CAUSES OF PERMO-CARBONIFEROUS GLACIATION 



The article by David White on " Permo-Carboniferous Climatic Changes 

 in South America" in a recent number of this Journal contains a discus- 

 sion of the causes of that remarkable period of glaciation. One section of 

 the article is headed, "Exaggerated Temperature Effects of Elevation" — 

 a heading that seems to me more appropriate than the author presumably 

 intended it to be; for the temperature effects of elevation, as a cause of the 

 Permian glaciation of South Africa at least, appear to be greatly overrated 

 in the argument there set forth. 



It is not to be questioned that elevation of that part of South Africa 

 from which the Permian ice sheet spread across the neighboring lower 

 lands would have been an efficient cause of lower temperature; but at 

 present the chief evidence of elevation is the need of it in the climatic argu- 

 ment; and we are not yet in the position of having so effectually excluded 

 all other causes of glaciation as to warrant the acceptance of the elevation 

 of the area of glacial dispersion on the ground that no other possible cause 

 remains. It is true that^the idea of great elevation of the area of dispersion 

 was current among geologists in South Africa when I was there in 1905; 

 but on inquiring more particularly for the evidence in favor of this idea, 

 there appeared to be nothing more than its assumed necessity. 



Two reasons are assigned by White as indicative of land elevation: 

 "The enormous accumulations of coarse conglomeratic material in the 

 eastern regions testify to the steep gradient of the drainage systems," 

 and "The presence in nearly all regions of the great unconformity is itself 

 evidence of the vigor of the post- Carboniferous uplift" (p. 631). As I 

 see the case, neither of these reasons can be applied with force to South 

 Africa. 



As to the first of these reasons : the Dwyka tillite or glacial conglomerate 

 of South Africa is itself the chief conglomeratic deposit of great series of 

 continental formations in which it is the basal member; the other members 

 are largely sandstones and shales; the Ecca formation, which follows the 

 Dwyka more or less conformably, contains coal seams, which testify to 

 gentle gradients, not to steep gradients in the stream systems. The great 

 total thickness of these continental formations indicates a long-continued 

 progressive depression of their basin, and this is quite as consistent with a 

 long-continued uplift of never-lofty neighboring lands as with a great 



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