REVIEW OF GEOLOGICAL STEUCTUEE. 31 



tljere are some, who have not had the opportunity of making the com- 

 parison, who still cling to the theory that the distribution of the Drift, 

 as well as the glaciation of the surface rocks, were produced by ice- 

 bergs, which floated over the continent during a period of submergence, 

 grounding, dragging, and grinding as they progressed. 



Principal J. W. Dawson, of Montreal, is generally credited with the 

 authorship of the iceberg theory of the Drift, but he is too good an 

 observer, and too well-read a geologist to exclude glaciers from participa- 

 tion in the great mechanical effects produced during the ice period. 

 That he differs from the writer in the reading of the history of the Drift 

 phenomena in the basin of the great lakes, is simply due to the fact that 

 he has not had the opportunity of studying on the spot the inscriptions 

 upon which our conclusions have been based. If he could come to Ohio, 

 and examine our Drift deposits, and the peculiar and characteristic 

 glacial markings on the rock surfaces, he would find here the same un- 

 mistakable evidences of glacial action that he has seen in those portions 

 of the country where he concedes that glaciers did exist. It is ako prob- 

 able that, if he had examined the Till or Bowlder clay which so generally 

 "covers the glaciated surface in the lake basin and Upper Mississippi 

 Valley, in which there are no marine fossils and no eastern Canadian or 

 Adirondack bowlders, he would abandon the view which he once enter- 

 tained that our glaciation was effected, and our Drift deposits were dis- 

 tributed, by icebergs floating from the north-east through the submerged 

 lake basin and down the Mississippi Valley. 



The arguments against the glacial, and in favor of the iceberg hypo- 

 thesis, advanced by Professor E. B. Andrews, in his report contained in 

 Volume I (page 447, et seq.), would hardly have been written if his dis- 

 trict had not been outside of the Drift area. These arguments are : 



First. That an ice sheet could not cover a large part of Ohio without 

 there being local glaciers in the Alleghanies. 



Second. That grounding icebergs could produce the planing, groov- 

 ing, and striation of the rocks. 



Third. That the clays, gravels, and sands of the Drift bear evidence 

 that they were deposited and arranged in water. 



Fourth. That the Canadian highlands were not high enough to afford 

 sufficient fall to carry glaciers by gravity through the basin of the lakes, 

 and over the surface of Ohio. The conclusion is that, in a general sub- 

 mergence, ice rafts and water currents produced all the Drift phenomena. 



To which, it may be answered ; 



First. The traces of local glaciers have been observed by the writer 

 in the AUeghenies of "West Virginia, and by Professor Safford in the 

 Unaka range of Tennessee. (Geol. of Tenn., p. 438.) 



