MARYLAI^D GEOLOGICAL, SURVEY 



85 



though the granites are less schistose and the diorites show horn- 

 blende more abundant than mica. 



GABBEO. 



Gabbro is the second type of crystalline rocks found within the 

 eountv. It is exposed in the valley of the Patuxent near Laurel 

 and lias been quarried to some extent on the Howard County side, 

 just above the Laurel bridge. When fresh and umiietamorphosed it 

 is a dense bluish to greenish black rock of granular texture and 

 extreme toughness. When metamorphosed it loses its granular tex- 

 ture, becomes more and more platy through the development of 

 fibrous hornblende and chlorite. When the gabbro weathers it forms 

 rounded boulders with dark rusty surfaces, well known as "nigger 

 heads." The distribution of this type is usually marked by deep 

 red and brown clay soils, which, though somewhat heavy, are strong 



and fertile. 



The crystalline rocks, as a complex unit, extend southeastward 

 beneath the (Joastal Plain, and serve as the basement on which rest 

 the gravels, sands and clays which occupy practically all of the sur- 

 face of the county. 



The Lower Cretaceous Formations, 

 the potomac group. 

 The Potomac group of the Coastal Plain consists of highly colored 

 gravels, sands, and clays which outcrop along a sinuous line from 

 Pennsylvania to Kichmond, passing near the cities of Philadelphia, 

 Wilmington, Baltimore and Washington. The Potomac deposits are 

 of great value because of the excellent brick clays which they contain. 

 All three of the formations which are now recognized as composing 

 this group are represented within Prince George's County. 



The Patuxent Formation. 



The Patuxent formation received its name from Patuxent River, 



in the basin of which the deposits of this horizon were first recognized 



as an independent formation and systematically studied. Careful 



work show^ed that the deposits formerly included in the Potomac 



