TO QUESTION PERIODIC DIASTROPHISM 



609 



hand the idea of submarine diastrophism gives us a new method of 

 strengthening the case for continuous diastrophism. 



The periodic advance of e pi-continental seas. — It is generally 

 admitted among geologists that at certain times during geological 

 history the continents were widely submerged. It is notable that 

 such times were the occasions when diastrophic evidence is especially 

 scarce. On the other hand, where the evidence for diastrophism is 

 especially good, as at the end of the Paleozoic and in the late Ter- 

 tiary, the continents were widely extended. There may be more 

 significance to this occurrence than is commonly assumed. 



Fig. 2. — A cross-section of the ocean bottom. Vertical scale about 150 times the 

 horizontal. (Courtesy of the Scientific American.) 



T. C. Chamberlin explains these marine oscillations by a com- 

 bination of two factors.^ One of these is the periodic depression of 

 the ocean basins which causes the draining of the lands and the other, 

 the transporting of sediment out into the ocean, thus raising the sea- 

 level, which causes submergence of the lands until there is a new 

 depression of the sea bottom. 



While the idea of the periodic depression of the ocean basins is 

 quite commonly accepted, the basis for such an idea is difficult to 

 discover. Presumably it is supposed that the lands are eroded and 

 the ocean basins covered with sediment until they are so weighted 

 that they sink under the burden. However, it must be remembered 

 that the larger part of the oceanic basins is receiving almost no sedi- 

 ment, as is readily shown by the presence of Cretaceous fossils in 

 the oceanic dredgings. Could this slow sedimentation be expected 



I Chamberlin and Salisbury, Geology (1906), Vol. I, p. 57. 



