eties. When mica is absent, quartz is nearly always present.

Schist is easily separated along its foliationplanes, while across them it is tough, and breaks with from an uneven to a hackly fracture.

A. ARGILLYTE, or ARGILLITE, Argillous or Clay Schist. -A fissile more or less indurated argillous rock, having the planes of the most conspicuous structure in the original rock, glazed, mineralized, or micacized. Certain minerals, as chiastolite, phyllite, pyrite and such like, are developed in some, and peculiar structures in others; thus forming varieties.

Varieties in Composition.

a. Chiastolitic Schist.

b. Phyllitic Schist, or Ottrelitic Schist. c. Carbonaceous Schist.

d. Alum-schist.

e. Spilyte or Spilite, or Calcareous Schist, and f. Dolomitic Schist.

Chiastolitic schists are mostly argillous rocks; nevertheless some are micaceous. The crystals are usually disseminated through the mass quite irregularly, but in some varieties they are stellated, usually on the surfaces of the original structure of the rock. Phyllitic schist contains lamina of phyllite (Ottrelite). Carbonaceous schist is rich in carbon, and often is pyritous; of it Alum-schist seems to be a variety, being due to the decomposition of pyrite in pyrito-carbonaceous schist. Spilyte and Dolomitic schist are passage-rocks into schistose limestone and schistose dolomyte.

Structural Varieties.

g. Folded or frilled Argillyte, h. Contorted, and i. Fibrous.

Variety g has a folding or crumpling that gives it a frilled aspect: this peculiar folding is only found in some of the schists, and evidently is solely due to the metamorphic action, as it does not follow any structure in the original rocks. In variety i the particles are so linear and parallel-arranged as to give the rock a woody aspect, a weathered block looking like the butt of a sallow or some such

tree.

Argillite is the passage-rock between normal clay-slate, or shale, and mica-schist. It occurs in various stages of metamorphism. Sometimes the whole mass appears homogeneous, differing only from clay-slate, or shale, by its superior lustre; nevertheless it is rarely without traits, more or less. characteristic, of mica-schist; such as the crumpling, crushing, folding, twisting or contortion of the lamina; also the hardening of the joint and other lines, with the development of minerals, especially on the surfaces of beds or on the lamination and cleavage-planes.

B. QUARTZYTE, or QUARTZITE, Quartz-schist.-Chiefly consists of quartz with some mica; the latter being most conspicuous on conspicuous on the planes of foliation.

Varieties in Composition.

a. Itacolumyte (Eschwege), Pedra elastica (Anchieta), Micaceous Quartzyte." A fine-grained and at the same time schistose compound of

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 elastica, 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 (Eschwege), 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 ferri-
ferous 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 Hen-
wood describes as
66
an auriferous mica-
ceous 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.

Structural Varieties.

d. Quartz Rock (Jukes), Granular Quartzyte. e. Fibrous Quartzyte, and

f. Conglomeritic Quartzyte.

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 f 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

1

quartz-rocks appear to occur as masses in unaltered rocks.

Felsitic quartzyte is a remarkable variety; some would seem to be metamorphosed Petrosilex; while others merge into quartz-rock on the one hand, and into Felsitic granite or Felsite-rock on the other. Some also occur in bedded masses, when they may possibly be metamorphosed felspathic sandstones. C. FELSYTE SCHIST.-A felsitic fissile rock; colonr 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. Ribaned, 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.