29 
OF MICA SLATE, OR MICACEOUS SCIIISTUS. 
OF .^IICA SLATE, OR MICACEOUS SCIIISTUS. 
17. The characters hy which granite anil gneiss are determined 
and distinguished from other rocks, are drawn from their struc¬ 
ture, ratlier than their composition. The proportion of the in¬ 
gredients may vary, one of them may disappear and he replaced 
by anotlrer mineral, and still the application of the same common 
name, involves no absurdity, and produces no obscurity. The 
hornblende, chlorite, or talc, which takes the place of the mica, 
effects but an inconsiderable change in the appearance and cha¬ 
racters of these, as distinguished from other rocks, and forms, 
thei’efore, only an unimportant variety. Butin the combinations 
that ai’c immediately to follow, these substances, instead of occu¬ 
pying a subordinate place, become the pi-edominant ingi'edient, 
and give rise to well marked and important distinctions. There 
is therefore a necessity for the separation of rocks, apparently 
of the same age, alike in their structui'e, and differing in the sanie 
xi'Cty, that mere varieties of granite and gneiss differ fi'om each 
other, into five or six distinct species—mica slate, quartz rock, 
hornblende schist, chlorite schist, and talcose schist, to which 
IMcCulloch adds actinolite schist. All these have a schistose 
structure, more or less perfect. 
il//ca Slate, or IMicaceous Schist, succeeds to gneiss in the 
^V ernerian arrangement. Its essential ingredients are mica and 
quartz. By the loss of one of these, the substitution of another 
mineral, or (he acquisition of a third, it passes into quartz rock, 
the other varieties of schist, and gneiss. Like gneiss, it embra¬ 
ces subordinate beds of serpentine and limestone, and like it, is 
also rich in the metallic ores. The laminae of which it is com- 
joosed, often present numerous undulations and contortions. Of 
the earthy minerals, that in which it most abounds, is garnet, 
which is sometimes so abundant as to equal in quantity the in¬ 
cluding rock. Well characterized mica slate does not occur in 
North Carolina, except in the upper part of Cumberland, and in 
the western counties. It may be seen near the dividing line of 
Rockingham and Stokes on the south side of Dan river, in Sur¬ 
rey, Lincoln, Ashe, and Buncomb, where it sometimes contains 
octahedral crystals of the oxide of iron, in such quantities as to 
constitute a valuable ore of that metal. 
Quartz Rock, was regarded by geologists as a variety of mica 
slate, and treated of under that title until it was separated by Dr. 
IMcCullocb, and had assigned to it a place amongst the rocks, to 
which, from the space it occupies in the crust of the globe, it is 
well entitled. It is a stratified rock, consisting either of pure 
quartz, compact or granular, or of grains of quartz and feldspar, 
or quartz and mica. It seldom contains any imbedded minerals, 
whether metallic or earthy. The granular variety sometimes 
admits of a considerable motion amongst its particles, without 
being broken, constituting a flexible sandstone, of which Stokes 
