GENERAL PRINCIPLES — CONCLUDED 29 



due only or largely to sinking or gradation or to both is at present 

 often difficult or impossible to determine, though it is quite certain 

 that both processes have often been operative. 



Emergence may be caused either by (1) rise of the land; (2) 

 lowering of the sea; or (3) both combined. Except rather locally 

 as in the cases of mountain-making (orogenic) movements, it seems 

 doubtful if there is any good evidence for very considerable actual 

 uplifts of extensive land areas thus causing great sea retrogressions. 

 On the other hand, the earth is certainly a contracting body with 

 its whole surface approaching nearer and nearer to its centre. It 

 appears that "the rigidity of the earth may be such that its outer 

 parts are able to withstand for a time the strain set up by contrac- 

 tion. As the strain accumulates, it ultimately overcomes the resis- 

 tance, and the outer part of the earth yields. If the yielding results 

 in the sinking of the ocean basin, the surface of the water is drawn 

 down, and the surrounding lands seem to rise, unless they sink as 

 much as the surface of the sea does at the same time. The lowering 

 of the sea surface, because of the sinking of the sea-bottom, is prob- 

 ably the most fundamental single cause of the apparent rise of 

 the land. The periodic emergences of the continents, alternating 

 with periodic submergences in the course of geological history, are 

 perhaps to be thus explained. Periodic submergences, on the other 

 hand, might be explained by the sinking of the continental segments 

 of the earth, or by such sinking combined with the processes already 

 referred to which cause the rise of the sea." l 



Paleogeography 



Paleogeography literally means "ancient geography" and deals 

 with the geographic conditions of the earth during geologic time. 

 In making a paleogeographic map to represent North America at 

 a given time in its history, the attempt is made to show the rela- 

 tions of lands and waters with distinctions between deep and 

 epicontinental seas where possible, areas of continental deposition, 

 directions of ocean currents, etc. Until quite recent years there 

 were only crude attempts at making such maps for North America, 

 for the knowledge of the continent was not sufficient to form a 



1 R. D. Salisbury: Physiography, Advanced Course, pp. 401-402. 



