82 W. M. DAVIS 



can be given to Dwyka glaciation by areas of low pressure. It may be the 

 same with ocean currents. Until a reasonable cause can be given for the 

 occurrence of favoring ocean currents, their part in aiding the formation 

 of the Dwyka ice sheet is only a gratuitous postulate. However satisfactory 

 an explanation of a problem is, reached from such a postulate, it is not 

 safe to regard the explanation as fully demonstrated until the postulate 

 is independently established. The desirable thing in such a problem 

 as this is the publication of diagrams, on which the distribution of lands 

 should be indicated as described by White — "an Antarctic continent, of 

 which Australia, South Africa, and a part of South America were possibly 

 but lobes" (p. 630) — with a reasonable and warrantable distribution of 

 ocean currents and areas of low atmospheric pressure in proper relation 

 to the postulated lands and oceans. It would then be possible for a reader 

 to judge how much favoring assistance might be expected from these 

 contributive causes. 



The Permian glacial climate is one of the most remarkable problems 

 disclosed by geology. The addition of a South American glaciated area 

 to the others in Australia, India, and South Africa goes far, as White 

 points out, toward excluding any recourse to tempting explanations based 

 Qn the displacement of the earth's axis. The present limitation of precipita- 

 tion in at least the South African glaciated area chiefly to the warmer season, 

 while the colder season is prevailingly clear and dry, is a difiSculty that 

 must not be overlooked. The full explanation of the problem does not 

 involve only a moderate reduction of temperature; it involves either so 

 strong a reduction of temperature that snowy precipitation may prevail 

 in the warmer season, or a more moderate reduction of temperature with 

 a change of the season of precipitation from the warmer to the cooler part 

 of the year. Under the present understanding of atmospheric circulation, 

 the first of these alternatives is less puzzling than the second. 



W. M. Davis 



Harvard University 

 November 24, 1907 



