36 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



settling of an oceanic sector, will cause internal readjustments which 

 will manifest themselves in one way or another at the surface in other 

 places. The greater disturbances, which are appropriately recog- 

 nized as introducing and terminating periods and systems, are likely 

 to cause the most obvious movements of the strand line. It seems 

 possible that a downward mass-movement may be compensated in 

 part by the rise of a neighboring mass, but it seems improbable that 

 such compensating movements would be so nearly equal that the 

 strand line would remain unaffected. Inasmuch as the constant 

 tendency of rock masses under the influence of gravity is downward 

 toward the center of the earth, it is difficult to conceive of the actual 

 (as distinct from apparent) upward movement of any great mass of 

 the globe except as a result of some still greater mass-movement 

 downward. 



This is not the place to enter into a discussion of diastrophism. 

 But there are some questions which refer to the cause of diastrophic 

 movements and which bear so directly on the problems connected 

 with the Jurassic marine invasion that it seems advisable to at least 

 ask them, even though they cannot be answered. Were the great 

 disturbances which caused the major fluctuations of sea-level, move- 

 ments constantly in progress, or were they relatively short interrup- 

 tions of a state of general repose in the mass of the earth ? Locally 

 applied, was the advance of the water into the Jurassic sea caused by 

 subsidence in western North America, or was it caused by a decrease 

 in the capacity of the ocean basins, due perhaps to submarine vulcan- 

 ism or the discharge of sediments into the sea? Was the drainage of 

 the sea due to a rise in western North America of the land which 

 had previously subsided, or was it caused by an increase in the 

 capacity of the oceans, due perhaps to subsidence of some part of 

 an ocean bed? Are great land masses as fickle as some geologists 

 are wont to suppose, rising and falling frequently, or are many of 

 the supposed movements of land only apparent because of move- 

 ments of sea-level? Was the Jurassic sea drained quickly, or did 

 the retreat of its waters occupy an appreciable length of geologic 

 time? To be still more explicit, was the sea advancing during all 

 of lower La Plata time and retreating during all of upper La Plata 

 time? 



The Jurassic sea apparently represents the maximum advance of 

 sea water over the North American continent and therefore the 

 maximum rise of sea-level during the Jurassic period. But inas- 

 much as only small parts of the continent were covered, the trans- 



