52 



IOWA ACADEMY OF SCIENCES. 



GEOGRAPHIC DEVELOPMENT OF THE CRIMEA. 



BY CHARLES R. KEYES. 

 (A.bstract.) 



A year ago I attempted to emphasize the importance of hav- 

 ing definite and absolute datum-planes for purposes of exact 

 geological correlation, and I endeavored to show that strati- 

 graphic unconformities have a far greater significance than is 

 usually ascribed to them, especially when viewed in the light 

 of modern physiographic principles. The latter not only 

 imply the recognition of peneplains, or stages of approximate 

 base-level of the land surface, but also in the sea areas the 

 equivalent representatives in certain deposited materials. The 

 cause inducing the new cycle of deposition being the same as 

 that which produces mountains, the method of stratigraphical 

 correlation based upon it was termed orotaxis, or classification 

 of geological formations by mountain development. 



In general, the relations of erosion and deposition, and of a 

 grade-plain, or peneplain, and the great planes of sedimentation 

 are about as follows (figure 1) : 



,_v^,5^ct--- 



Figure 1. Relation of Grade Plain and Great Plane of Sedimentation. 



As a rule the phenomena thus represented can only be made 

 out clearly after a careful examination of the geological feat- 

 ures over a very considerable region. The unconformity is of 

 very frequent occurrence, and its equivalent representative of 

 strata is equally often observable, but the practical correlation 



