cos

MICA-SLATE.

texture is very fine, and the folise of mica so small, that they arescarcely discernible by the eye, unless their aggregation be previous-ly destroyed by heat.

This rock lias often a very high lustre, when viewed by the re-flected rays ol the sun. It is, however, subject to decomposition, bywhich its aspect is much altered.

(Geological remarks.) Mica-slate is always a primitive rock,and never contains any petrifactions. But, like the other primitiverocks, it appears to have been formed at different periods.—In someof the older varieties, which pass into gneiss, the texture is somewhatcoarse, and the mass imperfectly fissile; while, in more recent varie-ties, which approach very nearly to argillite, the texture becomesvery fine, and the mass completely fissile.

When Mica -slate occurs in the same mountain or district withgneiss, or with granite and gneiss, it ordinarily rests on the gneiss,or, if granite only be present, it rests upon that. This relative situ-ation gives it the third rank, in point of age, among primitive rocks.Its strata are often highly inclined, and sometimes even vertical.

When this rock exists in the same mountain with gneiss, it ap-pears at a lower level, than that of the gneiss; and of course it or-dinarily occurs at a considerable distance from the principal summitof the mountain.—It sometimes alternates with gneiss, or other prim-itive rocks.

Mica -slate seldom appears in high and steep cliffs, like those ofgranite. When it forms whole hills, the summits are usually muchrounded ; and the general aspect of those districts, in which thisrock abounds, is often undulated, presenting long elevated ridges,separated by vallies of a moderate breadth.

The older varieties of Mica -slate sometimes contain a little feld-spar, and very frequently embrace garnets, among which the precioussarnet is often found. Schorl or tourmaline, hornblende, staurotide,cyanite, and carbonate of lime are sometimes disseminated throughthis rock.

Beds of various simple minerals, among which are granular lime-stone, hornblende, hornblende-slate, serpentine, and sulphate of lime,sometimes occur in Mica -slate, or even alternate with it.

Mica-slate abounds with ores, which exist both in beds and veins,but more frequently in beds. Garnets and actynolite often occur inthese metallic beds.

This rock sometimes passes into chlorite-slate, and sometimes re-sembles hornblende-slate, or an argillaceous slate containing horn-blende.

(Remarks.) This rock is abundant, but less so than gneiss. It