CHAPTER IX.
OF THE MATERIALS OF MOUNTAINS:—SECONDLY, SLATY CRYSTALLINES.
§ 1. It will be remembered that we said in the last chapter (§ 4) that one of the notable characters of the whole group of the crystallines was the incomprehensibility of the processes which have brought them to their actual state. This however is more peculiarly true of the slaty crystallines. It is perfectly possible, by many processes of chemistry, to produce masses of irregular crystals which, though not of the substance of granite, are very like it in their mode of arrangement. But, as far as I am aware, it is impossible to produce artificially anything resembling the structure of the slaty crystallines. And the more I have examined the rocks themselves, the more I have felt at once the difficulty of explaining the method of their formation, and the growing interest of inquiries respecting that method. The facts (and I can venture to give nothing more than facts) are briefly these:—
§ 2. The mineral called mica, described in the course of the last chapter, is closely connected with another, differing from it in containing a considerable quantity of magnesia. This associated mineral, called chlorite, is of a dull greenish color, and opaque, while the mica is, in thin plates, more or less translucent; and the chlorite is apt to occur more in the form of a green earth, or green dust, than of finely divided plates. The original quantity of magnesia in the rock determines how far the mica shall give place to chlorite; and in the intermediate conditions of rock we find a black and nearly opaque mica, containing a good deal of magnesia, together with a chlorite, which at first seems mixed with small plates of true mica, or is itself formed of minute plates or spangles, and then, as the quantity of magnesia increases, assumes its proper form of a dark green earth.
§ 3. By this appointment there is obtained a series of materials by which the appearance of the rock may be varied to almost any extent. From plates of brilliant white mica half a foot broad, flashing in the sun like panes of glass, to a minute film of dark green dust hardly traceable by the eye, an infinite range of conditions is found in the different groups of rocks; but always under this general law, that, for the most part, the compact crystallines present the purest and boldest plates of mica; and the tendency to pass into slaty crystallines is commonly accompanied by the change of the whiteness of the mica to a dark or black color, indicating (I believe) the presence of magnesia, and by the gradual intermingling with it of chloritic earth; or else of a cognate mineral (differing from chlorite in containing a quantity of lime) called hornblende.
| Fig. 5. |
Such, at least, is eminently the case in the Alps; and in the account I have to give of their slaty crystallines, it must be understood that in using the word "mica" generally, I mean the more obscure conditions of the mineral, associated with chlorite and hornblende.
§ 4. Now it is quite easy to understand how, in the compact crystallines, the various elements of the rock, separating from each other as they congealed from their fluid state, whether of watery solution or fiery fusion, might arrange themselves in irregular grains as at a in [Fig. 3], [p. 106]. Such an arrangement constantly takes place before our eyes in volcanic rocks as they cool. But it is not at all easy to understand how the white, hard, and comparatively heavy substances should throw themselves into knots and bands in one definite direction, and the delicate films of mica should undulate about and between them, as in [Fig. 5] on page 114, like rivers among islands, pursuing, however, on the whole, a straight course across the mass of rock. If it could be shown that such pieces of stone had been formed in the horizontal position in which I have drawn the one in the figure, the structure would be somewhat intelligible as the result of settlement. But, on the contrary, the lines of such foliated rocks hardly ever are horizontal; neither can distinct evidence be found of their at any time having been so. The evidence, on the contrary, is often strongly in favor of their having been formed in the highly inclined directions in which they now occur, such as that of the piece in [Fig. 7], [p. 117].[45]