28 THE PHYSICAL FEATURES OF PRINCE GEORGE S COUNTY 



the literature on the subject is still scanty. The earliest reference 

 is found in a work by Martin entitled "A Comprehensive Descrip- 

 tion of Virginia, and the District of Columbia," published in 1835, 

 in which is given a brief description of the gneiss in the vicinity of 

 Eock Creek. Featherstonaugh in an article published in the follow- 

 ing year also refers to the gneiss of that region and states that it 

 dips to the east. Merrill in 1884 was the first geologist to describe 

 the crystalline rocks in any detail. Additional brief information is 

 given in articles by the same author published in 1886 and 1895. 



Williams, in 1891, in "Geology of Washington and Vicinity," Avas 

 the first to differentiate and describe the leading rock types occurring 

 in the vicinity of Washington and to explain their general structure. 

 The best and most complete account of the crystalline rocks of this 

 region is contained in ''The Washington Folio" already mentioned. 

 In that publication Keith describes all the different kinds of crystal- 

 line rocks of the District of Columbia and some of those of Prince 

 George's County. The data for the descriptions of the crystalline 

 rocks included in this report are principally taken from the Wash- 

 ington Folio. 



The Lower Cretaceous. — The first published statement regarding 

 the materials of the Potomac formations is contained in a book by 

 Finch on "Travels in the United States of America and Canada," 

 published in 1833, in which reference is made to the red soils exposed 

 in ravines between Bladensburg and Washington. The iron ores of 

 the County which occur in deposits of this age were described by 

 Ducatel and later by Tyson. A great advance in our knowledge of 

 these formations was made by Rogers in 1875, who published a fairly 

 good description of the Potomac strata which he correlated with the 

 Purbeck beds of England. In 1885 McGee proposed the name 

 Potomac for these deposits. The iron ores were discussed by Ben- 

 ton in Volume XV of the Tenth Census Reports and localities 

 and analysis of the ores are given. Plant remains have been found 

 at various localities which have been described by Knowlton, Fon- 

 taine, and Ward, while !^[arsh has described a number of vertebrate 

 fossils, principally dinosaurian remains, obtained near Muirkirk. 

 The question of the age of the deposits was long a disputed one 



