192 F. H. KnoivJton — Evolution of Geologic Climates. 



Under this postulate cooled, if not glaciated, areas 

 could have been developed at any time and place where 

 the conditions were favorable, and these could easily have 

 been reflected in the growth-rings. But again it seems 

 necessary to call attention to the fact that growth-rings 

 are not dependent on changes in temperature, but are 

 produced with equal facility by changes in the supply of 

 moisture. 



The so-called seasonal banding of Pleistocene clays is 

 an attractive study, and it seems to many to have been 

 demonstrated by De Geer and others with reasonable 

 certainty as the result of seasonable changes in sedimen- 

 tation. It must be admitted that the resemblance is 

 striking between these Pleistocene clays and certain 

 banded shales and slates of earlier geologic ages, but on 

 consultation with a number of physical geologists it 

 appears that not all are as yet prepared to accept it at 

 its assumed face value. They point out that we still 

 know so little of the physical conditions surrounding the 

 sedimentation in which these bandings occur that it may 

 be hazardous to attempt to connect them with seasonal 

 fluctuations, especially when unaccompanied by evidence 

 of glacial activities as many undoubtedly are. These 

 << growth-rings ' ' of the rocks as they have been called are 

 obviously dependent on a periodic supply of moisture, 

 but does it necessarily follow that this fluctuation was 

 seasonal? 



A circumstance recently reported to me by Prof. A. F. 

 Foerste of Dayton, Ohio, may be of interest. All will 

 recall the disastrous flood that swept Ohio some years ago. 

 This flood was three days rising and four days receding. 

 When it was possible to enter the town Professor Foerste 

 found the floor of his laboratory covered with a layer of 

 fine mud some six inches thick. This mud was distinctly 

 stratified, there being not less than twenty-five distinct 

 layers that must have been formed within less than a 

 week. 



I shall have to leave this so-called seasonal banding an 

 open question. 



Under evidence of aridity Professor Coleman says : 



' ' To dispute the formation of salt and gypsum beds by evapora- 

 tion in times of dry heat without suggesting any other mode 

 of forming such deposits is surely unwarranted. ' ' 



Let us consider the formation of gypsum, pure deposits 



