Where found in Maryland.—The granitic rocks of Maryland have lately received special attention, both microscopically and in the field. They occupy small areas, a dozen or more in number, scattered through the eastern part of the central zone of the state, known as the Piedmont plateau. This belt is essentially a crystalline one, made up almost entirely of gneisses, which are broken through in numberless places by gabbros, pyroxenites and granites, and other closely related types of igneous rocks.
The different areas may be known from the chief places within their respective borders. They are Port Deposit, Texas, Windsor road, Relay Sykesville, Guilford, Garrett park, Woodstock, Ilchester, Ellicott City and Dorseys Run.
Their mineralogic Composition.—Microscopically the rocks under consideration are composed largely of quartz, feldspar and mica, with accessory plagioclase, microcline, magnetite, apatite, zircon, epidote, allanite, hornblende, sphene and often some other minerals.
Four Types represented. . . .