THE DATE OP THE GLACIAL PERIOD. 339 



between two hundred and two hundred and seventy-live 

 feet in the eleven years between 1875 and 1886." 



It will be perceived that the recession in the centre of 

 the Horseshoe is very much more rapid than that nearer 

 the margin ; yet this rate at the centre is more nearly the 

 standard of calculation than is that near the margin, for 

 the gorge constantly tends to enlarge itself below the falls, 

 and so gradually to bring itself into line with the full- 

 formed channel. Taking all things into account, Mr. 

 Woodward and the other members of the Geological Sur- 

 vey thought it not improbable that the average rate of 

 actual recession in the Horseshoe Fall was as great as five 

 feet per annum ; and that, if we can rely upon the uniform- 

 ity of the conditions in the past, seven thousand years is 

 as long a period as can be assigned to its commencement. 



The only condition in the problem about which there 

 can be much chance of question relates to the constancy 

 of the volume of water flowing in the Niagara channel. 

 Mr. Gilbert had suggested that, as a consequence of the 

 subsidence connected with the closing portions of the Gla- 

 cial period, the water of the Great Lakes may have been 

 largely diverted from its present outlet in Niagara River 

 and turned northeastward, through Georgian Bay, French 

 Eiver, and Lake Nipissing, into a tributary of the Ottawa 

 Eiver, and so carried into the St. Lawrence below Lake 

 Ontario. Of this theory there is also much direct evi- 

 dence. A well-defined shore line of rounded pebbles ex- 

 tends, at an elevation of about fifty feet, across the col 

 from Lake Nipissing to the head waters of the Mattawa, a 

 tributary of the Ottawa; while at the junction with the 

 Ottawa there is an enormous delta terrace of boulders, 

 forming a bar across the main stream just such as would 

 result from Mr. Gilbert's supposed outlet. But this outlet 

 was doubtless limited to a comparatively few centuries, and 

 Dr. Robert Bell thinks the evidence still inconclusive.* 



* See Bui. Geol. Soc. Am., vol. iv, pp. 423-427, vol. v, pp. 620-626. 



