Page:Popular Science Monthly Volume 9.djvu/453

Rh feldspar, somewhat opaque and cloudy as they usually are in granite, but now and then clear and beautifully striped, and also the crystals of mica, imbedded in the clear quartz, which will be at once known by its bright clear colors and by the margin of rainbow-like tints which border its patches. Ordinary orthoclase feldspar is usually some-what opaque and dirty-looking under the microscope, and by this it may be distinguished from the clear, glassy sanidine which is frequently found in igneous rocks, and presents under the microscope, when polarized, pure rich colors as well as sharply-defined crystals similar in form to those of the common orthoclase. The orthoclastic feldspars may be very readily distinguished from the plagioclastic by their structure, as revealed by the polariscope; the latter invariably are seen to be striped with variously-colored bands, showing what is called twin crystallization; and the orthoclase, though often forming twins on a larger scale, does not present the minutely-banded



of the plagioclastic feldspars. The mica in the granite section will not be difficult to recognize, especially if Biotite; often we shall observe it as forming fairly-shaped hexagonal crystals, and the polariscope will also help us to know it by its thinly-laminated structure, giving rise to fine parallel striæ on the surface of its crystals. Its colors, also, when polarized will be duller than those of the quartz, for which it might sometimes be mistaken at first sight, should it be a light-colored mica; and then, again, it will frequently be found that when the prisms of the polariscope are crossed the mica becomes perfectly opaque, its sections having been formed across the optical axis. But let us now look at the quartz. We shall observe that this quartz is generally not crystallized in definite forms, as are the feldspar and the mica; it appears as a matrix which has been at some time or other soft and so is penetrated by the other crystals, the interspaces of which it fills up: this shows us at once that it must have been solidified after them, and so was unable to assume its regular forms. This is a very remarkable fact, and helps us toward the secret of the formation of the granite. We know that quartz requires a higher temperature to melt it than does either the feldspar or the mica, and so, had the granite been formed as are regular volcanic rocks in the ordinary way of igneous fusion, we should certainly have found that the quartz would have crystallized before either the feldspar or the mica, and it