LAND AND WATER 33 



preceded our own, the rapidity with which sediments were laid 

 down was the same as that of to-day. As we have only the 

 vaguest notion of the reduction in thickness undergone by the 

 oldest deposits during their transformation into gneiss, it is 

 quite likely that estimates made by tins procedure are too low 

 and that the error due to this fact will compensate, in large 

 measure, for possible exaggerations due to a difference in the 

 rapidity with which certain deposits are formed. On the basis 

 of the foregoing facts, Dana estimated the duration of the 

 primary period as 15 million years ; that of the secondary 

 as 4 million, and that of the tertiary as i| million years, 

 making the total of 20 millions at which Lord Kelvin 

 had already previously arrived. The duration of our present 

 period has been calculated from an entirely different starting- 

 point. The Niagara River, emerging from Lake Erie, originally 

 fell, after a short course, into Lake Ontario. But gradually the 

 cliff from whose height it falls has been eaten away ; thus 

 the Falls to-day are n kilometres distant from the lake. 

 Taking as his basis the rate at which the cliff is retreating 

 to-day, Lapparent estimated the duration of our present 

 epoch as 40,000 years. This estimate is confirmed if we measure 

 the present rate of increase of coral reefs and try to discover 

 how long it has taken to unite to the peninsula of Florida 

 the four coral reefs attached to its primitive coasts ; this has 

 required 35 to 40,000 years. The calculation of the time 

 necessary for the formation of the present peat-bogs gives the 

 same results. We may, therefore, regard it as exceedingly likely 

 that about 40,000 years have elapsed since man began to spread 

 over the earth. 



From the agreement of all these facts we may at least 

 conclude that the surface of the earth has been solidified 

 for from 1 to 2,000 million years. Other calculations 

 indicate that it is at least a trillion years since the earth was 

 separated from the sun and that life is already an extremely 

 ancient phenomenon. 



The prodigious changes which have occurred in the course of 

 ages in the configuration of the continents and the seas has 

 necessarily influenced the mean temperature of any given 

 region. According to whether the sea washing the coasts was 

 in extensive communication with tropical or polar waters, its 

 temperature rose or fell, and that of the adjacent continent 



