§ 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 colour, 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.