REVIEW OF GEOLOGICAL STRUCTURE. A3 
is capable of demonstration. One suchcase proves the presence of one ice- 
berg, and the many others which have been observed prove many icebergs. 
Hqually conclusive arguments to the same end are the thousands of 
bowlders which lie wpon the laminated clays; for no glacier or water cur- 
rent could have transported them to their present positions, without cut- 
ting away the delicate layers of impalpably fine clay on which they rest, 
and without leaving Bowlder clay or some other material beside these 
large, solitary erratics as tokens of their presence. 
Another fact confirmatory of the view that icebergs took part in the 
transportation of the Drift, is that the bowlders found resting on strati- 
fied sand and clay—often the most recent of our Drift deposits—are unl- 
formly masses of crystalline rocks, granite, greenstone, slate, etc., which 
have been brought from the Canadian highlands, while the bowlders of 
the Bowlder clay are in Ohio oftener than otherwise derived from indi- 
genous rocks. 
Prof. Jas. Hall reports the same thing in regard to the bowlders which 
are scattered over the surface about Albany, and which rest upon sand or 
laminated clays. In the Natural History of New York, part IV, p. 319, 
he says: 
In the vicinity of Albany and Troy I have searched in vain for a bowlder or pebble of 
granite or any other rock older than the Potsdam sandstone in the deposits below the 
clay, while in a period subsequent to the deposits of the sands and clays, bowlders of 
granite are by no means rare. 
Mr. Thos. Belt, who has carefully studied the Drift deposits of many 
countries, speaking of the erratics of our northern States (Quarterly 
Journal of Science, April, 1875), says : 
Only one satisfactory explanation has been given of the presence of these far-traveled 
blocks on the surface of the undisturbed loose beds of sand and clay, namely: that they 
have been dropped from floating ice. 
The evidence that the waters of Lake Hrie once stood two hundred or 
three hundred feet higher than now is indisputable, and given this great 
body of water filling the lake basin, and a retreating glacier resting on 
the flanks of the Canadian highlands, icebergs are a necessary con- 
' sequence. ‘Whether the continent was depressed at the time the lake basin 
was filled is altogether another question, with which this has no logical 
connection. The fact of the filling of the basin is recorded in the old 
beach lines and Lacustrine clays, and the discussion of the causes, conse- 
quences, or concomitants of this submergence, can not affect the ,validity 
of that record. We may say, however, in passing, that the proofs of 
alternations of elevation and depression, either of the land or ocean sur- 
face, during the Quarternary age, are unmistakable and striking. It 
has been shown conclusively, that since the deposition of the Champlain 
