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quartz with some mica, talc, or chlorite."— Cotta. Henwood says of the Itacolumyte of Minas Gerais, "quartzose talco-micaceous slate."
These flexible quartzytes were first noted among the metamorphic sedimentary rocks of Brazil, and were named after Itacolumi Peak. Burton says there are three different kinds of rock named after this peak: 1st, the flexible quartzyte, or Pedra clastica, under which name it was described nearly three centuries ago by Padre Anchieta; 2nd, Diamantine itacolumyte. the matrix of the diamond, "a hard talcose rock of distinctly laminated quartz, white, red, or yellow, granular, with finely-disseminated points of, mica;" and 3rd, in Minas the name is applied to "refractory sandstone grits, a fine crystalline rock evidently affected by intense heat." The peak itself consists of none of the three kinds, although all are called after it.
Jukes thus describes Itacolumyte :—" A genuine unaltered sandstone, more or less micaceous, like other sandstones, but the mica in worn spangles, not in connected flakes." The rock, of which this is a description, came from India. In it are lines that appear due to deposition; when placed under the microscope, it is found to be full of long drusy cavities, which lie in lines rudely parallel with the structure of the rock. The cavities open and shut when the slab is bent. According to Dana, Itacolumyte pertains to the talcose series, and is the matrix of the diamond; this evidently is Burton's second kind of rock.
b. Itabiryte (Bschwege), Jacotinga.—Ferruginous quartz-schist, a variety of Itacolumyte, named after Itabira, in Brazil. Eschwege
who first described the rock, makes it the matrix of the diamond. Some very ferriferous varieties are said to be worked in Brazil as iron-ores; they are more or less auriferous. Itabiryte, Dana says, " contains much specular iron-ore in grains or scales, or in the micaceous form." Jacotinga Henwood describes as an "auriferous micaceous iron-schist."
c. Felsitic Quartzyte—An aggregate of quartz and
felsite, sometimes also felspar and a little mica; usually more or less massive, rarely well foliated.
d. Quartz Rock (Jukes), Granular Quartzyte.
e. Fibrous Quartzyte, and
Variety d is fine-grained, homogeneous, and more or less saccharoid in aspect, often merging into felsitic quartzyte (c). e. has an arrangement in long rude prisms like coarse wood; while /. contains pebbles, usually ovate, but sometimes more or less angular; this is evidently a metamorphosed conglomerate, the normal form of the pebbles having been elongated by the subsequent metamorphic action.
Quartzyte or Quartz-schist (B) is an undoubted metamorphic sedimentary rock, originally a highly siliceous sandstone or grit. There are, however rocks described as Quartzyte or Quartz-rock that seem not to be metamorphic rocks, but rather normal rocks, either deposited from a solution, or perhaps a variety of intrusive rock. Some of these quartz-rocks appear to occur as masses in unaltered rocks.
Felsitic quartzyte is a remarkable variety; some would seem to be metamorphosed Petrosilcx; while others merge into quartz-rock on the one hand, and into Felsitic granite or Felsite-rook on the other. Some also occur in bedded masses, when they may possibly be metamorphosed felspathic sandstones.
0. Felsyte Schist.—A felsitic fissile rock; colour whitish, greyish, or greenish; from compact to mealy or granular in texture.
Note.— Some of the compact varieties of Felsyte schist are identical with rocks named by Krantz as Leptinyte or Whitestone; on the other hand, Cotta classes Leptinyte with Granulyte, apparently considering both as intrusive rocks.
Varieties in Composition. a. Micaceous, b. Quartzose, and c. Pyritous.
Varieties in Structure.
d. Bibaned, when the mineral constituents are in
layers or seams, alternating with one another.
e. Mealy, having a leafy or scaly structure; and f. Gneissoid or granular, a granular felsitic rock,
slightly fissile, with grains of quartz disseminated through the mass, also a little mica.
Note.—Gneissoid or granular felsyte schist merges into felsitic quartzyte. In some localities it is an undoubted bedded or interstratified rock, while in others it is evidently intrusive. Perhaps in the first case it may be a metamorphosed stratified felstone or tuff (which could only be determined by a microscopical examination), while in the latter it may be a metamorphosed intrusive felstone. The latter variety of granular felsyte schist appears to answer the description of the rock called granulyte by Cotta.
D. Mica-schist. —When typical, a foliated or fissile aggregate of quartz and mica. Usually the foliation is regular; nevertheless it may be crumpled, crushed, folded, twisted, contorted, curled, nodular, or spheroidal.
Varieties in Composition.
a. Two-mica Schist, b. Chloritic, c. Talcose, d. Garnetiferous, e. Chiastolitic, f. Andalusitic, g. Amphibolic, h. Epidotic, i. Quartzose, j. Felsitic, k. Calcareous, I. Schorlaceovs, m. Pyritous, n. Pyrrhotitic, o. Mematitic, and p. Graphitic. The varieties in composition are named after conspicuous minerals, locally essentials. Most of these rocks, however, are only passage-rocks into the other subgroups of schist; some even (such as Epidotic mica-schist) are not normal rocks, but due to a secondary change in the mineral constituents.
g. Fine Mica-schist, where all the ingredients are small and even-grained.
r. Ribaned, with the minerals in layers or bands.
s. Fibrous, with a woodlike arrangement.
t. Nummoid, having in it small discs of quartz, giving the rock an appearance as if studded over with pieces of small coin.
v. Frilled, having the foliation folded or crumpled on itself.
w. Curled or Spheroidal.
x. Knotty or Concretionary.
y. Nodular; and
z. Gneissoid; the last-named variety being the passage-rock between mica-schist and gneiss.
HI. Basic Schists.— Schist in which such minerals as amphibole, ripidolite, talc, and the like, replace the whole, or nearly the whole, of the mica, and become essentials of the rock.
Varieties are,—a. Hornblende,or Amphibole Schist; b. Actinolite Schist; c. Chlorite or Ripidolite Schist; d. Ghloritoid Schist (Hunt); e. Potstone; f. Talc Schist; g. Garnet Schist; h. Tourmaline, or Schorl Schist; and i. Rhcetizitic or Magnesia Schist.
It is evident that with more mica, any of these rocks may pass into one of the varieties of micaschist; or that one of the varieties of mica-schist, by a loss of its mica, may pass into one of the basic schists. Sterry Hunt describes a dark-coloured Canadian subvariety of chlorite-schist as largely composed of chloritoid, a mineral allied to ripidolite and phyllite; and Cotta describes potstone as a felt-like web of ripidolite, and rarely foliated. Perhaps these two varieties of schist would be more properly classed as varieties of argillyte. Ehsetizitic schist is described by Forbes as remarkable for the predominance of silicates of alumina and magnesia, rhastizite, iolite, chiastolite, &c.
The basic schists have numerous structural varieties. They are, however, very similar to those mentioned when describing mica-schist; therefore it is unnecessary to enumerate them here.
F. Metallic Schists.—Schists, in which a mineral ore replaces part or the whole of the mica, and gives a character to the rock mass.
The principal varieties are,—a. Mico-iron Schist, b. Pyrite Schist, and c. Pyrrhotite Schist: other ores may also give a character to a rock.