462 T. C. CHAMBERLIN 
in the periodic development and emergence of circumcontinental 
terraces and their critical effects on the evolution of life, and (3) 
in the successive depletions and enrichments of the atmosphere. 
For subdivisions of lower order the migration of faunas and the 
special features of continental development will furnish appropri- 
ate bases, and below these again, the local phases of sedimen- 
tation and faunal adaptation will afford the provincial terms of 
a natural classification. If this hope be well grounded, the arbi- 
trary divisions that now vex our system may be largely elimi- 
nated. 
I. CC. (CaAwngiaRicion. 
