66 ulrich: major causes of oscillations 



nently "sunken areas." Suess, however, believed that the 

 median areas of the continents are essentially stable, a view 

 adopted by Schuchert, who holds "that the continent (North 

 America) is a horst, that the great medial region remained un- 

 moved, while the margins were often folded and elevated. The 

 seas periodically flowed over this medial land — in fact, were 

 elevated over it — owing to the detrital materials unloaded into 

 the oceanic areas, thus filling them and causing them to spill 

 over on to the lands." 



I can not subscribe to this opinion. On the contrary, though 

 accepting the idea of permanent oceans and continents, it seems 

 to me that the crust of the lithosphere was subject to periodic 

 movement away from the poles; that the surface of the lands 

 was exceedingly unstable in the median areas as well as along 

 the borders of the continents. Schuchert's paleogeographic maps, 

 indeed, offer convincing proof of such instability; and the more 

 detailed maps made since his appeared, further substantiate 

 my claim. 



In reaching these conclusions I am mainly influenced by a 

 lifetime study of Paleozoic formations and their faunas. The 

 criteria and principles used in the course of these stratigraphic 

 investigations are defined and discussed in my Revision of the 

 Paleozoic Systems published in 191 1. In this work more than 

 100 previously undescribed instances of differential vertical 

 movements of lands and consequent shifting of seas are dis- 

 cussed in varying detail. Since 19 10 much additional informa- 

 tion has been gathered concerning such oscillations in North 

 America. 



On this occasion I shall mention briefly some of the more 

 convincing of the published cases and in greater detail a few of 

 the more recently determined instances — enough of both to 

 show that from the beginning of Cambrian time the surface 

 of the continents was exceedingly unstable and subject to fre- 

 quent oscillation, and that the epicontinental seas were corre- 

 spondingly inconstant, shallow, relatively small and frequently 

 withdrawn in part or entirely. Even in the same geological 

 provinces the outlines of the new sea may agree essentially and 



