A specimen of granite from Owlshead was studied with a microscope after it had been cut and ground to a thickness of only 0.03 millimeters. Many minerals, which ordinarily appear to be opaque, are transparent when ground this thin. By their various optical properties, the different minerals can be identified and the composition of the rock can be determined. Figure 1 shows a photograph, taken through a microscope, of one of these thin sections of granite. By careful examination of the thin section and by measuring the areal extent of the different minerals present, the rock was determined to contain, by volume, 35 percent quartz, 60 percent feldspar (in proportions of 25 percent _microcline_ feldspar, KAlSi₃O₈ and 35 percent _plagioclase_ feldspar, NaAlSi₃O₈) and 5 percent mica (in proportions of 4 percent biotite and 1 percent muscovite). Although it is a member of the granite family, this rock should, in strict terminology, be called a quartz monzonite rather than a granite to indicate more precisely the mineral composition. Because of slight differences in composition, granite from the same body may elsewhere be correctly called granodiorite, quartz diorite or granite proper, depending on the relative amounts of the two feldspars and quartz. In this report these close distinctions have not been made and the rock is simply called granite. _Cracks in the granite_ Two kinds of natural breaks, or cracks, occur in the granite in the State Forest area. _Joints_ are breaks which occur along plane surfaces and _exfoliation_ is the name given to the breakage along curved surfaces related to the exposure of the rock. Granite, as contrasted with other rocks, is characterized by its uniformity of texture and massiveness, so that any cracks present are conspicuous. [Illustration: Figure 1. Photomicrograph of a thin section of granite from Owlshead Mountain. The mineral with the grid pattern (upper left) is a feldspar named microcline which has the composition of KAlSi₃O₈. The one with the indistinct striped pattern (lower center) is a feldspar named plagioclase, variety oligoclase, which has the composition of approximately NaAlSi₃O₈. The patterns for these minerals result from different portions of the same mineral grain having different orientations, called twinning, so as to give a different optical appearance. The clear white mineral (right center) is quartz. The dark gray mineral with the fine lines (upper center) is biotite and the smaller, lighter-colored, elongate mineral to the right of the biotite is muscovite. The other minerals are feldspar and quartz in different orientations. The actual diameter of the clear white quartz grain (right center) is about four-tenths of a millimeter so that the photograph is a magnification of about one hundred.]