CHAPTER X. GEOLOGY AND MINERALOGY OF THE CHANNEL ISLANDS. ANCIENT FORMATIONS. IN no part of Europe, and in no group of islands readily accessible, are the physical geography and geology more closely related than in the Channel Islands. The tongues and spurs of porphyritic rock, of which almost all the promontories of the Atlantic coast of Europe consist, are well illustrated in Western Britany and in Cornwall, and not less so in the intervening rocky groups. Better able than most of the secondary rocks to resist the constant action of the sea, these rocks show themselves, partially denuded and generally rugged, in the neighbourhood of Cherbourg and Cape la Hague, but they appear in perfection in a succession of spurs pointing westward, forming the groups which together compose the islands we are considering. Exposed from their isolated position to the never ceasing beating of the waves during their elevation, these islands have been deprived of most of the covering of stratified rocks they may once have possessed, and except a patch here and there in the more sheltered positions, scarcely any indication exists of those deposited rocks that abound on all sides of them, and give variety to the geology of England and France. None of the old fossiliferous rocks appear, not a particle of stratified limestone of the secondary period is to be seen, and not a single patch of the old tertiaries that are so nobly represented on the opposite shores of Hampshire, and in the Isle of Wight, has ever been discovered. All is rude and bare and the mere skeleton of that rich and varied structure to which we are elsewhere accustomed; so that at first sight it may seem as it has indeed been generally stated that the geologist has here little opportunity for investigation, and need only record the fact of the existence of such and such varieties of rock in certain localities. But the comparative anatomy and natural history of the earth, like that of its inhabitants, must be learnt from the skeleton. It is where the old, hard, nether-formed rocks frowningly appear above the water, where they are exposed in broad tabular masses laid bare by the waves, unclothed with soil, uncovered by sand and pebbles, that this history can best be learnt. It is there that natural sections of them may be studied, showing all varieties of early structural change, and the later action of water, and of constant change of temperature. Under such circumstances, when they form hills rather than mountains, and cliffs that may be approached rather than precipices that can be looked on only from a distance :—where they have been exposed to all varieties of change and disturbance-where they have been undermined and torn into shreds, but are still struggling with apparent success against their inevitable fate, that we read most of the great lessons they were intended to teach. For such reasons, and used in such a manner, the Channel Islands afford an admirable school for the geologist. The field is one which has not too many cultivators, but it will amply repay any amount of labour. There is, probably, no area of equal extent even in England, that presents more variety of detail, or from which more may be learnt. The points of interest, although numerous and important, NATURE OF THE ENQUIRY. 249 are concentrated at the two extremities of the geological scale. They involve inquiries concerning the mode of production and subsequent change of most varieties of crystalline rock and mineral veins, and thence they pass at once to the latest changes that have taken place upon the earth. On the one hand, we have numerous phenomena exemplifying the laws of composition and the original structure of the rocks called igneous, such as the syenites and porphyries, and other hypogene rocks. We learn the history of the stratified form of these rocks; the nature, extent, and cause of the various systematic clefts or fissures by which they are very generally intersected; the minerals and rocks by which the fissures have since become filled. We may learn, also, the nature of those modifications by which the rocks and systematic fissures or veins have been affected since their original formation, and the external or mechanical changes, whether of squeezing, elevation, depression, or weathering, either by sea wave or atmospheric exposure, to which they have been subjected. The consideration of questions of this kind is in itself of the highest importance in geology, and it leads to results of great practical value connected with the presence of metaliferous wealth, the applicability of stones for various purposes, and the origin and nature of soils and subsoils. There is another department of geology, in regard to which the Channel Islands are full of instruction. This relates, as we have stated above, to the last changes that have taken place upon the earth; it considers the disturbances and deposits whose date is historical rather than geological, and which result from causes still in operation at the same place. The deposit of peat, or of rolled pebbles and stratified sand; the removal of other similar deposits already bedded; the breaking through of barriers, more or less natural, and the introduction and operation of various forces, visible and calculable; the mode in which rocks are undermined, weathered, broken up, and carried off in fragments by the sea; and the small upheavals or depressions imperceptible to the eye, but of recent date, which have helped or hindered the sea and the weather in their destructive course: these together form a class of phenomena, which are, as it were, the very grammar of geology; phenomena which cannot be studied too closely, or known too familiarly, and which afford subjects for investigation, not less interesting and useful than any others. Since, however, the two classes of geological phenomena in the islands are so distinct, they may with advantage be considered separately. Let us begin with the group first alluded to, the crystalline and metamorphic rocks, including syenites and porphyries, together with shales, schists, slates, sand-stones, and conglomerates, of whatever date. Metamorphic Geology of the Channel Islands. It may be well to state, that by the term metamorphic, is here meant not that the rocks so called have lost all evidence of originally mechanical origin, but that their early history, mechanical or otherwise, has been either masked, or is obliterated by subsequent chemical action on a large scale. No doubt most, if not all, were originally mere mixtures of various common minerals, quartz or flint, clay, lime-stone, magnesia, iron-oxides, potash and soda, manganese and fluorine, in various proportions, compacted together under certain conditions of temperature and pressure. These materials have, since their original accumulation, been exposed for a long time to the action of uniform high temperature in the presence of water, and have been acted on by currents of magnetic electricity circulating near the surface of the earth. In this way they have in time been elaborated into rocks, and the whole compound mass has subsequently undergone important changes. Out of the materials just named, or some of them, have been |