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, l. Schorlaceous, m. Pyritous, n. Pyrrhotitic, o. Hematitic, 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.

Structural Varieties.

q. 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.

u. Wavy.

v. Frilled, having the foliation folded or crum

pled 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.

E. 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. Chloritoid Schist (Hunt); e. Potstone; f. Talc Schist; g. Garnet Schist; h. Tourmaline, or Schorl Schist; and i. Rhætizitic 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. Rhætizitic schist is described by Forbes as remarkable for the predominance of silicates of alumina and magnesia, rhætizite, 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.

G. BURNT SHALE.-Altered argillous rocks, due to the burning of carbonaceous beds.

Varieties are,-a. Burnt Clay, b. Rock Slag, and c. Porcelanyte.

H. MINERALS FORMING ROCKS.

a. Quartz and Jasper, generally in veins. b. Corundum, in subordinate layers.

c. Apatite, sometimes forms compact spheroidal

masses.

d. Magnesite, in subordinate masses.

e. Orthoclase.

f. Pistacite; and

g. Pycnite, generally in veins.

h. Lepidolite, rarely forms independent rocks.
i. Lievrite or Ilvaite, in subordinate beds.
j. Magnetite.

k. Anthracite. 1. Graphite.

Normal graphite is pure carbon with a little oxide of iron mechanically mixed. Graphite occurs in beds and imbedded masses, laminæ, or scales, in granite, gneiss, schist, and schistose limestone. Of it Dana says, "It is, in some places, a result of the alteration by heat of the coals of the coal formation."

m. Eisenkiesel, "a concretionary rock, made up of red and brown iron-ore, and fragments of bright red jasper."-Bauerman.

n. Garnet Rock.

One variety is massive, of a milk-white colour, very hard and heavy, with a peculiar graining through the mass. Others are brownish or reddish, and more or less crystalline; these usually pass into

a highly crystalline vuggy rock, that has as accessories, pyrite, marcasite, chalcopyrite, epidote, calcite, &c.: occurs in veins and subordinate masses.

N. Slate [according to Worcester, originally spelt "Sclate." Old Fr. esclat, a splinter, a shingle. Celtic scorlt, to split or cleave].- Originally shale or clay, into which a subsequent slaty cleavage has been introduced.

Some slates are said to belong to the Metamorphic series. The slaty structure, however, seems more connected with Sedimentary than with Ingenite rocks. Slates often occur associated with unmetamorphosed sedimentary rocks, and many metamorphic rocks are known to have been cleaved prior to their being metamorphosed. Slates are mentioned here, but they will be described hereafter among the Derivate rocks, as varieties of the subgroup Argillous rocks (see page 103).

O. Calcareous Rocks (Lat. cala, calcis, lime). A. SCHISTOSE LIMESTONE.-A crystalline granular or nearly compact aggregate of calcite, with more or less ripidolite, mica, felsite, quartz, pyrite, galenite, &c.; often more or less fissile, sometimes highly schistose; usually the foreign minerals are in more or less regular lines or layers, but in some varieties they are disseminated through the mass of the calcite. a. Cipollino, or Micaceous Limestone.- Rich in mica, which gives the rock a schistose texture; this variety graduates into Spilyte. b. Calciphyre (Brongniart), or Garnetiferous Limestone, containing garnets and pyroxene or felspar.

c. Hemitrene, or Amphibolic Limestone. In addition to the amphibole, it generally contains grammatite.

d. Felsitic Limestone.-The felsite often occurs in highly crumpled parallel layers.

e. Statuary Marble, or Saccharoid Limestone."A white, fine-grained rock, resembling loaf sugar in colour and texture, working freely in any direction, not liable to splinter, slightly translucent, and capable of taking a polish. Flakes of mica or chlorite sometimes exist in it."-Jukes.

Typical statuary marble should answer the above description, but the rock in mass is often more or less schistose, and also streaked with dark lines, caused by foreign minerals being associated with the calcite.

f. Granular Schistose Limestone. A granular crystalline, more or less schistose limestone; containing some mica, ripidolite, felsite, or even quartz. Colour white, pale grey, dove, light blue, greenish, reddish, purplish, or yellowish. It is often impregnated with microscopic crystals of pyrite, marcasite, galenite, or chalcopyrite.

P. Pseudomorph Calcareous Rocks.-Most of these. rocks are pseudomorph metamorphic rocks; some, however, may be derived direct from the original rocks.

A. SCHISTOSE DOLOMYTE.-A crystalline aggregate of calcite and dolomite, together with some ripidolite, talc, or mica: pyrite, marcasite, or chalcopyrite are usually present in greater or less quantities, but minutely disseminated