DRIFT, BOULDERS, etC. 423 



the mass of boulders, but it is possible tint both belong to one poiiod. I have been unable, how- 

 ever, to discover, between the tertiary and the polished surfaces, any beds of boulders, or 

 any formation like drift. 



From the preceding facts, the four following in([uirics naturally arise : 



1 . What were the agents immediately concerned in the transportation of drift and boulders ? 



2. What were the conditions of the surface immediately preceding and during the drift 

 period, compared with the present ? 



3. What relations exist between the boulder system and the scorings of rocks ? 



4. Were any of the causes which were concerned in the transportation of boulders and drift' 

 concerned also as causes in the excavation of valleys ? 



The preceding inquiries, though stated under distinct heads, are so intimately related to 

 each other, that neither can be discussed without involving the whole collectively, or without 

 anticipating to some extent the answers which belong to each of the others : 



1. The agents concerned in the transportation of drift and boulders are water and ice. The 

 fine materials in which the boulders are imbedded consist moslly of the debris of the rocks in 

 the immediate neighborhood, and hence are not as a whole of foreign origin. The boulders 

 proper are foreign, and have been brought to the places where we now find them. The agent 

 concerned in this work of transport is ice, either local or general. Icebergs, according to the 

 opinion of the geologists of the present day, wore the immediate agents concerned in trans- 

 porting and distributing boulders. There is no necessity for occupying time in the discussion 

 of this point, as no fact in geology is better established. But, 



2. What then was the condition of the surface of that portion of New-York which lies be- 

 tween Lake Champlain and the St. Lawrence ? Without giving a direct answer, I make this 

 hypothesis, namely, that it was depressed ; and that preceding the deposition of the tertiary, 

 the country was low, and connected at the north with a wide and extensive region giving origin 

 to large rivers, \vhich flowed in succession over different parts of the region lying between 

 the Champlain and the St. Lawrence. These rivers were wide, shallow, and swift in some 

 parts of their course, and frequently formed new channels. They communicated with the 

 Atlantic on the south, through the Champlain, Hudson and Mohawk valleys. They bore along 

 ice loaded with sand, pebbles, etc., which scratched and grooved the surfaces of rocks over 

 which they flowed, and were the agents also of perforating the rocks in the form of pot-holes. 



I will state here, before I proceed further, why I prefer this hypothesis to others which 

 have been advanced : — 



1. Icebergs, though loaded with boulders, are very poorly adapted to polish, groove or 

 score rocks. The bottom of the ocean is not a bare rock, but is covered with mud and sand; 

 and \fhen the icebergs ground, it is in these soft materials, which, if they do not completely 

 protect the surface of the rock beneath, yet by no means leave it to be marked or scratched 

 in long parallel lines, like the surfaces we are considering. The motion of the icebergs, 

 when they ground, is rotatory : they are not driven through the ocean, ploughing the surface 

 of its rocks ; but they carry boulders, and drop them in their course, and hence become pro- 

 bably the principal agents of distributing and transporting those masses. 



