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a-nd spread far beyond into adjoining regions. Some of them are 

 plainly traceable far southward along the Alleghanies through 

 Virginia, southwestward to Tennessee, northeastward to the 

 Lower St. Lawrence, or northwestward beyond the Upper Missis- 

 sippi. Thus far, at least, spread the ancient sea on the floor of 

 which these successive sheets of sediment were deposited ; and 

 over all this vast extent the same groups of fossil forms prevail, 

 proving that the living population of that old ocean was much 

 the same over its whole extent at each successive period. 



We say its living population was much the same in all parts 

 at the same period : it was so, but not entirely the same. In 

 existing oceans the living population varies more or less in differ- 

 ent parts : for instance, out of an hundred shells gathered on the 

 shores of Massachusetts, and an hundred gathered on the coast 

 of Great Britain, only about thirty-five per cent will be found 

 identical ; while of the remainder, a large proportion are very 

 similar on both sides of the ocean, so that they may be called 

 " geographical representatives " of each other : a small number, 

 beside, will be found quite peculiar to one coast or the other. 

 We may therefore reasonably expect to find a similar degree of 

 correspondence among the organic relics from different parts of 

 any of the rocks so widely spread ; and this is exactly the fact, 

 as proved by observation. A particular bed of limestone, for 

 example, will be found in Ohio to contain not entirely the same 

 fossils found in it in New- York, but a great proportion of them ; 

 while in each of these distant localities, some will be discovered 

 which are not known in the other. This is even true where the 

 character of the rock itself changes in different districts. A 

 certain series of strata may in New- York be slates or sandstones : 

 when examined further west, it is found that they become more 

 and more calcareous, until in Kentucky they may be almost pure 

 limestones, varying from their New- York aspect not only in tex- 

 ture,, but perhaps in color also. Yet the imbedded fossils will 

 be found so much the same, that a practised observer will at once 

 recognise the rock by them ; and having thus found one portion 

 of the series of strata to be identical with a particular set of 

 layers known here, he is able to guide himself with great cer- 

 tainty in the search for other parts of the series which it may be 

 desirable to discover. This principle is often applied with great 

 advantage in searching for such minerals as may be known to 



