CHAPTER V.

MICACEOUS ROCKS.

71. Griesen.-An assemblage of Quartz and Mica with a granular texture. It is a granite without Feldspar. The Mica is yellowish-gray or greenish, and has a brilliant lustre. The Quartz is crystalline and gray.

Accessory minerals: Oxide of Tin, in black grains, vitreous fracture, slightly resinous lustre ; Feldspar (passing to Granite). Accidental minerals: Fluorine, Apatite, Topaz, Tourmaline Emerald, Copper Pyrites, Mispickel, etc.

72. Mica Schists.-A crystalline, granular assemblage of Mica and Quartz of schistose texture. Mica forms, generally, one third or one half of the mass, but sometimes the quantity of Mica is so small as to merit only the name of Micaceous Quartzose Schist. In other cases, Quartz is nearly wanting. The proportion of Silica varies from 40.7 to 69.45. It attains 82.38% at Mont Rosa.

Mica presents itself in rocks in the condition of independent scales disposed in thin layers. Quartz, crystalline and granular, fills the space which separates these layers, which exist in parallel bands, plane, undulated, or folded in zig-zags. The more abundant the Mica, the more schistose the rock. The rock is easily split along these bands of Mica. Such a fracture is often wrinkled. It reflects light brilliantly and shows no Quartz. It is only in a fracture across the layers that one can see the true proportion of Mica.

When the Quartz is very abundant, the schistose character is only slightly apparent, and the Quartz forms nodules surrounded by the Mica. The Mica is usually one of the potash series, having two optical axes. It is generally clear gray,

green, yellow, or white.

Certain Mica schists have a black

color, owing to a ferro-magnesian Mica of this color.

Damourite of a pearly lustre and containing water, sometimes replaces true Mica.

Accessory elements of the Mica schists: (1) Graphite. It replaces Mica more or less completely and gives the rock its characteristic lustre (Graphite Mica schist).

(2) Calcite. This replaces the Quartz in Calcareous Mica Schists (passage to Cipolins).

(3) Garnet. Sometimes as abundant as the essential elements, it forms rhombododecahedrons of red or brown grains. It seems to have crystallized after the Mica.

Mica Schists are associated with Feldspar, Talc, Chlorite, Amphibole, passing to Gneiss, to Talcose, Chloritic, or Hornblende Schists. Red oxide of Iron establishes transition to Itabirite.

Accidental minerals are extremely numerous. We will cite only Tourmaline, Andalusite, Staurotide, Emerald, Apatite, and occasionally Cubic Pyrites, Mispickel.

:

Mica Schists cover or envelop the Gneisses. They form sometimes high mountains intersected by great crevasses. The alteration consists chiefly in the per-oxidation of the Iron contained in the Mica. When complete, it results in an ochrey clay, sandy and micaceous.

Micaceous Sand.-Mica in Scales mixed with Quartz, in grains. 73. Minette (Syn. Fraidronite).—

Essential elements: a paste of Orthoclase, a nearly equal quantity of Ferro-magnesian mica, and, occasionally, Hornblende. It contains Silica, varying from 50 to 65%. M. Delesse, who made a study of this mineral, considers it Eurite with an excess of Mica; richer in Magnesium and Iron than Porphyry. The rock is black, sometimes brown, in consequence of alteration, and often brilliant. That of Alsace has a density of 2.65. Easily fusible before the blow-pipe. The paste is deep gray or reddish-brown, sometimes porous. It often contains laminæ of red

dish Orthoclase. Sometimes distinct crystals of Orthoclase give to the rock a porphyritic appearance. The Mica is of a blackish-brown. The two axes diverge, at most, 5o. Hornblende of a grayish-green or deep green color; it contains about 10% of water. It is altered.

Accessory elements: Chlorite, Oxide of Iron, Carbonate of Iron. Accidental elements: Calcite, Quartz rare, Epidote, etc.

When the Orthoclase appears in globules, the rock becomes globuliferous. It separates naturally, and the Minette assumes a schistoid character and sometimes divides into parallelopipedons. It forms veins in Granites and in the Syenites of the Vosges. It is found in Saxony, called Trap glimmer by Naumann. The Minette of central France was called Fraidronite by Dumas.

74. Leptynolite.-This name, Cordier has given to rock which bears the same relation to Minette as Gneiss bears to Granite. It is of a gray color, more or less blackish; schistose or tabular; and is formed of Mica and Feldspar. It is generally speckled with a black material externally, but is white in stripes and is regarded as a macle imperfectly developed. It belongs with the crystalline schists and is classed by German lithologists among the older argillaceous schists.

75. Kersanton.-Grayish or greenish-gray rock, composed of a green or gray paste of Oligoclase, with a much smaller quantity of Mica of a copper-brown or blackish color, containing Manganese and Iron; and, finally, of Carbonate of Iron mixed with Calcite, visible under a slight magnifying power. The Calcite often forms nodules which are enveloped by the Mica.

Oligoclase is sometimes found in separate distinct crystals, white or greenish-white, giving the rock a granitoide texture; it has generally been altered, which renders it easy to crush or cut. Kersanton is discolored when treated with hot hydrochloric acid.

It bears the chisel of the sculptor, as may be seen in the monuments of Brittany. M. Delesse has made a study of it.

He shows that the rock does not contain Amphibole. It is, according to this same lithologist, sometimes found in an analogous rock of Visembach. This rock, of a blackish-green color contains more than 70% of Oligoclase, generally in the state of paste, and less than 30% of Mica in microscopic particles mixed in the paste. It effervesces in acids and contains as accidental minerals: Cubical Pyrites; Magnetic Pyrites; Chalco-Pyrites; and Amygdaloidal forms of Quartz, Chlorite, Epidote and CalcSpar.

M. Delesse regards these characters as sufficiently different from those of Kersanton, to warrant the name of Kersantite.

CHAPTER VI.

CHLORITIC ROCKS.

76. Chloritic Schist.-Aggregate of a more or less greenish color and schistose texture of chlorite, generally mixed with Quartz in grains. Generally, notwithstanding their schistose character, these rocks are not easily separated into laminæ.

Accessory minerals: Feldspar, Actinolite, Mica, and Talc. Accidental minerals: Carbonate of Magnesia: Magnetic Iron ore in octahedrons; Garnet in rhombododecahedrons, often of a hyacinth-red color; Diopside; Epidote; Sphene; Actinolite; Tourmaline; Corundum; Iron and Copper pyrites; Calcite in mass; Quartzite in layers; Serpentine. The Chlorite schists alternate with the other crystalline schists-Talcose schist, Mica schist, Hornblende schist, etc. Allied to these rocks is Topfstein. Associated with the Chlorite schists also are Satin Phyllades of Canada, which contain numerous crystals of Pyrophyllite which is doubtless an essential element.

77. Glauconyte.-This name is given to a Silicate of Iron with Potassic hydrate. These rocks differ chemically from the Chlorites in containing much more Silica (40 to 54%); and, generally, much less Alumina (2 to 7%). It is known only in grains mixed with calcite or sand which give it an oölitic appearance. Examples: Glauconyte or Chloritic chalk of the Paris basin; Glauconyte from Silurian beds in Russia; Glauconyte Sandstone, generally green or brown when the Ferrous oxide of the Glauconyte passes to the condition of a hydrate.

Grains of Glauconyte often present themselves in the form of shells fitting one on the other. They are found at St. Petersburg, in the Devonian system; they characterize the green Sandstone of the Paris basin and the Tertiary formation at Vienna and abound in the Swiss Molasse.