AGE OF MAMMALS. 289 



tooth and other bones of a mastodon found in these de- 

 posits show that they were laid down in the Champlain 

 epoch, for this animal flourished at that time. 



400. Length of the Post-tertiary Age. If we can not 

 make some approximation to an estimate of the length 

 of time consumed by the Post-tertiary period, by calcu- 

 lations from the rate of progress in the recession of the 

 Falls of Niagara, we can, at least, see that the age was a 

 very long one. The recession is still going on, and va- 

 rious observations have been made in regard to it, and 

 estimates have been based on those observations. The 

 results, as worked out by different observers, vary great- 

 ly. Some estimate the recession as high as three feet in 

 a year, which undoubtedly is far beyond the fact. Lyell 

 estimates it as averaging one foot a year. This, which 

 Dana says is certainly large, would give over 31,000 

 years for the whole recession. Mr. Desor, another ob- 

 server, inferred from the data which he ascertained that 

 it was " more nearly three feet a century than three feet 

 a year." Taking it at three feet a century, the whole 

 would require over a million of years. 



401. Post-tertiary Rivers. Rivers are a part of the 

 grand apparatus for the circulation of water on the earth. 

 Evaporation carries up water in the atmosphere from 

 over the whole surface, from the land as well as the sea, 

 and the vapor thus dissolved in the air is condensed into 

 rain, or snow, or hail, in order to be brought back to the 

 earth. Falling, it gathers in little streams, which, uniting 

 together, make the great rivers that run to the ocean. 

 Now, in order to have very large rivers, two things are 

 necessary a considerable extent of land to afford room 

 for the union of many streams in one, and the presence 

 of great mountains, to condense the vapor rapidly with 

 their tall, cold peaks. Neither of these conditions was 

 present when the continents began to form. When that 

 long island, with its low mountains, the germ of the 

 North American continent, was lifted up in the Azoic 



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