Occurrence and eruptive Origin.—Very conclusive evidence has been found recently showing that certain granites of Maryland are eruptive in origin. The granitic masses occur in irregular bosses breaking through gneiss, gabbro and other crystalline rocks. In the central part of the state, at Dorseys Run station, Woodstock, Ilchester and Ellicott City, the acid eruptives are true granitites, granular aggregates, consisting essentially of quartz, feldspar and black mica, with considerable amounts of epidote and allanite as accessory constituents. These rocks vary considerably in color, from dark iron-gray to nearly white, according to the percentage of ferro-magnesian silicates they contain.
Microscopic Characteristics.—Under the microscope thin sections sometimes show that the incipient stages of dynamic metamorphism have set in. In other occurrences the granites show little or no signs of mechanical deformation. The occurrence of the two prominent accessory minerals in the Maryland acid rocks was first noted . . .