12 /. Clifton "Ward — On Geological Time. 



going far beyond Mr. CroU's Grlacial period ; and it seems to me 

 very possible that the subsidence commenced long before tlie Post- 

 tertiary, the continent about tbe commencement of the Glacial Epoch 

 having reached a condition somewhat similar to that represented in 

 the figure where the highest point might be some 3000 feet. 



The following is an actual example of the wearing away of solid 

 materials since a given period. The Falls of Niagara have been cut 

 back at least six miles since the Champlain Epoch, for lake-deposits 

 formed by the old extension of Lake Ontario, and containing similar 

 shells to those now living near the entrance of the lake, are found 

 both at Goat Island and on either side of the gorge near the whirl- 

 pool ; six miles then at least of the gorge have been excavated since 

 the formation of these deposits. Dana says,^ " Taking the rate at one 

 foot a year, the six miles will have required over 34,000 years ; if at 

 one inch a year — which is 8|- feet a century — 380,000 years." The 

 former was Sir Charles Lyell's estimate, which, if considered too 

 great, is probably outdone in the other extreme of one inch a year ; 

 if, however, we take the mean of these two estimates, namely, six 

 inches a year, the time would then be 62,000 years since the probable 

 close of the Champlain Epoch. 



Going still further north in the same quarter of the globe, raised 

 beaches of Champlain Epoch age are found in the Arctic regions at 

 an elevation of 1000 feet above the sea ; ^ if it be not far from the 

 truth that the probable average rate of elevation as well as depres- 

 sion is about one foot per century, then 100,000 years would seem 

 to have elapsed since the Champlain Epoch, instead of some 60,000 

 as just estimated. But is there any good reason to think that the 

 average rate of continental elevation (I speak not of local and 

 paroxysmal elevations connected with volcanic phenomena) is the 

 same as that of depression ? It seems to me probably not ; for take 

 the case of the west coast of South America, which has undergone 

 within comparatively modern times an elevation of at least 1200 feet ; 

 if we suppose that the elevation of one large tract is connected with 

 the subsidence of a neighbouring tract, and we take into considera- 

 tion that for the most part the highest portions of continents face the 

 largest seas, and are generally not far removed from the coast line,^ 

 then in the case of South America, for instance, we see why the 

 Andes on the west coast should be at a greater elevation than those 

 ranges on the east of the same continent. Inasmuch also as the tract 

 over which elevation has taken place is far less than that over which 

 subsidence has been going on (supposing elevation on a large scale 

 to be due to the sinking of certain portions and the squeezing up of 

 the portion between them), it follows that the elevatory force having 

 to act within narrow limits will produce a greater upward effect than 

 the subsiding force, acting over a larger area, will produce in a down- 

 ward direction. Thus, supposing the subsidence of the great Pacific 

 area to be connected with the elevation of South America, it might 



1 Manual of Geology, p. 590. 2 Dana's Manual, p. 551. 



3 Dana's Manual of Geology, p. 731. 



