CAUSES OF PERMO-CARBONIFEROUS GLACIATION 8i 



In view of all this, is it not premature to assert that "the elevation of 

 the southern land masses " in Dwyka time is ' ' fully demonstrated" ? There 

 certainly were times when parts of the South African land masses were 

 deformed and elevated; but one of these times of elevation was so long before 

 the Dwyka period that the elevated areas had been worn down to small 

 relief when glaciation occurred; and the next occasion of strong deforma- 

 tion and elevation was after all the continental formations, of which the 

 Dwyka is the basal member, had been deposited; and moreover, this eleva- 

 tion occurred well to the south of the area of glacial dispersion. But in 

 Dwyka time, all du-ect evidence suggests that South Africa was a low- 

 lying continental area. It is of course permissible to postulate an elevated 

 continental area in a region concerning which we have no direct informa- 

 tion as to Dwyka topography, north of the northernmost Dwyka tillite 

 patches in the Transvaal; and this postulated highland may be regarded 

 provisionally as the source of the Dwyka ice sheet; but postulating an 

 elevation and demonstrating it are very different processes. 



White says also that "the occurrence of glacial phenomena within 

 the tropics was presumably due in part to an extension of the southern cold 

 with the favoring assistance of ocean currents and perpetual atmospheric 

 'lows,' resulting in part from continental relations and topography " (p. 631). 

 Here again, it is perfectly legitimate to postulate favoring ocean currents 

 and perpetual atmospheric "lows," if one wishes to do so, and then to 

 deduce the consequences of the postulates; but the value of the deductions 

 will necessarily depend upon the validity of the postulates themselves; 

 hence they must be examined. Ancient currents in the ocean and areas of 

 low pressure in the atmosphere should not, in the present state of scientific 

 inquiry, be arbitrarily assigned to this or that part of the world. The exist- 

 ing currents of the ocean and areas of low pressure in the atmosphere are 

 so systematically arranged that one may fairly object to any assumed 

 ancient distribution of these phenomena that is inconsistent with the con- 

 trols by which they are determined today. For example, a continental 

 area of low pressure in latitude 25° can occur only in the warm season, 

 when the high temperature on the land may suffice to reverse the 

 tendency to high pressure perpetually induced in that latitude by the 

 processes of planetary (atmospheric) circulation; and the relatively high 

 temperature by which a low-pressure area is formed in such a region is 

 evidently unfavorable to the occurrence of snowy precipitation. In the 

 cold season of the same region, when the temperature is more favorable to 

 snowfall than at other times, the atmospheric pressure will be high, but 

 precipitation in the area of high pressure will be small. Hence little aid 



