250 J. Bar r ell — Upper Devonian Delta of the 



not so pronounced as to mask the constructional nature of the 

 terrace, as shown by the relation of its upper surface to wave 

 action.* To what extent then are the continental terraces 

 constructional features controlled by erosion and sedimentation, 

 and what are the limits which isostasy places upon them ? The 

 answer must satisfy the evidence of both geologic and geodetic 

 nature. It will depend to a considerable degree upon the prevail- 

 ing conceptions on the rigidity of the crust, as measured by the 

 area and thickness of load which may be piled up before 

 sinking of the foundation begins to keep pace with further 

 loading. In order to gain such an answer the writer has 

 investigated the subject under a separate title, f The results 

 derived from both geologic and geodetic evidence, contrary to 



Fig. 5. 



Fig. 5. Section of the Coastal Plain and Continental Shelf through Eich- 

 mond and Fortress Monroe, Va. ; vertical scale multiplied by 26. 



some recent statements, indicate that the crust now sustains in 

 places loads represented by several thousand feet of rock over 

 areas of several tens of thousands of square miles. Judged by 

 this measure it is quite possible that the continental shelves may 

 be in places constructional features built far outward and 

 upward ; changing what would otherwise be a gentler slope 

 from continental platform to ocean basin into a pronounced 

 subaqueous shelf and steeper distal slope. 



In the light of this possibility, attention may be turned to 

 the actual evidence and the inferences from that evidence 

 as given by the Atlantic Coastal Plain and its extension 

 in that part of the continental shelf southeast of Pennsylvania 

 and Maryland. It is a region which has been free from 

 folding since the Permian, from fracturing since the Middle 



* Professor T. C. Chamberlin has discussed in current articles the signifi- 

 cance of this surface of wave planation as showing a lack of control by dias- 

 trophism. See especially Jour, of Geol., vol. xxi, pp. 523-533. 1913. 



f To be published in the Journal of Geology under the title, — "The Strength 

 of the Earth's Crust." 



