REVIEW OF GEOLOGICAL STRUCTURE. ol 
there 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 Ohion, 
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 also 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 EK. 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 Alleghenies of West Virginia, and by Professor Safford in the 
Unaka range of Tennessee. (Geol. of Tenn., p. 438.) 
