548 F. H. KXOWLTOX EVOLUTION OF GEOLOGIC CLIMATES 



practically tlirougliout the entire geologic colmnn. Hardly less than a 

 dozen of these are now known, ranging in age from Huronian to Eocene. 

 It seems to be a very general assumption by those who hold this view that 

 these evidences of glacial activities are to be classed as ice ages, largely 

 comparable in effect and extent to the Pleistocene refrigeration, bnt as a 

 matter of fact only three are apparently of a magnitude to warrant such 

 designation. These are the Huronian glaciation, that of the "Permo- 

 Carboniferous/"' and that of the Pleistocene. The others, so far as avail- 

 able data go, appear to be explainable as more or less local manifestations 

 that had no widespread effect on, for instance, ocean temperatures, dis- 

 tribution of life, etcetera. They might well have been of the type of 

 ordinary mountain glaciers, due entirely to local elevation and precipita- 

 tion, of which we have many examples now existing. And in this con- 

 nection it may not be out of place to call attention to the difficulty of 

 accurately fixing in the time scale these often widely scattered areas 

 exhibiting glacial phenomena. It seems to me not only possible, but 

 entirely probable, that there is a considerable element of error in the 

 determination of the stratigraphic position and assumed or implied sjn- 

 chroneity of certain of these ancient glaciated areas. They are often 

 unaccompanied by fossil remains, and hence their position can only be 

 fixed within very broad limits. 



PHYSIOLOGICAL ACTIOX OF IXCREASED CLOUDIXESS OX PLAXT LIFE 



As already abundantly set forth in preceding pages, the conclusion 

 seems inevitable, if the data has been correctly interpreted, that there 

 was increased cloud production during the early geologic ages which shut 

 out access of the sun to the earth's surface. As the question is almost 

 certain to be raised as to the probable physiological effect of this cloud 

 mantle on plant life, I have taken occasion to ascertain recent observa- 

 tions and opinion on this point. Inasmuch as a very considerable per- 

 centage of existing plants grow under conditions where they seem to 

 demand, or at least tolerate, strong sunlight, it has seemed to many a 

 fair inference that similar condition must have applied to the earlier 

 floras — that is to say, that their presence is proof positive of the presence 

 of full sunshine — but the facts now available do not bear out this con- 

 tention. 



In this connection I consulted with Dr. John M. Coulter, of the Uni- 

 versity of Chicago, and through him with Dr. AVilliam Crocker, of the 

 same institution. Among other things. Dr. Crocker states : 



"Late work is indicating very strongly that the main effect that light has 

 on the development of the plant is its effect on photosynthesis and through 



