THE PERMIAN PERIOD. 677 



India, Australia, and South Africa, it would be farther away from any one of 

 them than is any present center of continental glaciation from the present poles. 

 In most of the ancient periods, the polar regions do not seem to have been affected 

 by glaciation, but rather to have been frequented by ferns, corals, figs, and mag- 

 nolias. A general climatic stage verging toward glaciation and furnishing an 

 appropiate climatic environment for a regional concentration of frozen precipi- 

 tation, is prerequisite, and this is the primary problem. 



Local elevation has been postulated, but the stratigraphic and other geologic 

 evidences do not seem to support it. Marine beds are inter stratified in the gla- 

 cial series without evidences of extraordinary elevation and intensified erosion 

 between their respective depositions. A case has been cited in which a bivalve 

 was found standing on edge, as in life, in clay among glacial bowlders. Other 

 evidences of iceberg deposition have been adduced, implying that the glaciers 

 reached the sea. 



The Crollian hypothesis 1 assigns this as well as other glaciations to a tem- 

 porary increase in the ellipticity of the earth's orbit. If the fundamental con- 

 ception is valid, ellipticity might be a cooperating agency, but, as already 

 remarked, the general conditions that render any areal glaciation on lowlands 

 possible would seem to be a prerequisite to its effective action. The hypothe- 

 sis is virtually reducible to a change in the distribution of heat between sum- 

 mer and winter, the sum total of the heat remaining the same. In those periods 

 in which the polar night was not fatal to ferns, figs, and magnolias, the elliptical 

 variation could scarcely be effective. 



Among the purely astronomical hypotheses, the variability of the sun's heat, 

 which is supported by the recent observations of Langley 2 and Abbot, is the 

 most substantial. Obviously, however, short-period variations in heat emission 

 are inapplicable, as the glacial epochs can scarcely have been less in length than 

 some few tens of thousands of years each; and the problem of localization, that 

 thorn in the flesh of all hypotheses, is left untouched. 



1 Climate and Time. 



2 The solar constant and related problems. Astrophys. Jour. 1903, pp. 89-99. 



