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 pro- montories 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 our- selves, 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 appear- ance 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 horn- blendic 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.