24 DRIFT DEPOSITS OF ILLINOIS. 



In the early part of our investigations in Illinois and Iowa, 

 the writer was inclined to accept the "glacial theory," as a prob- 

 able explanation of the conditions under which the boulder clays 

 were deposited, mainly in deference to the ability of its able 

 advocates, but later and more widely extended observation have 

 compelled its rejection as a reasonable or necessary explanation 

 of the phenomena presented, after a careful study of the super- 

 ficial deposits of this State. 



When the advocates of the "ice cap" theory are able to pre- 

 sent some tangible evidence of the former elevation of the 

 northern portion of the American continent to the height of 

 several thousand feet above its present level, independent of 

 drift phenomena, or can point to some other plausible fact 

 going to show the prevalence of arctic conditions at a former 

 period over the temperate regions of North America, that theory 

 may be generally accepted, but until then, it is but fair to 

 assume that diverse opinions will prevail in regard to the 

 agencies by which the drift deposits of the Mississippi valley 

 were deposited. 



It has seemed probable to the writer that many of the advo- 

 cates of the "ice cap" theory became converts to the theory 

 before commencing field investigation, and their researches were 

 consequently devoted to obtaining such facts as were believed 

 to have a favorable bearing upon the accepted theory. If they 

 have gone into the investigation of the drift phenomena of the 

 Mississippi valley entirely free from all preconceived ideas as to 

 its origin, it is a question whether either "glaciers" or an "ice 

 cap" would have been suggested as the most probable theory to 

 explain the phenomena of the drift, as it is distributed over the 

 vast plains of the northwest. 



