The metalliferous beds that occur in this rock contain magnetic iron-ore, argentiferous lead-glance., blende, copper and iron pyrites.
The lead-mines of St•ontian, in Argyleshire, are situ ated in gneiss.
Geographical distribution.—It is a very widely distri buted rock. It is found in almost every country where granite occurs ; and is often inlet posed between granite and mica-slate, or is contained in mica-slate, or even in elay•slate. It is an abundant rock in Scotland, forming extensive tracts in the middle and northern divisions, and also in the islands. It is the principal rock in Swe den and Norway. It occupies almost the whole of the Saxon metalliferous mountains : it abounds in Bohemia and Silesia ; it is not uncommon in the Black Forest, the Upper Palatinate, in Carinthia, in the Southern Alps, the Pyrenees, and the Vosges. It occurs also in Greece ; and, in the vicinity of Athens, the old mine works of the ancients are situated in it. It is an abun dant rock in the United States of America, and in South America : Humboldt met with it in the high chain of the Andes of Quito, in the mountains of Parima, and Venezuela.
HI. Mica—Slate.
Glimmer-Schiefer.— IVerner.
Micaceous Schistus.—Kiman. Mica-Slate.—Jameson.
Schiste micace —Brochant.
Rocke Feuilletee, et quartz. on Schiste micace.— Daubuisson,—Saussure.
1. Constituent Parts.—This rock is composed of mica and quartz, and, like gneiss, has a slaty structure. The mica is generally the predominating ingredient : its colour is grey, sometimes inclining to green, sometimes to yellow, and more rarely to brown. It is often dis posed in continuous plates, not in distinct scales, as in gneiss. The quartz is grey, with its usual vitteous lustre, and is disposed in thin lenticular masses, inter posed between the plates of mica. Sometimes these masses increase in magnitude, and become globular. and then the rock acquires a conglomerated structure. Although the mica forms the principal and predomi nating ingredient in mica slate, yet it sometimes hap pens that the quartz is the most abundant, and thus a transition is formed into quartz rock.
2. Varietics.—We can distinguish different kinds of mica-shte. These are Common, Undulated, "l'alcky,and Fine. Slaty. The common is straight, and rather thick
slaty, and contains garnets. and sometimes felspar. The undulated has a wa% ed structure, and contains neither garnets nor felspar. The talcky is straight slaty ; con tains thick layers of quartz, and the mica has a green colour, and inclines to talc. The fine slaty borders on clay-slate, (the next rock in the order of succession,) has a light yellowish grey colour, and contains extreme ly little quartz ; it passes imperceptibly into clay-slate. Of these, the oldest is the Common, and the newest the Fine-slaty.
3. Imbedded Illinerals.—It frequently contains imbed ded minerals of different kinds. The principal of these arc the following : (I.) Garnet, either in grains or in crystals, and is so frequent and abundant, that it may almost be considered as a characteristic and principal ingredient of the rock. It abounds in mica-slate districts in Scotland, and on the continent of Europe.
(2.) Tourmaline and Schorl.—These are met with in the mica-slate of Scotland, and in other countries.
(3 ) Grenatite —This mineral occurs in the mica slate of the county of Wicklow in Ireland. and in dis tricts of the same description on the continent of Europe.
(4.) Chiastolite.—This curious mineral is found in mica-slate in the Pyrenees.
(5 ) Kyanite —Is found in the mica-slate of the Shet land isl. cls, and also in a similar rock in Banffshire.
(6.) beautiful emerald, found in Egypt, occurs in mica-slate.
Besides the minerals already enumerated, many others, as vesurian, rutile, graphite, &c. occur in mica-slate.
4. Stratificatian and Position.—It is very distioctly stratified. The strata are sometimes variously convolut ed, and the same character occurs in the substance of the strata. It often rests on gneiss, and is covered by clay-slate. It passes, on the one hand, into gneiss, and the transition is made by the common kind ; and on the other into clay-slate, and the transition is made by the fine-slaty kind. The out-goings of the strata are fre quently lower than those of the gneiss, on which they rest, and higher than those of the clay-slate that cover them. It sometimes also occurs in beds in gneiss, and clay-slate, and even in granite.