the other. Small irregular grains of quartz are easily recognized by this means, either in sandstone, where it is almost the only mineral, or in granite or porphyry where it is associated with Feldspar.

Moreover, the color exhibited by a crystal belonging to any other than the cubic system, varies with the surface which is presented to the light. This explains why, in basalts, the crystals of Feldspar, distributed parallel to their planes of lateral cleavage, present, when observed between two Nicol prisms, alternate bands of different colors.

III. Another fact of which it is necessary to take cognizance in microscopic analyses, is the dichroism of minerals. Often a crystal affords colors which change with the direction in which it is regarded by transmitted light. We can, with a fragment of Calc-spar obtained by cleavage, observe two colors, or, at least two different shades, in nearly all colored crystals except those of the cubic system. M. Tschermak has been able to distinguish Augite from Hornblende by means of these colors.

IV. I shall recommend, finally, for this kind of research, a process which has often served me with advantage, and which consists in the employment of a lamina of Selenite of sensible thickness, but still of such thinness as to afford colors between two Nicols. Superposed upon a thin section of any substance which affords a color between the Nicol prisms under the microscope, this Selenite heightens the brilliancy of the colors.

PART FIRST.

PROPERTIES OF THE PRINCIPAL MINERALS WHICH COMPOSE THE ROCKS.

(A description of the leading minerals which constitute rocks, is alone afforded in this first part.)

FELDSPARS.

(Silicates of Alumina, with either soda, potash or lime). Hardness 6, being scratched by Quartz; generally cleavable in two directions having an angle between them of from 860 to 900; all more or less easily fusible.

Five different Feldspars with well-determined characteristics, are known: Orthoclase, in which potassa predominates; Albite, in which soda predominates; Anorthite, having a base of calcium or lime; and two or three other Feldspars of more complex composition; Oligoclase, having a base of soda and lime; and Labradorite, having a base of lime with a smaller proportion of soda.

Orthoclase (Aluminum potassium silicates).- Form: an oblique prism with a rhombic base, mod

ified generally upon the lateral faces. mm 118° 48'; pm = 1120 16'; ph1 = 90°.

= 1160 7'; pg1

Direction of cleavage: at first, parallel to p, then to g1.

Dominant forms: elongated in the direction of the heights (Fig. 1); elongated, also, in the direction of their bases (Fig. 2).

The crystals exhibit many different modes of grouping, the principal of which are the following: (1) coupled in

Fig. 1.

Fig. 2.

inverted manner upon a facet which modifies the lateral angles of their bases (and inclined 45° 3' to the vertical axis), so that the two bases P become adjacent to each other, also the lateral faces g1; (2) two crystals joined in a reverse manner, like an object and its reflection in a mirror, having their vertical edges parallel and partly interpenetrating each other, having one part common and the other part free (Fig. 3). The two bases form a re-entrant angle of 127° 46'; (crystals of granite and porphyries are examples).

If a fragment of one of these rocks be made to reflect a beam of light, in order to distinguish the basic cleavage of one of its feldspar crystals, the light from one portion of the base only is obtained; the reflection from the cleavage of the other portion is obtained only by turning the rock through an angle supplementary to the preceding. In granites and pegmatites, the small crystals are simple and exhibit the faces p and g1, and a face h1 with very unequal fractures.

Fig. 3.

Colors: white, reddish-white, gray, green, bluish-green. Powder grayish-white. Lustre: vitreous; sometimes pearly upon the cleavage faces; all degrees of transparency. Hardness 6. Specific gravity 2.5 to 2.6. Not acted upon by acids, except hydrofluoric and ammonium fluoride. It contains from 64 to 67% of Silica.

Varieties: Adularia, colorless and nearly transparent; Ama

zon-stone, green or greenish-blue, colored by copper: Loxoclase, containing more Soda than Potash; Mikroklin, containing about equal proportions of Soda and Potash; Sanadin, being the form found in Pumice and Trachyte.

Modifications of structure: laminated; granular (Leptynite); compact (Petrosilex); vitreous (Obsidian and Perlite).

The general characteristics exhibited by Orthoclase in rocks are its vitreous or slightly pearly lustre, its laminated fracture, its hardness and its fusibility before the blow-pipe. In order to observe this latter characteristic, it is necessary to break it into as small fragments as possible; then choose from among them those which have the sharpest edges or points, and place one in the platinum pincers; apply the hot flame of the blowpipe to the thin edge of the fragment; after a few seconds, the edges may be seen, by the aid of a lens, to be slightly rounded. Albite (Aluminum-sodium, silicate).-Form: a doubly oblique prism, often modified. The two cleavages, generally the most marked, have, between them an angle of 86° 24′ and corresponding― the first, pearly, to the base p; the second, more vitreous, to the face g' (Fig. 4). The crystals are generally reversed, the one upon the other, giving rise to the form exhibited in the figure. Hardness, somewhat greater than Orthoclase. Specific gravity 2.6; sometimes as high as 2. 63. The color varies from milkwhite to green or even to red. It is generally white in the rocks. In order to analyze it, it is necessary to treat it with barium hydrate or ammonium fluoride. It contains about 68% of Silica.

Oligoclase (Aluminum-sodium-calcium silicates). The principal cleavage-plane has a vitreous lustre and is marked by parallel striæ. A cleavage-plane of the second order, makes with one of the first order, an angle of 860 10'; the fracture in other places is somewhat splintery. The striae of the base arise