from a grouping analogous to that which we have described in Albite, but which is much more fully represented in this mineral. The color is a dull or greasy white, grayish-white, greenish, yellowish, or reddish. Hardness, about 6; specific gravity, about 2. 7 (just enough higher than Albite to admit of identification). Before the blow-pipe, Oligoclase fuses more easily than Albite to a colorless glass, coloring the flame yellow. It is readily attacked by acids. Oligoclase contains about 80% of Silica.

Andesine (Oligoclase, very rich in Lime).- Oligoclase is generally accompanied, in the rocks, by silicates of magnesia and iron more or less aluminous: Micas, Chlorites, Amphiboles, Pyroxenes. It represents, in a manner, the average of the other Feldspars, and is found associated with all; with Orthoclase in the syenites; with Albite in the diorites; with Labradorite in the melaphyres. Sometimes it forms in syenite the nucleus of a crystal of which Orthoclase furnishes the envelope. In the granites of Finland it surrounds the Orthoclase. Oftentimes the crystal is enveloped in a yellow crust produced by the carbonic acid of the air.

Labradorite (Aluminum - calcium - sodium, Silicates).Form: doubly oblique prism. Crystals are rare. The face of the principal cleavage exhibits a vitreous lustre and makes, with the secondary cleavage-plane, an angle of 86° 40'. The facet is marked by striæ, occasioned by grouping as in Oligoclase; but to this grouping is added another, intersecting it as in Orthoclase. The color varies from gray and grayish-white to greenish-gray. Labradorite often exhibits a beautiful play of colors of various hues, from the faces g'. Hardness 6. Specific gravity 2.68 to 2.74. Before the blow-pipe, Labradorite fuses more easily than the preceding species to a white enamel. colors the flame yellow, and is imperfectly dissolved by hydrochloric acid even when in powder. Concentrated sulphuric acid dissolves it with a separation of Silica.

It

Saussurite is found with Diallage in Gabbros; very tenacious and very heavy; greenish-gray or greenish-white, rather

dull. Hardness 5.5 to 6.5. Specific gravity 2.79 to 3. Labradorite gives rise to a calcareous clay or marl, and a soluble calcium carbonate.

a

Anorthite (Aluminum - calcium Silicate). Form: doubly oblique prism. Hardness 6. Specific gravity 2.75. Two very decided cleavages making an angle with each other of 85° 48'. Crystals: colorless and transparent, very clear, or white and translucent; exhibit a pearly lustre upon the cleavage faces. Easily fusible before the blow-pipe and completely decomposed by hydrochloric acid, with separation of Silica.

MINERALS ALLIED TO THE FELDSPARS.

1. Nepheline (Aluminum-sodium-potassium-calcium, Silicates). Hexagonal prism, some

Fig. 5.

characters slightly different from those of Nepheline. Has a

greasy lustre in the fracture, tending to pearly on the exterior. Color varies from brown, to green or red.

3. Sodalite (Aluminum-sodium Silicate). - Crystals with a vitreous lustre; colorless, or from grassy green to azure blue, losing this color by heat; fusible to a colorless glass; dissolving to a gelatinous mass in acids. Hardness 5.5. Specific gravity 2.29. Form: dodecahedron, generally elongated in the direction of one of the octahedral axes. Cleavages parallel to the faces of the dodecahedron.

4. Hauyne (Composition same as preceding with Sodium and Calcium Sulphates).-Crystals generally blue, belonging to the cubic system. Cleavable along the faces of a dodecahedron. Fusible with difficulty before the blow-pipe.

MICAS.

(Silicates of Alumina and Potash with or without Magnesia.) Many Micas also contain Fluorine, Lithium, Sodium, and in some, very small quantities of Cæsium or Rubidium. The

primitive crystalline form is a right prism with a rhombic base (Fig. 7). The figure is nearly always modified upon its lateral edges and has the appearance of a regular hexagonal prism. The cleavage is quite perfect, parallel with the base, and so easily affected that all sands and rocks which have resulted from the disaggregation of crystalline rocks, of which Mica is an essential element, exhibit the thin hexagonal laminæ of this mineral.

The lamina are flexible and elastic; the lustre is bright, perhaps a little oily, and sometimes pearly. When examined by the microscope, the laminæ readily exhibit the colored rings

which belong to crystals of two optical axes. The angle of the axes varies from 0° to 4o in the Biotites, from 50 to 200 in the Phlogophites, 45° to 750 in the Muscovites.

Muscovite Micas, very rich in magnesia, fuse to a white enamel; they are not easily attacked by hydrochloric acid. These are the Micas of brightest lustre. They are of various colors, silver-white, yellowish, golden-yellow, brown, red, green, rose, black. They occur as an essential element in Granite, Gneiss, and Mica Schist. With this group should be ranked Lepidolite, which contains Lithium and oxide of Rubidium, and which colors the flame, at the moment of fusion, a purplish red. After fusion it is attacked by hydrochloric acid with a gelatinous deposit of Silica.

Biotites (Micas rich in magnesia). — Of a black or deep brown color, attacked by concentrated sulphuric acid, leaving a residue of pearly scales of Silica.

One variety of this class is Rubellan, of a rose-red, opaque or black. The Magnesian Micas alter more readily than the Potash Micas, and exfoliate in innumerable scales.

A certain number of minerals resemble the Micas in possessing an easy cleavage nearly in one plane; a bright lustre, often pearly upon the cleavage-face; and presenting hexagonal laminæ which contain water, easily recognized by heating in the open tube.

Damourite (Aluminum Silicate and Potassium Hydrate).– In whitish or yellowish scales with a pearly lustre; before the blow-pipe it swells up and fuses with difficulty to a white enamel. It is entirely decomposed by concentrated sulphuric acid. Specific gravity 2.79.

Paragonite (Aluminum Silicate and Sodium Hydrate).— In schistose masses formed of fine scales which are yellowish, grayish-white, greenish; lustre rather dull, though pearly and translucent. Infusible before the blow-pipe. Specific gravity 2.89.

Sericite.—Mica in silky scales, forming the undulating

laminæ of some of the schists. Hardness 1. Specific gravity 2.897. Silicate of Aluminum and Potassium, and much resembles Damourite.

CHLORITES.

(Talcose Micas). Substances intermediate between Mica and Talc. Those which enter into the composition of rocks, forming, often, foliated masses. Their crystals are nearly always laminated, the scales being hexagonal, having a perfect cleavage as easy and brilliant as Mica and Talc; divisible into fine scales, flexible, slightly elastic; leek-green or olive-green color, more or less blackish, somewhat rarely, bluish; greasy, brittle; the powder unctuous to the touch. Density, about 2.8. Yields water in the closed tube. Decomposed by concentrated sulphuric acid, also in boiling hydrochloric. Before the blow-pipe it is more or less easily fused to a black enamel, which is magnetic for the reason that it contains more or less iron. Composition of Chlorite: Silica 26.88, Magnesia 13.84, Alumina 17.52, Oxide of Iron 29.76, Water 11.33.

The Chlorites comprise also the Ripidolites and Delessites. The others (Clinochlores), much less rich in oxide of iron, having a density always less than 2.8. They are less easily fusible before the blow-pipe where several of their varieties exfoliate; their color is green, resembling that of the emerald. Their optical characters are of the oblique system, with a rhombic base, but their exterior properties are not distinguished from the preceding. They are decomposed by the aid of concentrated hydrochloric acid after a prolonged boiling; more easily by sulphuric acid.

Finally we have the Pennines, having optical form and properties of the rhombohedral system; they have also a basic cleavage, greasy lustre and the general properties of the Chlorites.

Ottrelite (Hydrated Silicate of Alumina, Iron and Magnesia).-Crystallizes in hexagonal tabular plates; cleavage par allel to the base; blackish-gray color; lustre a little greasy,