(3.) This, the third variety, is very slaty, and very micaceous. The scales of mica, from their smallness, appear indistinct, and form continuous plates. The fel spar and quartz arc in very small grains, and arc some times so enveloped in the mica, that it is difficult to dis tinguish them. It is also sometimes glandular, and in some instances almost an aggregation of balls of mica.
The gneiss which passes into granite belongs to the first variety, as that which passes into mica slate does to the third.
5. Stratlfication.—It is distinctly stratified, and the strata are parallel with the slaty structure. But when the beds rest upon granite, they sometimes follow all the sinuositics of the irregular surface of that rock, and form the mantle-shaped stratification ; in other cases the stra tification is saddle-shaped, or it does not appear aff.xted by the granite, the strata passing without change of di rection from one mass of granite to the other.
6. Deeomposition.—This rock, like granite, decays on exposure to the atmosphere, but the decomposition is in general more rapid. The felspar is at first changed into kaolin, and, owing to the greater abundance of mica, the disintegration of the mass is more rapidly effected. Hence it is that this rock does not occur so often in great isolated blocks as granite ; and hence also it is that mountains of gneiss are often less sharp in their outline than those of granite, that their summits are generally roundish, and that they rarely shoot into needles, or are formed into denticulated ridges.
Sometimes the decomposition, in penetrating the gneiss, loosens the adherence of the parts; the folic are then easily separated by the finger, and the mass appears as if rotten.
7. Foreign beds.—These beds are more considerable and more numerous in gneiss than in granite. The fol. lowing may be enumerated.
Limestone.—It is generally highly crystallized. Occurs in Aberdeenshire, Perthshii e, and other parts of Scot land ; and on the continent of Europe, in the Pyrennees, in Dauphiny, &c.
Trap —This rock, in the form of hornblende rock, hornblende-slate, and greenstone, occurs in beds, and in imbedded masses, and is often very much intermixed with the gneiss. When the gneiss abounds in horn blende, it is named bornblendie gneiss.
Pornbyry.—I1,:ds, imbedded masses, and veins of por phyry, sometimes of great magnitude and extent, are not (infrequent in sonic gneiss districts. Pet thshirc, Aber
deenshire, and Inverness-shire, afford fine examples of pot phyry in beds, imbedded masses, and veins.
Compact and granular felsnar.—The white stone. Wcisstein of Ilrerner.—This rock sometimes occurs in layers, which are not more than a few inches thick ; in other instances in beds, so thick as to form whole moun tains. The felspar is white, and is very fine granular, like dolomite : it contains numerous grains of red gar net, even grains of quartz, and sometimes scales of white mica. In short, it is to be considered as a granular fel spar, generally containing scales of mica, with some grains of quartz, and of other minerals. It forms beds and whole hills in Saxony, Moravia, and Sweden.
Quartz Rock.—This rock occurs in great beds, and sometimes also in veins in gneiss districts.
Besides the beds above described, gneiss contains occasionally granite, mica-slate, and clay-slate.
Glance Coal—The slaty kind sometimes occurs in beds in gneiss.
8. Metalliferous Minerals —Gneiss is one of the most metalliferous of the primitive rocks. The metals occur in veins, beds, and imbedded masses, but in greatest va riety in veins. There arc few metals that do not occur in it. Most of the Saxon, Bohemian, and Saltzburg,ian mines, are situated in this rock. The oldest gneiss in the Saxon Ergebirge, that with reddish-coloured fel spar, is the least productive in ores ; but the newer, with white-coloured felspar, is the most productive; and the veins, though small, are numerous. The oldest venigenous formation appears to be that which contains tinstone. The tin-ore is accompanied with wolfram, molybdena, arsenic-pyrites, fluor-spar, chlorite, topaz, and opal. The second venigenous formation appears to be lead-glance. The third formation consists principal ly of copper, and the ores are grey copper-ore, copper glance, copper -pyrites, and variegated copper-ore. The fourth formation, which is very extensive, contains ores of cobalt. The newest formation is that which contains ores of silver. Veins, containing antimony and red ironstone, occur in gneiss, and these are supposed to be newer than any of the preceding. Veins of quartz with gold also occur in gneiss.