Two other types of igneous rocks called aplite and pegmatite occur sparingly in Groton State Forest. Both of these are productions of crystallization of residual fluids or late stage magma related to the granite. These were emplaced along cracks or planes of weakness in the granite after the granite had solidified. When viewed from the surface the aplite or pegmatite generally appear as bands cutting through the granite. However, when the third-dimension is considered it is easily realized that they are tabular or sheet-like in shape. Igneous rock masses having these dimensions are called _dikes_. At Groton most of the dikes are nearly vertical with a thickness ranging from less than an inch to more than several feet and extending for considerable distances. On Owlshead, one of these dikes is nearly three feet thick. The extent of these dikes is not known because they are only partly exposed, in that they extend beyond the limited areas of rock exposure. [Illustration: GROTON STATE FOREST] DREW MTN NIGGERHEAD MTN BLAKE HILL NIGGERHEAD BROOK KETTLE MTN SPICER MTN OWLSHEAD MTN KETTLE POND STILLWATER BK. HARDWOOD RIDGE BEAVER BROOK SILVER LEDGE LITTLE SPRUCE MTN PEACHAM POND DEER MTN DEVIL’S HILL PEACHAM BOG LITTLE DEER MTN OSMORE BK. COLDWATER BK. GROTON POND JERRY LUND MTN RICKER POND RICKER MILLS EXPLANATION GRANITE EXPOSURES SCHIST EXPOSURES TRAIL RAILROAD SWAMPY AREAS CONTOUR LINE WITH ELEVATION CONTOUR INTERVAL IS 100 FEET TOPOGRAPHY FROM U. S. GEOLOGICAL SURVEY MAPS BY ROBERT CHRISTMAN The _pegmatite_ dikes are coarse-grained, in some cases consisting of individual mineral grains as much as two to four inches in diameter. The mineral composition of the pegmatites is nearly the same as the granite, except that biotite is usually absent. Because of their larger grain size, the minerals can be recognized more easily in pegmatites than in either granite or aplite. Quartz is glassy and breaks with smooth curved fractures. Feldspar is chalky white, or pink, and may occur as tabular crystals with straight-line contacts. It tends to break along definite intersecting planes which can be seen in their reflecting position. Muscovite occurs as “books” of semi-transparent leaves. The large “books” of muscovite are particularly interesting because of the fascinating fact that a mineral sheet can be split along a given planar direction into thinner and thinner sheets until they are too thin to handle. Theoretically the mineral might be split into sheets only as thick as one layer of atoms. The ability of a mineral to break along definite planes is related to its atomic structure and is called _cleavage_. The cleavage in mica is perfect, whereas the cleavage in feldspar is only poorly developed, and quartz does not possess cleavage at all.