RETIEW OF GEOLOGICAL STRUCTURE. 43 



is capable of demonstration. One such case proves the presence of one ice- 

 berg, and the many others which have been observed prove many icebergs. 



Equally conclusive argumehts 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 uni- 

 formly masses of crystalline rocks, granite, greenstone, slate, etc., which 

 have been brought from the C.anadian 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 tlie 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, 1876), 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 di opped from floating ice. 



The evidence that the waters of Lake Erie once stood two hundred or 

 three hundred feet higher than now is indisputable, and given this great 

 body of watfer 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 



