F. H. KnowUon — Evolution of Geologic Climates. 195 



that deposits of gypsum necessarily imply aridity is 

 * ^ surely unwarranted. ' ^ That gypsum may sometimes be 

 precipitated from impounded sea water is undoubtedly 

 true, but that all or even a very considerable part is so 

 formed is not proved, and, I may add, that to my mind 

 it is not provable. 



As a supposed sure indication of aridity the presence 

 of red beds furnish another case in point. It now seems 

 to be acknowledged that the formation of red rocks 

 (except when derived from rocks originally red) is not 

 now known to be going on under desert conditions at the 

 present time, and hence there is little reason to suppose 

 that they were so deposited in the past. 



The presence of so-called sun cracks seems also a ques- 

 tionable indicator for aridity. They might better be 

 called shrinkage cracks, for they are developed whenever 

 and wherever mud dries out, whether the sun is shining 

 on it or not. 



Professor Coleman ^^ finds it difficult to believe in a 

 warmly humid world enveloped in rain clouds that never 

 parted to let in the sun until the Pleistocene.'' 



Again I must call attention to the fact that I have 

 nowhere stated that the sun never shone through the 

 cloud envelope, but rather that it did not gain permanent 

 control of earth temperatures until or approaching 

 Pleistocene time. 



Both Professor Coleman and Professor Schuchert 

 appear to have overlooked or perhaps failed to appre- 

 ciate one of the principal objects I had in mind in writing 

 the paper on geologic climates, namely, the search for the 

 explanation of certain of the fundamental principles that 

 must have operated in determining and delimiting the 

 climates of the past. 



For example, has the sun dominated earth tempera- 

 tures throughout all geologic time as it is acknowledged 

 to have done during and since Pleistocene time? 



If the sun causes a zonal disposition of temperatures 

 on the earth's surface, what caused or permitted the non- 

 zonal disposition that all agree to have obtained for at 

 least vast stretches of time when climates were undoubt- 

 edly equable over the whole earth? 



What was the source or sources of heat that warmed 

 the early oceans ? 



