WASHINGTON ACADEMY OF SCIENCES 115 



shore lines, he made an attempt to ascribe the various land forms and 

 the peculiarities of drainage and shore line to the causes that had 

 produced them. These lines of investigation led to a proposed genetic 

 classification of physiographic features. The influence of Powell, 

 Gilbert, and Chamberlin is acknowledged in this phase of his work. 

 The genetic principle was also fundamental in his stratigraphic work. 

 He was not satisfied with the mere description of stratigraphic units 

 and physically tracing them as far as possible, but he also tried to 

 reconstruct the conditions under which sedimentation took place, 

 and to ascertain the source whence the sediments came. He was led 

 to study the record of sedimentation with reference to the degrada- 

 tion of adjacent land masses and to consider the diastrophic move- 

 ments whereby the limits of areas of sedimentation and degradation 

 are reciprocally shifted. He developed the principle of homogeny 

 as a basis for geologic correlation and applied it extensively. He 

 endeavored to write geologic history in terms of diastrophism, sedi- 

 mentation, and degradation, his aim being to reconstruct the physical 

 history of the Coastal Plain. 



McGee is largely known to geologists because of the attention he 

 paid to those Coastal Plain formations in which marine fossils are 

 not known. He proposed the name Potomac formation for the non- 

 marine Lower Cretaceous sediments of Virginia and Maryland; he 

 gave the name "Appomattox" formation to certain gravels and 

 loams unconformably overlying the Miocene, and unconformably 

 overlain by geologically later deposits of Pleistocene age in Virginia; 

 and he applied the name Columbia formation to extensive terrace 

 deposits along rivers or facing the coast and to sands or loams on 

 divides. 



He attempted, in his consideration of the Potomac formation, which 

 has subsequently been subdivided into four formations, to recon- 

 struct the geologic history antecedent to its deposition and the phys- 

 ical conditions under which its deposition took place. According 

 to him, the deposition of the " Appomattox" (for which the subse- 

 quently proposed name Lafayette of Hilgard was substituted in 1891) 

 took place during a period of regional depression which lowered the 

 land in amounts ranging from 100 to 1,000 feet below its present 

 altitude. The depression, which was accompanied by crustal warp- 



