32 CORRELATION OF GEOLOGICAL FAUNAS. [bull. 210. 
Such movements of faunas may be assumed to have been more fre- 
quent and more apparent in such portions of the ocean bed as were 
near the shore, and thus where the sediments were in process of rapid 
accumulation and were expressed by varying classes of sediments. 
The Devonian formations of the upper portion of the Appalachian 
Basin were on this account particularly fitted to tell the story of shift- 
ing of faunas, and in the following pages evidence of the shifting, 
recurrence, and modification of faunas is reported, and it will be 
shown that the movement or migration of a fauna may occur with 
only slight evolutional mutation of the species. 
