84 TIJE GEOLOGY OF PKIXCE GEOKGE S COU-XTY 



it occupied during the previous submergence. There are a few ex- 

 ceptions to this, however, that will be noted in the descriptions which 

 follow. The traveler passing from northwest to southeast crosses the 

 outcrops of the formations in the order of their deposition. The 

 general sequence is shown in the accompanying table . 



The Crystalline Rocks. 



The exjxisures of crystalline rocks within the limits of Prince 

 George's County are confined to the deeper valleys along the northern 

 border of the county. Elsewhere the crystallines are completely 

 covered by the unconsolidated deposits of the Coastal Plain. Of the 

 different varieties of rock developed in the contiguous portions of 

 the Piedmont, only the granite gneiss and gabbro are exposed within 

 the confines of the county. Granite gneiss, diorite, serpentine, 

 gneisses, and schists are exposed within the District of Columbia and 

 in near-by portions of Montgomery and Howard counties. 



The oldest rock of the region is the Baltimore gneiss, or (Carolina 

 gTieiss, as it was earlier named by Keith in his description of the rocks 

 of Washington and vicinity. According to Keith — 



"The formation is composed of alternating layers of gneiss and schist of 

 a prevailing gray color, dark bluish gray where fresh, and greenish or 

 yellowish gray where weathered. Individual bands vary from a lew inches 

 up to several feet in thickness, with an average of perhaps less than a foot 

 . . . . The original nature of the gneiss, whether igneous or sedimentary, 

 is quite unknown." 



GRANITE GNEISS. 



The granite gneiss is the first of three classes of granite which 

 intruded the Baltimore gneiss and one of the two crystalline types 

 found within the limits of the county. Like the gneiss, it is almost 

 always gray in color, and the minerals have been arranged in approxi- 

 mately parallel lines as a result- of the metamorphism which the 

 original granite suffered. There is, however, no true banding, as 

 in the Baltimore gneiss, and the degree of schistosity varies from 

 place to place. The black patches found in the granite gneiss are 

 usually regarded as inclusions of Baltimore gneiss torn from the 

 latter by the former at the time of intrusion. 



Outside the county, within the District and elsewhere, are other 

 granites and diorites which might be confused with the latter, al- 



