68 ulrich: major causes of oscili.ations 



ment of the lands as the immediate cause. If the submergences 

 had been occasioned solely by rise of the waters, the successive 

 submergences would have been always similar in geographic 

 pattern and different only in lateral extent. In fact, a general 

 similarity or repetition of old patterns is recognizable, but there 

 is also exceeding diversity of expression; and often the differ- 

 ence is greatest when directly succeeding stages are compared. 

 Often again, when one stage appears to have been very differ- 

 ent from the next, the following third or fourth may be very 

 much like the first. Only oscillatory movements or warping 

 of the land surfaces could produce such results. The area af- 

 fected by such movements may be very large, as, for instance, 

 during the middle Ordovician and middle Silurian, when nearly 

 half of the continent of North America was involved. During 

 these periods the Gulf waters seem at certain times to have 

 been completely withdrawn from the southern part of the con- 

 tinent, the middle and northern parts at such times being tilted 

 so that the boreal sea extended southward beyond Chicago and 

 occasionally as far as northern Tennessee. 



Strictly, these widely operating movements hardly fall un- 

 der the category of epeirogenic movements. On the other hand, 

 they are not truly orogenic, if that term is to be confined to 

 movements originating in shrinkage of the centrosphere. Ap- 

 parently they indicate a combination of causes, perhaps begin- 

 ning or ending v^ith the play of orogenic factors that built moun- 

 tains in the submarginal areas whereas the warping and deforma- 

 tion of the more stable interior areas was mainly occasioned 

 by the necessity of isostatic readjustments to stresses incident 

 to the greater deformations of the orogenic movements. 



Then there were many relatively local changes in the strand- 

 line of continental seas that may be explained only by assuming 

 correspondingly local differential, vertical movements of the 

 lithosphere. I do not refer to movements connected with 

 volcanism. On the contrary, the best examples of the kind in 

 mind are found in areas but rarely or not at all directly affected 

 by volcanism. These dififerential movements indicate actual 

 elevation of one area while another nearby was sinking. More- 



