552 F. H. KNOWLTON EVOLUTION OF GEOLOGIC CLIMATES 



I submitted this statement to Dr. W. C. Alden, who writes me as follows : 



"It seems like a far cry from banded pre-Cambrian argillites to solar radia- 

 tion and seasonal temperatures, yet there may be a direct and logical relation. 

 The reasoning may be correct, but I would hesitate to jump directly to the 

 conclusion stated. There seems to be no doubt that variation in stream-flow 

 due to variation in precipitation resulting from seasonal changes will produce 

 lamination in fine sediments, but I doubt if all lamination is produced in this 

 way. Any other recurring condition that affected precipitation (rainfall) and 

 the load of sediment carried by the streams might produce such lamination. 

 The factor of recurrence at more or less regular intervals, however, strongly 

 suggests seasonal recurrence. This might be without the occurrence of winter, 

 meaning by winter a cold season such as we experience in the temperate zone 

 and higher latitudes. ... I am not sure the lamination of sediments asso- 

 ciated with glacial deposits might not be produced under other conditions. It 

 requires close analysis to eliminate alternative possibilities when one is con- 

 sidering what might have happened in pre-Cambrian times." 



Dr. E. S. Bassler, with whom I have discussed this question, has 

 expressed the opinion that the banding in the pre-Cambrian argillites 

 may possibly be due to segregation from shales more or less homogeneous 

 when originally deposited. As examples he cites the many banded 

 shales — all unaccompanied by evidences of glaciation — in Ordovician, 

 Silurian, and Devonian rocks. Dr. E. 0. Ulrich, with whom I have 

 also discussed the problem, doubts the efficiency of the segregation 

 hypothesis and considers the banding as purely a question of sedimenta- 

 tion. He is of the opinion that there are so many factors influencing 

 the deposition of fine sediments that it is unwise or unsafe to pin much 

 faith to variation in stream-How and consequent deposition as dependent 

 on seasonal changes, especially at a time so remote as the early pre- 

 Cambrian. 



In view of these conflicting opinions, it does not seem that this conten- 

 tion can be considered as much more than a possibility. It will have 

 to be supported by more exact information before it can be accepted. 



Barrell^s fifth objection reads as follows: 



"The abundance of carbon in the clays of early pre-Cambrian time suggests 

 the presence of sunlight sufficient to carry forward photosynthesis in plants 

 and therefore the absence of an extremely dense cloud envelope." 



This is a generalization on a very insufficient and insecure basis; for 

 while pure carbon or graphite may be produced by the metamorphosis of 

 coal or other vegetable debris, it by no means follows that it was alwajs 

 so produced, especially in the older rocks. Carbon is formed in meteor- 

 ites in a purely inorganic manner, as by the decomposition of metallio 



