38 McGee — Southern Extension of Appomattox Formation. 



lows that the sculptured forms on the one hand are genetically 

 related to the deposited beds on the other hand ; and it is 

 indicated not only by theory but by observation that such 

 relations sometimes extend not only from land province to sea 

 province, or vice versa, but across a sea province from one to 

 the other of the land provinces by which it is bounded, or vice 

 versa, and thus throughout great areas and perhaps continents. 

 In short, it is evident that when geologic agency was at work 

 in any spot it must have been doing something throughout the 

 neighborhood ; and properly conducted field work commonly 

 shows what this something was. 



Now the universal interrelation of geologic processes has 

 been constantly recognized in the researches into the genesis 

 (1) of the deposits forming the Coastal Plain and (2) of the 

 correlative sculptured forms found in the contiguous Piedmont 

 and Appalachian regions; and it has been ascertained that 

 each category of phenomena commonly represents two classes 

 of genetic conditions — one general and the other local. Thus 

 the topography of the Piedmont and Appalachian regions 

 gives a record of a certain elevation above base level during 

 each of three or four well defined periods, as a general con- 

 dition whose effects may be traced over immense areas ; but it 

 also gives a record of local conditions varying from place to 

 place with the volume and declivity of streams, the obduracy 

 of the rocks, the strike and dip of the strata, the homogeneity 

 or heterogeneity of the terranes, etc. Thus, too, the sedimen- 

 tary formations of the Coastal Plain give a record of submer- 

 gence beneath the ocean, during each of several well defined 

 periods, as a general condition affecting immense areas ; but 

 they also give records of local conditions varying from place 

 to place with the proximity to and volume of rivers, the ex- 

 posure to prevailing winds, waves, and currents, the depth of 

 submergence, the proximity to shores, etc. But the effects of 

 the general and local conditions respectively can commonly be 

 discriminated with confidence, and the products of the general 

 conditions traced throughout their extent. This is true specific- 

 ally of the littoral and marine formations of the Coastal Plain, 

 all of which represent widespread submergence with local 

 variations in depth of water, proximity to shores, activity of 

 deposition, vicinity of rivers, activity of waves, etc. The 

 Coastal Plain formations may therefore vary from place to 

 place in composition, texture, and structure ; but the different 

 phases are intergraduating parts of an indivisible unit, the 

 local variations are repeated from place to place, and the atti- 

 tude of each formation indicates a like relation between sea 

 ancLiand in all parts. 



y^w discriminating the general and local genetic conditions, it 

 is necessary to ascertain the relations between each formation 



