A CLOCK OF RECENT GEOLOGICAL TIME 365 



at others, and so does not work at a uniform rate. The cataract 

 of Niagara, charged with the pent-up energy of the waters of all 

 the Great Lakes, can rush its work as it is clearly unable to do at 

 times when the greater part of this energy has been diverted. 

 Units of distance measured along the gorge are therefore too un- 

 reliable for our use, with the unique exception of the stretch from 

 the railroad bridges to the site of the present cataract, within 

 which stretch the gorge cross sections are so nearly uniform as to 

 indicate an approximation to continued application of uniform 

 energy. This energy we may actually measure in the existing 

 cataract, and so fix upon a unit of time that can be translated into 

 years. 



In order to secure the normal rate of recession of this Upper 

 Great Gorge, we should add to the volume of water in the Canadian 

 Fall that now passing over the American; and for the reason that 

 the blocks which fall from the cataract cornice and are the tools 

 of the drilling instrument approximate to a definite size fixed by 

 their joint planes, the effect of this added energy it is not easy 

 to estimate. We may be sure, however, that the drilling action 

 would be somewhat increased by the junction of the two Falls, 

 and thus are assured that the average rate of recession within the 

 Upper Great Gorge has been somewhat in excess of the five feet 

 per year determined by Gilbert for the present Canadian Fall. 

 The Upper Great Gorge is about two miles in length, and its begin- 

 ning may thus be dated near the dawning of the Christian Era. 

 The Whirlpool Gorge was cut when the ice vacated the North Bay 

 Outlet in Canada, and still lay as a broad mantle over all north- 

 eastern Canada. For the earlier gorge and lake stages, the time 

 estimates are hardly more than guesses, and we need not now con- 

 cern ourselves with them. 



The horologe of late glacial time in Scandinavia. A glacial 

 timepiece of somewhat different construction and of greater refine- 

 ment has been made use of in Scandinavia to derive the " geo- 

 chronology of the last 12,000 years. " Instead of retreating over 

 the land and impounding the drainage as it did so, the latest con- 

 tinental glacier of Scandinavia ended below sea level, and as it 

 retired, its great subglacial river laid down a giant esker known as 

 the Stockholm Os, which was bordered by a delta and fringed on. 

 either side by water-laid moraines of the block type. These re- 



