McGee — Southern Extension of Appomattox Formation. 39 



and its newer and older neighbors, and to interpret the record 

 of each unconformity in terms of continent growth. By this 

 means the different parts of a formation may be found to rep- 

 resent not only general community of genesis but community 

 of beginning and ending — in short entire community of struc- 

 tural relation. Each part of the formation then records in 

 similar terms the same episode in continent-building and world- 

 growth. 



So, when a Coastal Plain formation is found to represent 

 general community of genesis and structural relation in its 

 various parts it is considered homogenic and accepted as a 

 record of an episode in geologic history. The parts may or 

 may not be homotaxial ; one part may be slightly older than 

 another part; but in a general way it is contemporaneous 

 throughout. Homogeny implies not only equivalence but 

 synchrony. S 



The value of homogenic correlation is illustrated by the 

 Columbia formation : in the Middle Atlantic Slope the forma- 

 tion consists of two diverse phases, one of which is composed 

 of dissimilar parts ; in the Southern Atlantic and Eastern Gulf 

 slopes the brick clays, bowlder beds, and veneers of local debris 

 of the north are replaced by the "second bottoms" and coast 

 sands ; and in the Mississippi embayment the formation is rep- 

 resented by the stratified clays, silts, and sands of the Port 

 Hudson, and the strongly individualized loess with its basal 

 gravel beds. Yet these widely diverse deposits represent a 

 general condition of submergence varying in amount from 

 region to region, in a gradual manner ; the basal unconformities 

 are alike, and represent like conditions of continent-growth ; and 

 the degradation suffered by the formation in its various parts 

 is indicative of like antiquity. Again, one local phase found 

 in the Middle Atlantic Slope tells of local activity of the 

 rivers, the other tells of a general activity of estuarine waves 

 and currents, and both tell of glacial cold ; the testimony of 

 glacial cold fails in the Southern Atlantic and Eastern Gulf 

 slopes, but there one phase tells of river work and estuarine 

 conditions, and another of wave work and marine condi- 

 tions, both operating on a distinctive local formation. So, too, 

 the loess and the subjacent gravel beds in the Mississippi em- 

 bayment tell of glacial cold in the upper reaches of the river, 

 accompanied first by a stimulated transportation and subse- 

 quently by such submergence as to slacken the waters and 

 precipitate fine debris ; while the Port Hudson clays, silts, and 

 sands tell of submergence and estuarine deposition in the 

 brackish waters of an arm of the Gulf. Thus the general con- 

 dition represented by the deposit is everywhere the same ; 

 while the local variations may be ascribed to varying local con- 



