220 THE POPULAR SCIENCE MONTHLY. 



of the deserted channels are cut to a great depth ; but, whatever 

 their depth, they are inclosed by banks or bluffs that are still dis- 

 tinct and comparatively steep ; thus showing that relatively short 

 periods of time elapsed both during and since their occupation by 

 large rivers. The rock-cutting done by Niagara in post-glacial 

 time seems to be a much greater piece of work than that accom- 

 plished by any of the temporary lake outlets during the closing 

 phases of the Glacial period ; but none of them, as far as I have 

 read, had an opportunity for active work equal to that of Niagara.* 

 They are nearly all comparatively shallow ; but an exception to 

 this rule has been pointed out to me by Mr. Gilbert, to which a 

 paragraph may be devoted. 



Emmons, the geologist of the second district of New York, 

 long ago described what he took to be fissures in the Potsdam 

 sandstone of northern New York, but which to modern interpre- 

 tation appear to be gorges or chasms cut by rivers, presumably 

 constrained into that position by drift or ice obstruction. The 

 Ausable chasm is a well-known instance of these " fissures," but 

 one of the examples described by Emmons has no river running 

 through it. It lies close to the Canadian boundary in Clinton 

 County, sixteen miles west of Lake Champlain, and is thus de- 

 scribed in Emmons's report (Geology of the Second District, New 

 York, 1843, pp. 309, 310) : " The fissure or gulf, as it is called, is 

 three hundred feet deep and about sixteen rods wide. Its walls of 

 sandstone or conglomerate are perpendicular at the deepest part. 

 The small lake at the bottom is said to be one hundred and fifty 

 feet deep. The direction of this fracture is north, seventy degrees 

 west, and the rock dips at a small angle from each side of it. . . . 

 At Keeseville and Cadysville large rivers, the Ausable at the for- 

 mer and Saranac at the latter, still occupy these gorges as their 

 channels, and have sufficient force and power to sweep out, es- 

 pecially in the time of high water, all rocks of an ordinary size. 

 At this place there is merely a small rill discharging itself from 

 a small lake of dead water, insufficient in itself to accomplish any 

 perceptible change. To account for the present condition of this 

 rock, we have therefore to go back to a period when some current 

 swept through this gorge with great force and power ; for by no 

 other means could the materials which once filled the space be- 

 tween the present walls of the gulf be removed." 



Returning to the general features of the abandoned channels. 



* Russell's Geological Reconnaissance in Central Washington gives an account of a 

 temporary displacement of the Columbia River when its valley was obstructed by ice and 

 its waters ran through the Grand Coul6e : a basin was then excavated beneath a cataract 

 in the course of the river, and the basin now holds a lake, although the river and the cata- 

 ract have disappeared (Bulletin 108, United States Geological Survey, pp. 91, 92). 



