Geology of the Channel Islands

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J. A. Birds—Geology of the Channel Islands. 79 The deposits referred to occur now as little patches within the area bounded by the great terminal moraines. As physicists themselves are not yet quite agreed upon the subject of glacier-motion, it is not incumbent upon the geologist to explain the precise mode in which a thick mass of ice can creep over the surface of incoherent beds without entirely demolishing them. It is enough for him to show how the remarkable distri- bution of the interglacial beds, and the various phenomena presented by these deposits, indicates that ice has overflowed them. It is useless, therefore, to tell him that the thing is impossible. The \ statement has been made more than once that an ice-sheet several | thousand feet thick is a physical impossibility, but unfortunately for I this dictum the geological facts have demonstrated that such massive I ice-sheets have really existed, and there appears to be one even now I covering up the Antarctic Continent. We used also to be told, not so many years ago, that the abysses of ocean must be void of life for various reasons, amongst which one was that the pressure of the water would be too great for any living thing to endure. Yet many delicate organisms have been dredged up from depths at which the pressure must certainly be no trifle. Now there seems to be just as little difficulty in believing that these organisms existed in a perfect state at the bottom of the ocean, as that shells imbedded in clay would remain unbroken underneath the pressure of a superincum- bent ice-sheet of equal or greater weight. If the ice were in motion, the clay with its included shells might be ploughed out bodily, or be merely crumpled and contorted; or it might be ridden over with little or no disturbance ; or, on the other hand, it might become in- volved with subglacial debris, and be kneaded up and rolled forward—the shells in this case being broken, crushed, and striated, just as we find that the shells in certain areas of Till have been. The fate of the fossiliferous beds would, in short, be determined by the rate of flow and degree of pressure exerted by the superincum- bent quasi-viscous body—the motion of which would be largely controlled by the physical features of the ground across which it crept.

V.—GEOLOGY OF THE CHANNEL ISLANDS. By J. A. BIKDS, B.A. Part I.~The Older Bocks. N the most extended view, the Channel Islands may be regarded as fragments and relics of the Eastern or European coast of the AtlanticI , reckoning from the North Cape to Cape St. Vincent, and including the "Western shores of Scotland and Ireland, and the promontories of Pembrokeshire and Cornwall. They are excellent illustrations, says Professor Ansted, " of those spurs and tongues of porphyritic rock, of which almost all the promontories of the Atlantic coast of Europe consist."1 Very small and insignificant specks indeed they seem in such a length of coast, stretching from 1 The Channel Islands, by Ansted and Latham, 1862, p. 247.

Downloaded from https://www.cambridge.org/core. INSEAD, on 13 Oct 2018 at 17:28:50, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800146357 80 J. A. Birds—Geology of the Channel Islands. lat. 37° to 72°, or upwards of 2000 miles; but there is a charm in such wide horizons, and it is a very allowable indulgence so to connect the little with the great, and to consider the position of such little specks in relation to the geography of Europe; one might almost as well say, of the world at large. Having refreshed ourselves, however, with such a glance at their widest geographical relations, we must be content to confine our view within a very much narrower compass, and to consider these islands simply as relics of a tract, which once formed part of what is now Normandy and Brittany; just as the Scilly Isles and Lundy Isle are relics of an area which once was connected with Cornwall and Devonshire— the original and actual basis indeed of Arthur's legendary kingdom of Lyonnesse. This itself is no narrow view; but even if it were, still we are not under any necessity—as in geology one never is—of losing a great horizon and burying ourselves in a mass of details; the only difference is that we must change our point of view, and regard the area under consideration in the aspect, not of space, but of time. Viewed in this way the series of rocks become so many visible and tangible links, or landmarks rather—the chain being so broken—leading the mind back into an almost infinite past. It is not my purpose, of course, in this paper, to attempt any- thing like a complete account, or even a resume, of the geology of these isles; that may be fully gathered from such articles as Dr. MacCulloch's, and others in the Transactions, Proceedings, and Quarterly Journal of the Geological Society ; from Duncan's " His- tory of Guernsey"; and, above all, from Prof. Ansted and Dr. Latham's work on the " Channel Islands." All that I desire to do is, to ask one or two questions about points which seem to me of chief interest and importance, to bring together information from the above works and elsewhere upon these points, and to add to this the result of my own observation during the latter part of last year. The first question is as to the age of the granites, or rather syenites, ' granitals,' schists, porphyries, sandstones, etc., of which the islands consist. Besides Prof. Ansted's conjectures that the grits and sandstones of Alderney are probably of Permian or Triassic age,1 and the older conglomerate of Jersey of the age of the Cherbourg grits2 (Bunter or Lower Trias), I am not aware of any observations having been published as to the date of the rooks. Looking at a geological map of the north of France, or of Normandy and Brittany, with which the islands stand in closest connexion, one would conjecture a priori that the sedimentary rocks at least were either Silurian or Devonian, more especially when we observe what is probably a continuation of these rocks across the Channel in Devonshire and Cornwall.3 The only positive evidence in favour of such a supposition, that I have seen, is a small patch or 1 The Channel Islands, p. 269. 2 Ibid, p. 274. s See an article by the late Mr. Salter on " The Pebble Bed at Buddleigh Salterton," GEOL. MAG. 1864, Vol. I. p. 5, etc.

Downloaded from https://www.cambridge.org/core. INSEAD, on 13 Oct 2018 at 17:28:50, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800146357 J. A. Birds—Geology of the Channel Islands. 81 rather one or two bands of schistose strata which make their appearance in Rocquaine Bay, Guernsey ; and, again, the schists or shales of Jersey. In the former case the rocks are of a bluish-grey colour, weather- ing brown, and have at first sight a general Silurian or Devonian aspect. They are referred to by Prof. Ansted in a note concerning the absence of any deposits in Guernsey more recent than the fun- damental syenites and gneiss : " A small patch of clayslate," he says, " in Rocquaine Bay is hardly an exception." This so-called patch (where I examined it) consists of two or three bands of slate, which are imbedded in the midst of a felspathic syenite, and dip at a very high angle to the east. I traced them for about fifty or sixty yards on the land side of the fort called Rocquaine Castle till they became lost under the sea. Probably, at low water, thej' may be found in other parts of the bay. The cavities left by decomposed felspar have often a very deceptive resemblance to casts of Brachiopoda, and to encrinital remains; but I searched in vain for any trace of fossils. Possibly these may yet be discovered. Should this not be the case, the schists will not, of course, yield any evidence of their age, or of that of the felspathic and hornblendic rocks amid which they lie. In a mineralogical point of view, however, they would still be interesting, as showing a stage in the progress of metamorphism, viz. a passage from sedimentary schists into a greenish-grey porphyritic rock containing crystals of felspar and calcite (?). Alderney, according to Ansted,1 besides the portion of sandstone above referred to, consists entirely of syenite, with the exception of a single boss of hornblendic porphyry, upon which Fort Touraille is erected. Ortach and the Casquets, although few, if any, geologists have landed to examine them, are believed to consist of similar syenite or porphyry with cappings of the same sandstone as Alderney. It is said that the syenite enters and pierces the sandstone in the Casquets—a very important point—but one which requires confirmation. Of Guernsey I can speak from personal observation. It is di- visible geologically into two or three veiy unequal portions by a line drawn N.W. from some quarries just above St. John's Church, in the town of St. Peter, to some other quarries below Capelles School; and thence again S.W. to a quarry on the right or north side of the Cobo Road, close to Cobo Bay. About three-quarters, or rather four-fifths, of the island south of this line consists of a very felspathic syenite and gneiss, and the remaining quarter or fifth of a hornblendic ' granital'—a compound of quartz and hornblende. A third division consists of a syenite often charged with specks and flakes of mica. This micaceous syenite may be traced along the coast from near the centre of Cobo Bay and Grandes Rocques almost to Fort Doyle, at the north-eastern extremity of the island. In the centre of Grand Havre, however, and on Mont Cuet—between Grand Havre and Lancresse Bay—and perhaps at some other points 1 The Channel Islands, p. 268. DECADE II.—VOL. V.—NO. II. 6

along the shore, the syenite is interrupted by veins of the' hornblendic granital. It does not extend far inland, but is soon replaced by the mass of hornblendic granital constituting the north of the isle. Of the felspathic portion of Guernsey I have nothing to remark, more than that it furnishes magnificent crystals of felspar, especially in Moulin Huet and Petit Bot Bays, surpassing in size and brilliancy of colour any in the porphyritic granites of Cornwall, or Shap Fell, Cumberland. The cliffs are frequently intersected by veins of greenstone and felstone running both parallel with, and at right angles to the apparent bedding or jointing of the syenite. The hornblendic portion of the island also occasionally {e.g. at St. Sampson's) affords splendid crystals of black hornblende, finer indeed than any I have ever seen in England or Scotland. The principal veins in this portion are of felstone, serpentine steatite, and epidote, of which last I found some rather pretty crystals imbedded in quartz. Chlorite frequently occurs in the form of coatings and stains. The principal axis of elevation of the whole area of the islands'is said by Prof. Ansted to run W.N.W. and E.S.E., and is regarded by him as a continuation of the great east and west elevations affecting the Continent of Europe, which are best illustrated in the range of the Alps and Pyrenees.1 The little island of Sark consists of syenite, with similar veins, as in Guernsey, of greenstone, felstone, serpentine, steatite, etc., and of hornblendic rocks, along with porphyry and trap. The hornblende is said to occupy the extreme ends, and to form a belt across the centre of the island, while the felspathic syenite fills up the intervals. The axis of elevation here, and also in the little islands of Herm and Jethou, between Sark and Guernsey, is nearly at right angles to the axis of the latter, or N.N.E. and S.S.W., and corresponds, says Ansted, rather with recent elevations and depressions than with the original upheaval.2 I was not fortunate enough to find any, or at least good specimens of the minerals with which Sark is said to abound. The old mine- heaps appear to have been thoroughly ransacked, and the shores are inaccessible except at a few points; only on the beach at Epercherie —the original landing-place—I picked up specimens of fine black hornblendic porphyry containing well-defined crystals of felspar, and a few pebbles of agate and jasper, generally much decayed, as well as some of serpentine, and a green mineral, which I take to be actinolite. The vein of kaolin, stained purple and pink, crossing the north side of the Coupee, is very conspicuous. Dr. MacCulloch, in his map of the island, indicates three other veins,— 1, of quartz, chalcedony, jasper, and agate; 2, talcose schist with steatite; 3, chlorite with pyrites; crossing the Coupee parallel with the kaolin. None of these are visible from above, one side being covered with detritus, and the other plunging down perpendicularly into the sea. 1 Ansted and Latham's Channel Islands, pp. 256, 260. » Ibid, p. 263.

WEST EAST.

Length of section 11 miles. i. Firm syenite, b. Rotten syenite, c. Quartzite. d. Indurated shale, e. Shale. /. Hornstone schist, g. Old conglomerate, h. Newer conglomerate, i. Sand- stone, k. Raised beach. I. Pebble beach, m. Blown sand. The syenite, indeed, forms the basis of all the other rocks. It does not, however, as might be supposed from the section, protrude at three distinct points, the intermediate spaces being covered with 3hale, but rather appears as three separate masses, in the north-west, south-west, and south-east of the island. The first and largest of these may be followed continuously along the coast from L'Etac, round by Grosnez and Plemont Points to Point Sorel, and the quarries north of St. John's Church; it is traceable inland by the quarries of Mont Mado, and a little south of the Wesleyan Chapel Balled Les Freres, to the head of St. Mary's Valley, near the fifth milestone, and thence almost in a direct line to L'Etac. The second mass occupies nearly the whole of the parish of St. Brelade, and forms the coast-line from La Carriere opposite La Eoeca Tower, in 3t. Ouen's Bay, to the Corbieres rocks; and thence to a point a ittle below Noirmont Manor House. The boundary of this mass inland is generally covered on the west by blown sand, but it ippears again between Tabor Chapel and St. Aubin's in the shape of i granital of quartz and felspar, very subject to decomposition, in which state it forms a fine china-clay. Quarries have been opened here, and the clay is exported to England. From this point the boundary may be traced S.E. to the shore again under Noirmont Manor. The third and last mass of the syenite forms the S.E. corner of the island, and may be followed along the coast from Fort Regent at St. Helier's to La Eocque Point. It is interrupted at two points, viz. at La Collette under Fort Regent, and in the headland :>f Samares, by veins of hornblendic granital. Between La Eocque Point and Mont Orgueil Castle the rocks are covered by the sands )f Grouville Bay; but the syenite, which here passes into a granital [quartz and felspar), may be traced inland by Gorey Church, and 1 The Channel Islands, p. 270. Geological maps of the islands there are none, it least I have seen none, except the sketch-maps appended to Dr. MacCulloch's ' Account of Guernsey and the other Channel Islands " in the first volume of the Jeological Society's Transactions. The Ordnance maps of Guernsey and Jersey are >n a scale of six inches to the mile, and somewhat expensive; but very good and :heap pocket maps may be had of Guernsey, of Messrs. Staddon and Grigg, High Street, St. Peter Port; and of Jersey, at any of the stationers there.

Downloaded from https://www.cambridge.org/core. INSEAD, on 13 Oct 2018 at 17:28:50, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800146357 84 J. A. Birds—Geology of the Channel Islands. along the upper road to Grouville, to the windmill above the village, and thence to the neighbourhood of Sion House (where it is very muoh decomposed)—and last to the quarry east of Victoria College, and the rocks underneath the College itself. Between these three great masses of syenite there lies upon the west a thick formation of shale or schist (e), which extends without interruption from St. Ouen's Bay to the western or lower branch of the Val des Vaux, close to St. Helier's. At this point it is broken by an outburst of volcanic rocks, which have altered it into a claystone porphyry, but it appears again in an unaltered form a little north of Sion House, and between Le Bourg and Petit Catillon, north of Grouville Church. The general character of the rocks is that of a bluish-grey argillaceous schist, with bands of hard grit. Sometimes (as near St. Helier's) it is a brown, sandy, finely-laminated shale. Occasionally I observed it take a cherty character, as, e.g., near St. Aubin's, on the right of the main road to St. Brelade's. The north-eastern portion of the island is occupied by a formation (/ and g of Prof. Ansted's section), variously composed of porphyries, hornstone-schist, altered sandstone, quartzite and quartzose conglomerate, which extends in a wide belt or broken arc from Fremont Point, above Bonnenuit Bay, and L'Etaquerel, in Bouley Bay, southward to within about a mile and a half of St. Helier's; and thence eastward by the Grand Val Mill, in the Val des Vaux, till it appears upon the coast again between Anne Port and St. Catherine's Tower, in St. Catherine's Bay. The points where it may be best examined are the quarries at La Crete Point in the latter bay, in a quarry a little north of Grand Val Mill, on the road from the latter to La Boucterie, in Blanche Pierre Quarry, in the lower or western branch of the Val des Vaux, in Bonnenuit Bay, and in the quarries on the Jardin d'Olivet above Bouley Bay. The quartzite and quartzose conglomerate are extensively quarried all along the cliffs between these two bays. Besting upon this formation (f and g), and occupying the whole of the extreme north-eastern corner of the island, from a little south of St. Catherine's Tower, in St. Catherine's Bay, to between L'Etaquerel and La Tour in Bouley Bay, lies a newer argillaceous breccia or conglomerate (A), composed mainly of chloritic slate and ferruginous sandstone, but with blocks of syenite, granital, quartz, and many other kinds of rock scattered throughout the mass. The boundary of the deposit inland may be traced from a little west of Carmel Chapel, on the road between the Jardin d'Olivet and Eozel, and near the lodge of Rozel Manor, and thence to a point about half-way between St. Martin's Church and St. Catherine's Bay, where it may be seen resting upon the altered sandstone (/). The finest sections, however, are those of the quarries at Verelut Point. Both there, and all along the coast to Eozel—but particularly in Port Saie—there are ample opportunities for its examination. On the left of the road descending to Bouley Bay, and also, according to Mr. Ansted, at the back of St. Catherine's Bay, there are two small patches of a still newer formation, consisting of a fine-grained

Downloaded from https://www.cambridge.org/core. INSEAD, on 13 Oct 2018 at 17:28:50, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800146357 J. A. Birds—Geology of the Channel Islands. 85 red sandstone with clayey and ferruginous bands. In Bouley Bay this sandstone rests unconformably, and almost horizontally, upon nearly vertical strata of the older sandstone or hornstone (/). Prof. Ansted believes this latest sandstone to be " quite modern," and to have been " deposited before " (? shortly before), " or during the last great elevation."' He does not, however, assign any reasons for his belief. The age, indeed, of all these rocks is as yet a mystery. Not a trace of fossils has been discovered either in the schist (e), or in the sandstone portions of (/), or in the later sandstone (t); only among the pebbles on the beach at Bouley Bay I found many containing what certainly look like the remains of corals and portions of shells. These have probably been derived from the newer conglomerate (h); and if their organic character were determined, it might throw some light on. the age of the conglomerate. As yef, however, one cannot even say whether the syenites are older or younger than the schist (e), it not being known whether they ever enter and pierce the latter, as in Cornwall the granites do the Devonian and Carboniferous rocks, and are thereby proved to be younger than them. The direction of the principal joints, nearly N. and S., is the same as that of the granites in Cornwall, but, this, being due to later causes, proves nothing, of course, as to the age of the original formation of the rocks. It is eaBy enough to invent theories as to the age and history of the various formations; and, in the absence of further evidence, this is about all that can be done. Dr. MacCulloch, in 1811, described the schist as grauwacke/ i.e. of Pre-Carboniferous age, and we may perhaps pretty safely assume that it is of some Lower Silurian period, coeval with the Silurian rocks of Normandy and Brittany. (Llandeilo, Caradoc, or Lower Llandovery), Of the sandstone, and its accompanying felstone and cherty porphyries, and conglomerate (/ and g), there is almost as little evidence as to age. From their general appearance one might imagine them to be of Old Bed Sandstone equally as well as of Permian or Triassic age. Only at one point, just above Le Bourg, did I discover the shale (e) and the felstone porphyry (/) in contact, and here it seemed to me that the shale actually overlay the porphyry. Of course this might have been due to a fault or inversion of the strata. But I can hardly think so in this case.. Can the porphyries have been eruptive and pierced into the shale ? From the section it does not appear whether the shale is older or younger than the sandstones and porphyries. Prof. Ansted conjectures, however, from the similarity of dip, that the newer Jersey conglomerate (h) and the Alderney sandstone may be of the same age: and, if so, that the older conglomerate (g) may perhaps belong to the Cherbourg grits (Bunter or Lower Trias). In this case the underlying porphyries and sandstones (/) might be somewhat older, or intermediate between the schist (e) and the conglomerate (g). 1 The Channel Islands, p. 275. 1 Account of the Geology of Guernsey and the other Channel Islands, Geol. Trans. 1st series, vol. i. 1st paper.

Downloaded from https://www.cambridge.org/core. INSEAD, on 13 Oct 2018 at 17:28:50, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756800146357 86 Notices of Memoirs—Prof. Liveing on Metamorphtsm. This is about all that conjecture at present can arrive at. There would seem indeed to be little hope of determining the question as to the age of the rocks from a study of the island alone, or of all the Channel Islands together by themselves. It can only be by comparison of their geology with that of the neighbouring parts of the Continent—of Normandy and Brittany—that we can expect a solution of the problem. In addition to the syenites, and the sedimentary and porphyritic rocks above described, there is a very important accumulation of volcanic rocks (trap, porphyry, and amygdaloid), in the immediate neighbourhood of St. Helier's, and which does not seem to have been noticed either by Dr. MacCulloch or Prof. Ansted. Com- mencing at Gallows Hill, on the west of the town, the series may be traced for about two miles northward, across both branches of the Val des Vaux, to near the Grand Val Mill. It occurs again between the two houses called the Hermitage and Bagatelle, in the quarry east of Victoria College, and at the foot of St. Saviour's Hill. It may be well seen in the quarries at the bottom of Gallows Hill, and on the road up the western branch of the Val des Vaux, where the focus of the eruption seems to have been. There is nothing to indicate the age of these rocks except the alteration of the shale (e) in their neighbourhood (as, e.g., on the Trinity Eoad, and other places) into a claystone porphyry, proving them to be more recent than it. The mention of this altered rock leads to another and final question; namely, what is the cause of metamorphism in the various rocks of Jersey ? It is not the syenite: for at, or close to, its junction with the shale (as, e.g., on the ascent by the path from St. Aubin's into the St. Brelade's Eoad), the latter is quite unaffected. The volcanic rocks may account for the alteration of the shale (e) into claystone-porphyry ; but they do not account for the felstone and* hornstone-porphyries (/). which exhibit the same character, both in the immediate neighbourhood of the trap (e.g. in the quarry N. of Grand Val Mill), and miles away from it (e.g. at La Crete Point, in Blanche Pierre Quarry and in Bonnenuit Bay). These would seem to1 be porphyries belonging properly to the sandstone series (/), but whether contemporaneous or intrusive I am unable to say. Here again the study of the geology of Normandy and Brittany is essential to the understanding of that of Jersey and the other Channel Islands. KOTICBS O3? ZMZZEHyCOIIRS.

ON THE METAMOBPHISM OP THE BOCKS OP THE CHANNEL ISLANDS. By Prof. LIVEING, etc., etc. PAPER was recently communicated to the Cambridge Philo- A sophical Society (Oct. 29th, 1877) by Prof. Liveing, in which the author traced the connexion of the rocks in Guernsey, and pointed out the extreme variations in the amount of change these rocks and some of those in the other islands had undergone, from