 beach, 16. 
 Apatite, 5, 10, 13, 14. 
 
 Biotite, 5, 9, 13. 
 Blown sand, 28-30, PI. VII. 
 Borlase, Eev. W., 3, 10, 31, 35. 
 Bread and Cheese Cove, PI. V. 
 Bryher, fine granite, 4. 
 Bubbles in quartz, 5. 
 Building-stone, 2. 
 
 Came, J., 3, 12, 31, 35. 
 
 Cavities in quartz and felspar, 5. 
 Chalk-flints, 15, 21, 23-26. 
 Climate, 2, 27. 
 
 Conglomerate — see Raised Beach. 
 Cornish Coast, Raised Beach of the, 
 
 15. 
 Costean shafts, 10. 
 Cracks — see Fissures. 
 Cretaceous flint and chert, 15, 21, 
 
 23-26. 
 Cromwell's Castle, 10, 26. 
 
 De la Beche, Sir H. T., 1, 3, 4, 12, 13, 
 
 31, 35. 
 Drift, 15-31. 
 Druids' hollows in granite, 12. 
 
 Elvan, 12. 
 Eocene gravel, 15, 
 Escallonia hedges, PI. VII. 
 
 Felspars, in granite, 4, 5, 8 ; in 
 
 elvan, 13. 
 Pishing, 2. 
 Fissure lines, north and south, 11. 
 
 see also Greisen. 
 Flower-culture, 2, PI. VII. 
 Foreign stones, 23-25. 
 
 Ganilly, Great, 4, 8-11. 
 Giant's Punch Bowl, 12. 
 Glacial deposits, 21-31, PI. V. and 
 
 VI. 
 Granite and associated rocks, 4-14, 
 
 Granite, arrangement of felspars in, 
 
 4 ; coarser, 4, 5 ; finer, 6, 7 ; in- 
 clusion and intrusions in, 7-10 ; 
 oval patches in, 7 ; schorl bear- 
 ing, 5 ; veins in, 7, 8 ; waste, 
 See Blown sand ; weathering of, 
 12, PI. III. and IV. 
 
 Greensand-chert, 15, 23, 24, 26. 
 Greisen lines, 9-11, PL VI. 
 
 Haldon (gravels of), 15, 24. 
 
 Halos, pleochroic, in mica, 5 ; in 
 
 schorl, 8. 
 Head, 17-19, PI. V. and VI. 
 Heath,R., 3, 20, 35. 
 Hell Bay, 16. 
 Horticulture, 2, PL VII. 
 Hunt, R., 35 
 
 Industries, 2. 
 Interior sea, 1, PI. I. 
 Iron-cement, 19-21. 
 
 Joints, 9-11. 
 
 Kettle and Pans, 12, PL III. 
 Killas, encircling granite, 2, 13, 14 
 
 fragments in drift, 24, 25 
 Kitchen-midden, 29. 
 
 Limon of Brittany, 27. 
 Lithia in mica, 8. 
 
 Majendie, A., 3, 11, 13, 35. 
 
 McCullooh, Dr., 12. 
 
 Market gardening, 2. 
 
 Mermekitic pegmatite, 5. 
 
 Micropegmatite, 13. 
 
 Micropoikilitic, 13. 
 
 Morlaix, Limon near, 27. 
 
 Muff, H. B., 17. 
 
 Muscovite, in elvan, 13 ; in granite, 
 
 5 ; in greisen, 9. 
 
 Old Town, 11, PL II. 
 Oligoclase, 5. 
 Orthoclase, 5. 
 Oval patches in granite, 7. 
 
 " Peach '' in griesen lines, 7. 
 Peninnis Head, 40, 42, PI. in. and 
 
 Perthitic structure, 5, 
 
37 
 
 Pinite, 13. 
 
 Platform of Old Beach, 16, 17. 
 Pleochroic halos, 5, 8. 
 Pliocene age of table-land, 15. 
 Pollard, Dr. W., 11. 
 Population, 2. 
 
 Porthloo, el van near, 13 ; fine granite, 
 6 ; head at, 19. 
 
 Quarries, 2. 
 
 Quartz in granite, 4, 5. 
 
 Quartz-porphyry, 12. 
 
 Radley, E. G., 20. 
 
 Rainfall, 2. 
 
 Raised Beach, 15-17. 
 
 Ramifying sills, 8. 
 
 Recent movements, 32, 33. 
 
 Reid, C, 21, 24. 
 
 Richardson, C. T., 35. 
 
 Rivington, H., 35. 
 
 River gravels of Devon and Dorset, 
 
 15. 
 Rocks, list of, 3. 
 Rutile hairs, 5. 
 
 St. Agnes, section of drift deposits, 
 20 ; schorl films in granite, 11. 
 
 St. Brieux, Limon near, 27. 
 
 St. Helens, foreign stones on, 25 ; 
 well on, 2. 
 
 St. Martins, blown sand, 30, PI. VII. ; 
 PI. V. ; glacial deposit, PI. V., 
 23-25 ; granite veins on, 8 ; 
 iron-cement, 20. 
 
 St. Marys, blown sand, 29 ; granite 
 veins on, 8 ; iron-cement, 20. 
 
 Samson, fine granite on, 4 ; blown 
 sand on, 29 ; foreign stones on, 
 26. 
 
 Sandstone pebbles in drift, 22-24. 
 
 Schorl, 4, 5, 6, 8, 10, 11, 13, 14. 
 
 Scott, L., 35. 
 
 Smith, A., foreign stones, 27, 35. 
 
 Spectroscopic reaction for lithia, 6, 8. 
 
 Statham, Rev. F. T., 27, 30, 32, 35. 
 
 Strahan, A., 33. 
 
 Submarine contours, Frontispiece, 31. 
 
 Table-land, 13, PI. II. 
 
 Tean, fine granite veins, 8. 
 
 Tiddeman, R. H., 33. 
 
 Tin ore, 10-11. 
 
 Tourmaline, see Schorl. 
 
 Tresco Abbey, exotic plants at, 2 ; 
 iron cement, 21. 
 
 Tresco, blown sand, 28, 29 ; finer 
 granite, 4 ; foreign stones, 
 25 ; old tin workings, 10. 
 
 Ussher, W. A. E., 35. 
 
 Veins in granite, 7, 8. 
 Vermicular pegmatite, 5. 
 
 Water supply, 2. 
 
 White Island, killas on, 13, 14 ; 
 raised beach, 16 ; section of drift 
 deposits, 22, and PI. VI. 
 
 Whitley, N., 21. 
 
 Wright, W. B., 17. 
 
 Zircon, 5, 6. 
 
Plate II. — View taken a little south-east of Old Town, St. Marys, 
 looking west. Shows the north and south trending boundaries of 
 land and water, and of inland features. Also the table-land of the 
 central granite mass of St. Marys. 
 
Geology of Isles of Stilly. 
 
 Plate II. 
 
 
Plate III. — Kettle and Pans, near Peninnis Head, St. Marys. To show 
 weathering of granite. These hollows were claimed by Borlase as the 
 sacrificial altars of the Druids. 
 
Geology of Isles of Stilly. 
 
 Plate III. 
 
 D 
 < 
 s 
 
 <5 
 
Plate IV. — West aide of Peninnis Head. Shows vertical fluting of the 
 granite, and in addition the horizontal weathering so characteristic of 
 the "Cams" or "Tors" of the Mainland. 
 
Geology of Isles of Scilly. 
 
 Plate IV. 
 
Plate V. — Bread and Cheese Cove, west of St. Martin's Head. The 
 Glacial deposit and the Head are seen just above the shore line at the 
 foot of the bare granite slope on left of photograph. 
 
Geology of Isles of Scilly. 
 
 Plate V. 
 
 a 
 < 
 
 w 
 
 > 
 
 o 
 O 
 
 Q 
 
 
 < 
 
 re 
 
Plate VI. — Gully at south end of White Island, St. Martins. The gully 
 has been eroded along a " Greisen-line " in the granite, and the sea is tc 
 the left, or on the opposite side of the ridge forming the sky-line. On 
 top of the granite, to the right, and continuing across nearly two-thirds of 
 the photograph is a small band of rounded pebbles, forming the 
 " Old Beach." The angular material above is the main Head and on 
 this is the fine Iron-cement, containing patches of pebbles, some 
 glacially straited. One of these patches is shown near tbe right edge of 
 the photograph. 
 
Geology of Mw of Scill !t . 
 
 Plats VT. 
 
Plate VII.— South-west end of St. Martins. Showing bars of Blown- 
 sand, keeping out the sea from the low-lying land behind. This ground 
 is devoted to flower- culture, and a number of high hedges of Escallonia 
 are shown, grown to break the force of the wind. 
 
Geology of I.sfes of Sri lit/. 
 
 Plate VI 
 
■ „„, . GENERAL MEMOIRS. 
 
 SUMMARY OF PROGRESS of the GEOLOGICAL SURVEY for 189V, 1898, 1899, 1900, 1901, 1902, 1903, and 1904. Each Is. 
 
 PLIOCENE DEPOSITS of BRITAIN. By C. REID. 5*. 6(2. 
 
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53 SE - 
 53 NE • 
 
 65 • 
 
 66 SW . 
 
 67 NW- 
 67- - 
 
 68 E - 
 
 68 WW, SW 
 
 70- 
 71 NE 
 79 WW 
 
 79 SE 
 
 80 NW 
 
 80 SW 
 
 81 NW, 
 
 82 SE 
 82 NE ■ 
 
 SW 
 
 SW 
 
 81 - 
 
 86 • 
 
 87 NW 
 
 87 SW 
 
 88 SW 
 88 SE 
 
 88 NE 
 
 89 SE 
 
 90 SE 
 
 90 NE 
 
 91 SW 
 
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 93 NW 
 93 NE 
 93 NW 
 93 SE, 94 
 91 NW 
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 95 SW, SE 
 
 95 NW 
 
 96 SE - 
 
 96 NE - 
 96NW.SW 
 
 97 SW - 
 
 97 NW- 
 
 98 NE - 
 102 SW - 
 101 SW, SE 
 
 107 ' ■>. 
 
 108 SE - 
 108 NE - 
 108 SW - 
 
 no sw - 
 
 no nw.- 
 no ne - 
 
 110 SE - 
 
 123 
 
 111 
 
 165 
 156 
 
 218 
 219 
 
 311 
 326 and 
 
 317 
 
 331 
 350 
 355 & 356 
 
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