68 SURFACE GEOLOGY. 



drift, some of the boulders being from 10 to 20 feet in diameter. They still lie 

 along the borders of the gulf in ridges that correspond exactly to the lateral 

 moraines of Alpine glaciers, and at the end of the slide we have a terminal 

 moraine. The rock in place is laid bare most of the way, and although consider- 

 ably smoothed, it is not striated to any extent. I cannot conceive of a fairer 

 opportunity to test this matter than on this spot. The size and quantity of the 

 moving mass of detritus, and the rapidity with which it must have descended on a 

 slope of 10° to 38°, were all favorable to the production of an exact counterpart 

 of drift action, if water only was the transporting agent. But it failed just where 

 we should expect it to fail, viz. : in the formation of striae and furrows. 



Where now, save in glaciers, icebergs, and ice-islands, can we find agencies that 

 meet the conditions of the above principles respecting drift ? Glaciers, as every one 

 knows, who has observed their effects in the Alps, do produce phenomena corre- 

 sponding to those of drift in northern regions, in almost every respect. Nor can 

 we doubt that icebergs and ice-floes, large enough to grate along the bottom of the 

 sea, would do the same, although the proof is more difficult to obtain, because the 

 scene of the operation is beneath the ocean. But such icebergs and floes as I sup- 

 pose, would, it seems to me, operate almost precisely like glaciers. For I assume 

 that they are so large and thick that they reach and press heavily upon the bot- 

 tom: such icebergs and icefloes in fact, as northern voyagers have described, 

 whose surface was so large that they travelled for days upon them, or their vessel 

 was frozen into them, without their suspecting that they were in motion, till an 

 observation for latitude and longitude showed them that they were ujxm a drifting 

 mass. Let such masses be put into motion by currents and winds, ever so slowly, 

 and how powerfully would they scour the rocky bottom, wherever they reached it, 

 especially if their under side were armed with fragments of stone. 



To which phase of this glacial agency, then, shall we refer the phenomena of 

 drift? Before attempting to answer this question, I shall make a few remarks 

 upon another point, viz.: whether in such a country as the United States and 

 Canada, we can fix upon the geological period when the drift agency operated ? 

 "Was it previous to the last submergence of the surface, or during its subsidence, 

 or while it was emerging ? 



There is one fact that leads to the conclusion that the greater part of this work 

 was done before the continent had emerged very considerably from the waters. 

 In my paper on Erosions, I point out several instances in which the beds of rivers, 

 that existed before the submergence of the continent, apparently became so filled 

 with detritus, while beneath the ocean, that the postdiluvian rivers were forced to 

 leave these old channels and wear out new beds, sometimes through solid rocks. 

 True, this detritus is often made up of materials much comminuted, and formed 

 into terraces, and, therefore, may not have accumulated till the continent had been 

 lifted considerably from the ocean. But since these old beds of rivers often show 

 drift scratches beneath the detritus, they must have been made previous to its 

 accumulation, and, therefore, before the drainage had proceeded very far. 



On the other hand, there is a fact that leads the mind to the conclusion that the 



