590 HISTORICAL GEOLOGY. 



GENERAL OBSERVATIONS ON GEOLOGICAL 

 HISTORY. 



1. LENGTH OF GEOLOGICAL TIME. 



On former pages (pp. 386, 493, 568) estimates have been given of 

 the relative lengths of the ages and periods, or their time-ratios* 

 Future discovery will probably enable the geologist to determine 

 these ratios with far greater certainty and precision. 



Although Geology has no means of substituting positive lengths 

 of time in place of such ratios, it affords facts sufficient to prove 

 the general proposition that Time is long ; and a few examples are 

 here given. 



Niagara has made its gorge by a slow process of excavation, and 

 is still prolonging it towards Lake Erie. Near the fall it is 200 to 

 250 feet deep, and at the fall itself 160 feet, — the lower 80 feet shale, 

 the upper 80 limestone. The rocks dip 15 feet in a mile up stream, 

 so that the limestone becomes thicker as it recedes on its course. 

 The waters wear out the shale, and thus undermine the limestone. 

 The distance from Niagara to the Queenstown heights which face 

 the plain bordering Lake Ontario is seven miles. 



On both sides of the gorge near the whirlpool (three miles below 

 the fall), and also at Goat Island, there are beds of recent lake- 

 shells, Unios, Melanias, and Paludinas, the same kinds that live 

 in still water near the entrance to the lake, and which are not 

 found in the rapids. The lake, therefore, spread its still waters, 

 when these beds were formed, over the gorge above the whirlpool. 

 A tooth of a Mastodon has been found in the same beds. This 

 locates the time in the Champlain epoch. Moreover, the waters 

 would not have been set back to the height of these beds unless 

 they extended on below for at least six miles from the falls. Six 

 miles of the gorge have, then, been excavated since that Mastodon 

 was alive. There are terraces in the shell deposits showing changes 

 of level in the lakes. 



There is a lateral valley leading from the whirlpool through the 

 Queenstown precipice at a point a few miles west of Lewiston. This 

 valley is filled with drift of the Glacial epoch, as stated on p. 536 ; 

 and this blocking up of the channel may have compelled it to open 

 a new passage. 



If, then, the falls have been receding six miles, and we can ascer- 



