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vice is of the nature of a bleb or closed cavity, such as is seen frequently in glass or lava, the inside will be filled by crystals shooting inwards from the walls towards the centre. As, however, the uplifting of a mass of granite to the surface from the place of its formation slowly progresses, the very veins themselves that have been filled up will also become split open, and new fissures will be formed, rending asunder the material that has already filled chasms of older date. More frequently, systems of cracks will cross other systems, producing a great apparent complication. Many of the crevices of later date will, in time, become filled up from below; while others, not accessible directly from below, will be acted on indirectly by vapours, or by structural changes produced in the mass of the rock, tending to separate out the various accidental substances that it contains from the rock itself, and collect them in the empty spaces. It will also occasionally happen that a wide crevice becomes filled with fragments of rock fallen in from above. Thus is produced the vast net-work of veins or intersecting chasms, some empty, but most of them filled up with foreign substances, so characteristic of all districts where granite abounds. These phenomena are admirably illustrated in the Channel Islands.

But the mechanical force acting from below that has brought up the granite to the sea level must, in doing so, have squeezed its upper surface against a heavy overlying weight of rock and water before overcoming its inertia. Thus if formed ten miles beneath the surface, (a depth only double that of the highest point of land above the sea) the mere weight of the overlying material, supposing half that depth to be rock and the rest water, would amount to 2,500 tons on every square foot of surface; and the pressure from below sufficient to overcome this, and lift up the mass, would inevitably produce the greatest mechanical change in those rocks having the smallest amount of elasticity. The effect of pressure on plastic matter is known by actual experience to produce that fissile structure of which slate

is the best example, and it may, in this case, have originated the gneissic varieties of granite and porphyry especially common in Guernsey.

The systems of crevices and fissures traversing the granitic rocks; the compass-bearing of the principal veins, the materials with which they are filled, and the relation these bear to the enclosing rocks; the nature of the subordinate veins and their contents; the threads of quartz that form the final delicate interlacing; the passage of certain rocks into each other, and the transfer of materials originally contained in the larger fissures into those smaller crevices traversing them; the presence, in certain cases, of so large a proportion of metaliferous mineral as to give to the veins the character of ores ;-such are the chief points that will require notice in this division of our subject. They must be alluded to systematically in reference to each of the islands.

In a general sense, the rocks will be regarded as syenites (quartz, felspar, and hornblende), or other varieties of porphyry, according to their composition. The hornblendic rocks, consisting of hornblende and felspar (the former known by its dark green colour, and the latter by its flesh tint), pass into greenstones, and then, by the replacement of actinolite (a pale green mineral) and prehnite or epidote, assume new and very characteristic forms. It may be convenient for the reader to be reminded that magnesia is the elementary substance chiefly concerned in these modifications; and that asbestos, talc, serpentine, and that curious soap-like mineral, called steatite, or potstone (abundant chiefly in the middle of Sark, but found elsewhere), are all minerals in which the magnesia element prevails. In all these minerals there is also more or less of a green colour, and soft saponaceous touch observable. Chlorite is an abundant rock everywhere. It is a silicate and sub-aluminate of magnesia and iron.

No one can walk much along the shore, or climb the cliffs of

any of the Channel Islands, without being struck by the tendency in many of the rocks to assume either a tabular or terrace form. This is expressed sometimes by calling the rocks trap, from the Swedish word trappa, applied by the earlier geologists to basaltic and other volcanic rocks. It is, however, an expression that is not unlikely to mislead, as connecting the greenstones with rocks with which they have no other relation than that of form. There are, no doubt, in the islands many examples apparently indicating the presence of lava or recent volcanic

action, but these are deceptive, there being no relation between the blackened surfaces and cellular rocks alluded to and any true igneous rock.

Striking examples of intrusive horizontal veins of greenstone may be observed in all the principal islands. In Alderney, an example has already been figured in the title to the first part of this work (page 1). In Jersey, equally picturesque but very different illustrations are seen, both on the north coast, near la Houle, and on the north-west coast, near Cape Grosnez. In Guernsey, the engraving on the last page is an instance not less picturesque than instructive of an almost identical phenomenon.

In all these cases the nearly horizontal veins simulate very closely the appearance of steps, and are therefore trap rocks in one sense, though certainly not by any means in the sense of vesicular lava connected with a volcanic eruption.

The granites, syenites, and other porphyritic rocks of the coast of Britany recur, as we have already reminded the reader, in the Cotentin as well as in Cornwall. The bottom rocks of the Channel, if we judge only by the islands and rocks that rise above or approach near the surface, are of similar material; but it by no means follows that this is the case uniformly across the whole sea. The great east and west elevations affecting the continent of Europe, and well illustrated in the lofty chains of the Alps and Pyrenees, have no doubt been the main causes of the hard and rugged promontories that jut into the Atlantic ocean at so many points. But the dying out of the east and west elevatory force is marked by the diminished height of the granite as we advance westwards. A glance at the map accompanying this volume, and a moment's consideration of the meaning of those shadings which reveal the shape of the sea bottom within certain limits of depth, will form the best introduction to a detailed study of the geology of the Channel Islands. It will there be seen, that if a uniform elevation of any extent were now to take place in this part of the sea, the form of the French coast

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would be greatly modified; for such elevation, even if to a very small extent, would include the largest island and the largest shoals as parts of the main land of Europe. While, however, ten fathoms would do this, a much greater elevation would do very little more.

On the north side of the northernmost of the promontories that would exist if the elevation were sufficient to include all the islands, there yet remains in numerous patches a considerable part of a deposit of sand-stone that was once connected with the main land of France; but the rest of this northern promontory consists of a hard syenitic and porphyritic floor, on which the sand-stone has reposed,-the banks of sand in the sea adjacent representing all that remains of the rock once covering the floor.

The islands of Guernsey, Herm, and Sark, with the smaller islands and rocks adjacent, would, by the elevation assumed, form another promontory. No deposits more recent than the old syenites and gneiss, except a few modern sea beaches and some accumulations of sand and fallen rock on the side of a cliff, can now be found on any part of this tract.*

The island of Jersey, the Minquiers, and the Chaussey islands would be far within the coast line. Jersey is much less denuded than Guernsey, and many varieties of crystalline and metamorphic rock and some deposited sand-stones and conglomerates still remain, and indeed cover a large proportion of its surface; but the Minquiers and Chausseys are mere shreds of syenite.

Now, let the reader imagine, instead of a sudden elevation of the present sea-bottom, that at a certain period the whole district was 180 feet lower than at present, but was then partially covered with rocks concealing those now visible. Let him also suppose a gradual elevation to take place, exposing the surface as it emerged to the action of the waves. The result must be the production of the present state of things, namely, a number

* A small patch of clay slate in Rocquaine Bay is hardly an exception to this remark.

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