Shaler.] 288 [December 16, 



have their analogues in European species, but which generally show 

 some slight differences, should really be considered as distinct species 

 or as mere varieties, can never be fully determined until careful com- 

 parisons are made with numerous specimens from both continents, 

 and the larval habits and characters of each are known. But 

 whether the differences be considered varietal or specific, it seems 

 best to register them under different names. 



Notes on some of the Phenomena of Elevation and Subsi- 

 dence of the Continents. By N. S. Shaler. 



In endeavoring to account for the changes of level of the conti- 

 nents, it is necessary to consider not only the changes of the land, 

 but those of the sea as well ; the ancient view of the sea being the 

 mobile region was not without its truth, and was too quickly aban- 

 doned. In the following propositions I have endeavored to present 

 the possible cases and means whereby the sea can greatly change its 

 relation to the land. The order of succession by no means repre- 

 sents the relative importance of the action at work in the effecting 

 of change. My only object is, in a brief way to call attention to the 

 complication of agencies which effect this most important class of 

 actions. 



Propositions. 



1. The absolute change in height of the continent acting on the whole 

 mass thereof. 



This may conceivably be brought about if we suppose the conti- 

 nents to have been lifted to their positions by constant lateral pres- 

 sure arising from the more rapid loss of heat in the interior of the 

 earth, thus bringing about a folding of the outer beds to correspond 

 to the diminished interior. Whenever the secular loss of heat brings 

 about a strain sufficient to overcome the resisting friction and weight, 

 there will result an upward movement of the continent. This proposi- 

 tion is merely a brief statement of the prevailing opinion of geologists 

 as to the nature of the principal cause that produces continental folds. 



2. Absolute change of height of parts of the sea floor. 



Sudden and great changes in the contour of any continent may 

 occur without necessarily affecting the sea level, but all changes of 

 the sea floor, be they elevation or depression, unless the alterations of 

 each movement compensate each other, must be attended by modifi- 



