Science, Geology and Physics 



are all so modern in the earth's history as to belong to a period 1841 

 when the marine, the fluviatile, and terrestrial shells, were the L y e11 

 same, or nearly the same, as those now living. \et if we fix 

 our thoughts on any one portion of this period — on the lapse of 

 time, for example, required for the recession of the Niagara from 

 the escaipment to the Falls, — how immeasurably great will its 

 duration appear in comparison with the sum of years to which 

 the annals of the human race are limited ! Had we happened to 

 discover strata, charged with fluviatile shells of recent species,, 

 and enclosing the bones and teeth of a Mastodon, near a river 

 at the bottom of some valley, we might naturally have inferred 

 that the buried quadruped had perished at an era long after the 

 canoes of the Indian hunter had navigated the North American 

 waters. . . . But fortunately on the Niagara, we may 

 turn to the deep ravine, and behold therein a chronometer 

 measuring rudely, yet emphatically, the vast magnitude of the 

 interval of years, which separate the present time from the epoch 

 when the Niagara flowed at a higher level several miles further 

 north across the platform. We then become conscious how far the 

 two events before confounded together, — the entombment of. 

 the Mastodon, and the date of the first peopling of the earth by 

 man, — may recede to distances almost indefinitely remote from 

 each other. 



But, however much we may enlarge our ideas of the time 

 which has elapsed since the Niagara first began to drain the 

 waters of the upper lakes, we have seen that this period was one 

 only of a series, all belonging to the present zoological epoch ; or 

 marine, had already come into being. If such events can take 

 place while the zoology of the earth remains almost stationary 

 and unaltered, what ages may not be comprehended in those 

 successive tertiary periods during which the Flora and Fauna of 

 the globe have been almost entirely changed! Yet how sub- 

 ordinate a place in the long calendar of geological chronology 

 do the successive tertiary periods themselves occupy! How 



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