

Ch. XH.] SUBMERGENCE OF NORTH AMERICA. 1(J5 



great numbers of the Mytilus edulis, or our common European mus- 

 sel, retaining both its valves and its purple color. This shelly 

 deposit, containing among other marine shells Saxicava rugosa, 

 characteristic of the glacier drift of Sweden, 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 erratics and the polished surfaces of rocks, 

 and their striae and flutings, we must assume first a gradual submer- 

 gence of the land in North America, after it had acquired its present 

 outline of hill and valley, cliff and ravine, and then its reemergence 

 from the ocean. In order to account for the universal glaciation of 

 the surface of the solid rocks, on which the drift reposes in the neigh- 

 borhood of the great lakes, and north and south of the St, Lawrence, 

 it seemed necessary to assume the action of ice previous to all depo- 

 sition of drift or transportation of erratics. The general direction of 

 the furrows from north to south, for they rarely deviate more than 

 10° or 20° to the east or west of the meridian, seemed to favor the 

 idea of their being for the most part due to the running aground of 

 icebergs drifting from arctic latitudes. The absence in many regions, 

 as in the Niagara district, of high mountain chains, and the extension 

 of undiminished ice action as far south as the 40th parallel, made me 

 unwilling to appeal, save in some exceptional cases, to land glaciers as 

 the principal agents of this glaciation. I assumed, therefore, that 

 while the land was slowly sinking, the sea which bordered it was 

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

 they grounded 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 frag- 

 ments of hard stone, frozen into the lower surface of the ice, scooped 

 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 ma- 

 chinery, producing such long, straight, and parallel furrows, as are 

 everywhere visible in the Niagara district, and generally in the region 

 north of the 40th parallel of latitude.f 



This hypothesis of a slow and gradual subsidence of the land 

 enables us to imagine that the polishing and grooving action may 

 have been going on simultaneously with the transportation of the 

 erratics. During the successive depression 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 



* Travels in N. America, vol. it p. 141 

 f Ibid., vol. il chap. xix. p. 99. 



