Abstract

The Laurel gneiss lies in the vicinity of Laurel, Maryland, about 17 miles southwest of Baltimore. A cover of Cretaceous gravels obscures all but about 10 square miles. On the north the Laurel gneiss is bounded by Guilford granite and the Wissahickon formation, on the northeast by gabbro, on the east by Cretaceous gravels, on the south by gneiss, and on the west by the Wissahickon formation.

The Laurel gneiss is gray, medium-grained to fine-grained, and faintly foliated. Chief minerals are quartz, oligoclase, biotite, and muscovite. Remnants of schist and quartzite from the Wissahickon formation are common.

Although the Laurel gneiss looks like a true migmatite, it is believed to have originated by the granulation, flowage, and recrystallization of the Wissahickon formation under conditions of stress, high temperature, and abundant water. This is suggested by: (1) the mineralogical and chemical similarity of the two rocks, (2) the granulated and recrystallized texture of the gneiss, (3) the abundance of schist and quartzite remnants which do not show granulation or recrystallization, and (4) the complete gradation from Laurel gneiss into the Wissahickon formation across contacts. The writer thus suggests the name “pseudomigmatite” for this body.

The Laurel gneiss has most of the features of a plutonic igneous rock but it does not show intrusive contacts. It is thought, however, that if conditions of metamorphism had been more intense it might have developed even these. The origin of the Laurel gneiss appears to be different from that of other igneous-looking rocks.

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