10 Changes of Animals. 



centre iu Pensylvanla is almost four and a half miles, and it has been heaved 

 up not only so as to constitute extensive countries of dry land, but even the 

 long ranges of the Alleghany Mountains which extend from the Southern 

 States north easterly through Lower Canada to the mouth of the St. Law- 

 rence. 



The grandest discovery made during the examination of these old de- 

 posits, is, that the world has changed its inhabitants several times since 

 animated beings were fii'st placed upon it by the Creator. Certain beds 

 lying at the bottom contain the remains of particular species, few in number 

 at first, but sufficiently well preserved to enable the Geologist to make out 

 their form and structure. Higher up, there are other beds of rocks contain- 

 ing other species, but none of those that are found below. The sediment 

 which constitutes these diflPereut formations was deposited in the seas of 

 different ages, and the contained organic remains prove that the denizens of 

 the oceans of the first age were no longer in existence when the ocean of the 

 second period covered the earth. Li the same manner a third deposit lies 

 upon the second, with its fossils difierent from both of those below — above 

 the third there is a fourth, and over this many more, until we arrive at the 



Burface. 



As the deepest coal pits excavated by man do not penetrate to the 

 depth of half a mile, it would be almost impossible to ascertain these facts 

 were it not that the subterranean forces which cause the elevation and sub- 

 sidence of laud come in to the aid of the student of nature. Whatever may 

 be the reason, certain tracts of country are more violently acted upon than 

 others, and the earth is in such places so broken up that the sedimentary 

 rocks instead of lying in a horizontal position as originally deposited, are 

 tilted up and their edges clearly exposed upon the surface, where the Geolo- 

 gist may measure their thickness and study the organic remains contained in 

 each formation at his leisure. It is beyond a doubt that rocks are now 

 exposed in the full light of day which w'ere once several miles beneath it. 



As the whole of the series of sedimentary rocks is estimated at the 

 tliickness of ten miles ; there can be no doubt but that a prodigious period 

 of time has rolled away since the first strata were deposited on the bottoms 

 of the primeval oceans. There is evidence in many of the beds that the 

 materials of which they are formed were very slow^ly accumulated ; some of 

 them consist almost entirely of shells which lived and died upon the spot 

 where they are now found. Often these shells are overgrown with coral in 

 such a manner as to render it quite clear that after their death it was long 

 before they were covered by the sediment. Other facts demonstrate that 

 the process of accumulating matter upon the bottom proceeded with no 

 greater rapidity in olden times than it does at present ; to form ten miles of 

 stratified rocks nmst have required a vast period of time, but how great, 

 geology does not venture to say. All that this science can prove, with 

 respect to time, is that certain rocks were formed after or before certain 

 others, and tliis is shewn either by the superposition or the fossil contents of 

 the strata. From the accounts above given of the origin of sedimentary 



