CLIMATES OF GEOLOGIC TIME. 



285 



A glacial period does not appear to remain constantly cold, but fluctuates between cold 

 glacial climates and warmer interglacial times of varj'ing duration. During the Pleistocene 

 there were, according to the best glaciologists, at least thi-ee, if not four, such warmer 

 intervals. The Permic glacial period also had its warmer times, while the interbedded red 

 strata of the Proterozoic tillites seem to point to the same variabihty. It is this decided 

 temperature fluctuation during the glacial periods that is so very difficult to explain. 



In addition to the well-known Pleistocene and Permic glaciation, there is rapidly 

 accumulating a great deal of evidence to the effect that there were at least two and probably 

 three other periods of widespread glacial climates. All of these were geologically very 

 ancient, earlier than the Paleozoic; in fact, one was at or near the close of Proterozoic time. 



Fig. 90.— Chart, of Geological Climates. Paleometeorolog}'. 



while another was at the very beginning of that era and almost at the beginning of earth 

 history as known to geologists. 



The oldest of all glacial materials occurs at the base of the Lower Huronian and is of 

 great extent in Canada. Seemingly of the same time is the Torridonian glacial testimony 

 of northwest Scotland. The Proterozoic tillites of China in latitude 31° N. may also be 

 of this time. If these correlations are correct, then the oldest glacial evidence indicates 

 that a greatly cooled climate prevailed near the very beginning of the known geologic 

 record and that it was dominant in the northern hemisphere. 



Toward or at the close of the Proterozoic there is other evidence of a glacial climate in 

 Australia, Tasmania, and Norway. These occurrences of tillites Ue immediately beneath 

 Lower Cambric fossiliferous marine strata and probably are of pre-Cambric age. 



In India there is also evidence of late Proterozoic tillites in two widely separated places, 

 and it may be that the inadequately studied Keweenawan testimony of the Lake Superior 



