MICA-SLATE.

605

In the older varieties of Gneiss , the three ingredients are often indistinct layers, which have an undulated direction.—In varietiesmore recent, theyngredients are more mingledand, when passinginto mica-slate, the layers become thinner, the structure more per-fectly slaty, and the mass more fissile.

.Mountains, composed of Gneiss , are seldom so steep as those ofgranite ; and their summits are usually rounded.

Gneiss often contains schorl or tourmaline, garnets, hornblende,actvnotite, and sometimes emerald, beryl, zircon, chrysoberyl, talc,&c. The hornblende is sometimes very abundant; and some varie-ties of Gneiss gradually pass into hornblende-slate or greenstone-slate. ^

But, in addition to the aforementioned simple minerals, scatter-ed through the mass, Gneiss often embraces beds of both simple andcompound minerals. Among these are beds of granular limestone,hornblende, hornblende-slate, greenstone, greenstone-slate, porphyry,potstone, quartz, anthracite, &.c.

Few of the primitive rocks are so metalliferous as Gneiss . Itsores occur both in beds and veins; but more frequently in the latter.

Gneiss, like granite, is somewhat irregular in its composition.Sometimes the mica almost entire.lv disappears, and is replaced byhornblende or actynolite. In sonic cases beds of Gneiss pass into anaggregate of feldspar, mica, and hornblende—or quartz, feldspar, andhornblende, &c.

(Remarks.) This rock exists very abundantly in many coun-tries, It is very common in some parts of the United States .—'Alarge number of the Saxon and German mines are situated in Gneiss .—-It is a useful rock for many purposes, in consequence of the facili-ty, with which it splits into masses of a regular form.

5. Mica-Slate.* Jameson.

Mica-slate is essentially composed of mica and quartz, which are,in general, more or less intimately mingled; but sometimes thetwo ingredients alternate in distinct layers. Although the proportionsof mica and quartz are variable, the mica usually predominates.

The quartz is most frequently grayish white ; but the mica may-be whitish or gray, greenish or bluish gray, brownish or yellowishgray, deep blue, or nearly black.

Its structure is always distinctly slaty", usually more so than that,of gneiss; and its masses are often very fissile. The layers aresometimes straight, and sometimes undulated. In some varieties the

* Glimmer-Schiefer. Werner. Scliiste micac<!. Brochast. SchistoseMica. Kiravan. Quartz rule act. Havx. Micaceous Schistus.