682 T. C. CHAMBERLIN 
important class of marine deposits and a typical kind of life. So far 
then as the testimony of these highly important formations bear on 
the problem of continental rejuvenation, they imply that it was not 
a continuous steadily progressive process but that it recurred at 
such wide intervals as to permit great advances toward mature 
base-leveling as well as the evolution of shelf-seas, of parallel 
terranes, and of cosmopolitan faunas. All this seems in turn to 
imply an elastic, rigid, crystalline earth. The form of isostasy that, 
is compatible with this has its working method in periodic 
approaches to complete isostasy through the successive accumula- 
tion and easement of strains in the body of the earth. 
