BREAKS IN THE GEOLOGICAL RECORD. 45 



in different regions are "contemporaneous," we simply mean 

 that they were formed during the same geological period, and 

 perhaps at different stages of that period, and we do not mean to 

 imply that they were formed at precisely the same instant of time. 

 A moment's consideration will show us that it is only in the 

 former sense that we can properly speak of strata being " con- 

 temporaneous ;" and that, in point of fact, beds containing 

 the same fossils, if occurring in widely distant areas, can hardly 

 be " contemporaneous " in any literal sense ; but that the very 

 identity of their fossils is proof that they were deposited one 

 after the other. If we find strata containing identical fossils 

 within the limits of a single geographical region say in Europe 

 then there is a reasonable probability that these beds are 

 strictly contemporaneous, in the sense that they were deposited 

 at the same time. There" is a reasonable probability of this, 

 because there is no improbability involved in the idea of an 

 ocean occupying the whole area of Europe, and peopled 

 throughout by many of the same species of marine animals. At 

 the present day, for example, many identical species of animals 

 are found living on the western coasts of Britain and the 

 eastern coasts of North America, and beds now in course of 

 deposition off the shores of Ireland and the seaboard of the 

 state of New York would necessarily contain many of the 

 same fossils. Such beds would be both literally and geologi- 

 cally contemporaneous; but the case is different if the distance 

 between the areas where the strata occur be greatly increased. 

 We find, for example, beds containing identical fossils (the 

 Quebec or Skiddaw beds) in Sweden, in the north of England, 

 in Canada, and in Australia. Now, if all these beds were con- 

 temporaneous, in the literal sense of the term, we should have 

 to suppose that the ocean at one time extended uninterrup- 

 tedly between all these points, and was peopled throughout 

 the vast area thus indicated by many of the same animals. 

 Nothing, however, that we see at the present day would justify 

 us in imagining an ocean of such enormous extent, and at the 

 same time so uniform in its depth, temperature, and other 

 conditions of marine life, as to allow the same animals to. 

 flourish in it from end to end; and the example chosen is 

 only one of a long and ever-recurring series. It is therefore 

 much more reasonable to explain this, and all similar cases, as 

 owing to the migration of the fauna, in whole or in part, from 

 one marine area to another. Thus, we may suppose an ocean 

 to cover what is now the European area, and to be peopled by 

 certain species of animals. Beds of sediment clay, sands, 

 and limestones will be deposited over the sea-bottom, and 

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