derived the following simple minerals and rocks, found in more or less abundance throughout the Channel Islands-felspar, mica, chlorite, hornblende, serpentine, steatite, quartzite and horn-stone, syenite, granite, and various porphyries; besides argillaceous and chloritic schists, and many others. All these occur here, much as they are found elsewhere, and may be described in chemical language as compound silicates of alumina, magnesia, lime, and other bases. These, have, perhaps, originally formed sand-stones, lime-stones, and clays; but with their previous history we have nothing to do. We know them only as they are now presented to us, metamorphosed into crystalline rocks. When in these rocks the crystalline texture pervades the whole, a true porphyry is obtained; in other words, a number of crystals are seen embedded in a crystalline mass. Such rocks occur in large abundance in all the islands. Beautiful angular crystals of pale flesh-coloured felspar are embedded in quartz,— sometimes of a pure transparent kind, sometimes blackened with iron or carbon. Beautiful and perfect crystals of hornblende are embedded in milk-white or dark-coloured quartz. Delicate crystals of mica are found in quartz. Large and beautiful crystals of felspar, of the richest pink, are accumulated in rock All these may be seen in almost every one of these rocky bays indenting the coasts of all the islands, and on every pebble beach. In some places, beautiful pale green crystals of actinolite or epidote are either embedded in or plastered, as it were, upon quartz; and not unfrequently the crystals of hornblende or felspar are mixed in the quartz paste with sparkling flakes of mica. masses. When, however, the crystalline texture exists, but is overborne by a mechanical structure, causing the mass to appear as if in regular layers or strata, the rock that would otherwise be granite is called gneiss; and in proportion as the mechanical or crystalline appearance is the more distinct, the terms gneissic or porphyritic are applied to distinguish rocks essentially alike. The stratified or tabular appearance of the crystalline rocks in the Channel Islands is one of their most remarkable characteristics. There is ample evidence that all rocks of the kind we are now speaking of have been formed at a high temperature (though far below the melting point of any of the constituents), under the great pressure of a large superincumbent mass of rock, and in association with water. Any one, whether geologist or not, who will reflect a little on the probable mode of formation of crystalline rocks, and of what must happen before they are brought to the earth's surface, will have little difficulty in understanding the cause of some of the appearances that are most characteristic of them. Assuming a granitic rock to have been formed at a high temperature under great pressure, it may well be supposed that if the pressure is removed to any extent, but the heat diminished in a greater ratio, the rock, in cooling, must undergo contraction, the mass splitting as it becomes hard. Such fissures originate on the upper or cooled surface, but may be continued downwards to any depth. If, however, they are followed by an elevation of the whole mass, the widest part of the fissures will ultimately be below. Such changes taking place gradually, and while there is still communication with the less cooled matter below, the chasms and fissures formed during cooling and elevation will be filled up by minerals, which contain generally the same elements as before, though being crystallised in changed proportions, and at a slightly different rate of cooling, new combinations are introduced. There may be a complete filling up of the fissures in these cases; and owing to the difference alluded to, the mineral in the vein may be either more or less completely crystalline, more or less like in its mineral character, and harder or softer than in the original rock. Sometimes when the crust or enclosing rock contains particular minerals in small quantities, the vein will contain them in larger proportion. Sometimes, when the cre RESULTS OF ELEVATION. 253 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 sca) 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 |