remain deflected or curled in the immediate vicinity of inclosed blocks or nodules; while in some granites regular or irregular masses, differently constituted to the rest of the rock, will occur.

In West Galway, Ireland, conglomeritic gneiss is usually associated with metamorphosed igneous rocks, and probably is metamorphosed conglomeritic tuff, or agglomerate.

J. PORPHYRITIC GNEISS; K. GRANITOID GNEISS.— These subgroups are allied, nearly all porphyritic gneiss being also granitoid.

Gneiss may have large and prominent crystals of felspar disseminated through the mass. This usually occurs in the granitoid subgroup, but not always. In some granitoid gneiss there is a distinct foliation, while in others it is indistinct.

The degree of distinctness of the foliation of the granitoid gneiss appears due to the intensity of the metamorphic action to which it was exposed. Granitoid gneiss is the passage-rock between gneiss and granite.*

M. Schist [Gr. schistos, split or divided]. A crystalline fissile aggregate, consisting, when typical, of two well-developed minerals, occurring in thin plates or leaves. Mica is usually one of these minerals; however, various others also occur, sometimes as accessories, but locally as essentials, thereby forming different subgroups and vari

*Hereafter microscopists may prove that granitoid gneiss ought more properly to be classed with the Granitic rocks (Class No. 1), and taken out from the Transition rocks (Class No. 4), as in it the original structure is obliterated, except in a few rare exceptions. The mineral constituents seem to be altogether crystalline, no felsitic or felspathic tuff occurring in granitoid gneiss.

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 laminæ 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 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 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.

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