590 GEOLOGICAL HISTORY. 



GENERAL OBSERVATIONS ON GEOLOGICAL 

 HISTORY. 



1. LENGTH OF GEOLOGICAL TIME. 



On former pages (pp. 371, 575), 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. A few examples are here 

 given. 



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

 still prolonging it toward Lake Erie. Near the fall, the gorge is 200 

 to 250 feet deep, and 1 60 feet at the fall, — the lower 80 feet shale, 

 the upper 80 limestone. The waters wear out the shale, and thus un- 

 dermine the limestone. The rocks dip fifteen feet in a mile up stream, 

 so that the limestone at the fall becomes thicker, as retrocession goes 

 on. 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, 

 Uhios, 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 of deposition 

 in the Champlain period. 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 ter- 

 races 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, as stated on page 553 ; and this blocking up 

 of the channel forced it to open a new passage. 



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

 the probable rate of progress, we may approximate to the length of 

 time it required. Hall and Lyell estimated the average rate at one 

 foot a year, — which is certainly large. Mr. Desor concluded, after 



