50 THE PHYSICAL FEATURES OF PEIXCE GEORGe's COUXTY 



1885. 

 Chester, Frederick D. The Gravels of the Southern Delaware 

 Peninsula. 



Amer. Jour. Sci., 3d ser., vol. xxix, 1885, pp. 36-44. 



The high level gravels about Washington are said to be contemporaneous 

 with the high level or Bryn Mawr gravels of Delaware and southeastern 

 Pennsylvania which are questionably referred to the Cretaceous. 



McGee, W J. The Geology of the District. The Evening Star, 

 Washington July 11, 1885. 



In the article the author states that "but three essentially distinct forma- 

 tions occur within the District of Columbia. These are: (1) the Washington 

 gneiss, which includes the gneisses, mica schists, amphibolites (massive 

 green stones) and related crystalline rocks so well exposed along the Potomac 

 above Georgetown; (2) the Potomac formation, comprising the laminated 

 clays, sands and gravels spread over the highest lands of the District, and 

 (3) the Columbia formation, made up of the brick clays, sands and gravels 

 prevailing throughout the bluff-bound amphitheatre in which Washington is 

 located." The Potomac formation is said to be of Lower Cretaceous age and 

 to have been deposited in shallow water near the shore when the ocean stood 

 600 feet above its present level. The Columbia formation is said to be 

 "apparently a delta deposit laid down in the broad estuary of the Potomac 

 that existed when the waters of the ocean rose more than 100 feet above 

 present tide level." The various terraces about Washington are also 

 described. 



RoBiNSOx, T. The Strata Exposed in the East Shaft of the Water 

 Works Extension. 



Abst. Bull. Phil. Soc, Washington, vol. vii, 188.5, pp. 69-71. 



The section shows a thickness of about 144 feet of Columbia and Potomac 

 deposits representing 23 different strata. The Piedmont crystallines were 

 penetrated to the depth of 43 feet. 



Spexoer, J. W. Occurrence of Boulders of Decomposition at 

 Washington, D. C, and elsewhere. 



Amer. Nat, vol. xix, 1885, pp. 163-165. 



Briefly describes decomposition boulders of gneiss along the Potomac River 

 near Georgetown. 



