29 



The positive areas were seldom elevated enough to have great erosive 

 activity and the land masses were generally quite low. As a result there 

 are certain limestone formations which hold their lithologic character for 

 hundreds of miles. 



In his epoch-making work on the Eevision of the Paleozoic Systems, 

 published in volume 22 of the Bulletin of the Geological Society of 

 America, in 1911, Ulrich has given a very detailed account of the dia- 

 strophic and faunal criteria employed by him in his studies on strati- 

 graphic classification, and he has devoted much attention to the oscillatory 

 character of continental seas. The northeast-southwest troughs, separated 

 by barriers or areas around which warping took place, were invaded by the 

 sea many times during the course of a period. Other barriers or areas 

 extending at approximate right angles to these troughs, separated them 

 into basins. The general idea was that the sea advanced and retreated 

 many times during the course of a period within these comparatively 

 restricted basins. With each successive invasion sediments were deposited 

 over a larger area so that the final result was a series of overlapping 

 deposits thinning out on the sides of the trough. Each trough or basin 

 would contain series of formations marked off by diastrophic and faunal 

 evidence and a formation would be lithologically and f aunally similar only 

 in its own basin, except in periods of great submergence when the sea 

 advanced over the barriers. As the different basins of sedimentation at 

 times connected with different oceanic areas, the marine forms of life in 

 them would consequently be different. Therefore, formations of the 

 same age in adjoining basins may differ totally not only in their lithologic 

 characters, but also in their faunal contents. These troughs of deposition, 

 with their separating barriers, were greatly influenced by later and later 

 periods of diastrophism with the result that in the Appalachians, where 

 folding occurred, the barrier or anticlinal structure is now much dimin- 

 ished in width and is often represented by an overthrust fault. 



In addition to the north and south structural lines it has been found 

 that there were definite east and west lines or axes which serve as pivots 

 of oscillation for the continent. The formations thin from either side 



