Ch. XIL] SUBSIDENCE IN DEIFT PERIOD. 141 



Fifj. 124. 



a 



Astarte Laurentiana. 

 a. Outside. h. Inside of right valve. e. Left valve. 



also occurs at an elevated point on the mountain of Montreal, 450 feet 

 above the level of the sea."^' 



In my account of Canada and the United States, published in 1845, 

 I announced the conclusion, to which I had then arrived, that to explain 

 the position of the erratics and the polished surfaces of rocks, and their 

 striae and flutings, we must assume first a gradual submergence of the 

 land in North America, after it had acquired its present outline of hill 

 and valley, cliff and ravine, and then its re-emergence from the ocean. 

 When the land was slowly sinking, the sea which bordered it was covered 

 with islands of floating ice coming from the north, which, as they 

 gi'ounded on the coast and on shoals, pushed along such loose materials 

 of sand and pebbles as lay strewed over the bottom. By this force all 

 angular and projecting points were broken off, and fragments of hard 

 stone, frozen into the lower surface of the ice, had power to scoop out 

 grooves in the subjacent solid rock. The sloping beach, as well as the 

 floor of the ocean, might be polished and scored by this machinery ; but 

 no flood of water, however violent, or however great the quantity of de- 

 tritus or size of the rocky fragments swept along by it, could produce 

 such long, perfectly straight and parallel furrows, as are everywhere visi- 

 ble in the Niagara district, and generally in the region north of the 40th 

 parallel of latitude.f 



By the hypothesis of such a slow and gradual subsidence of the land 

 we may account for the fact that almost everywhere in N. America and 

 Northern Europe the boulder formation rests on a polished and furrowed 

 surface of rock, — a fact by no means obliging us to imagine, as some 

 think, that the polishing and grooving action was, as a whole, anterior in 

 date to the transportation of the erratics. During the successive depres- 

 sion of high land, varying originally in height from 1000 to 3000 feet 

 above the sea-level, every portion of the surface would be brought down 

 by turns to the level of the ocean, so as to be converted first into a coast- 

 line, and then into a shoal ; and at length, after being well scored by the 

 stranding upon it, year after year, of large masses of coast-ice, and occa- 

 sional icebergs, might be sunk to a depth of several hundred fathoms. By 

 the constant depression of land, the coast would recede farther and farther 

 from the successively formed zones of polished and striated rock, each outer 

 zone becoming in its turn so deep under water as to be no longer grated upon 

 by the heaviest icebergs. Such sunken areas would then simply serve as 

 receptacles of mud, sand, and boulders dropped from melting ice, perhaps 



* Travels in N, America, vol. ii. p, 141. f Ibid. p. 99, chap. xix. 



