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giving greater precision to our knowledge of the interest- 

 ing locality to which they relate, and adding many new 

 forms to the catalogue of our paleozoic fossils, but as 

 helping to illustrate geological questions of wide appli- 

 cation and interest in the philosophy of the science. 



The gradual passage from a Devonian to a carboniferous 

 fauna, previously noted in this locality by Prof. Hall, and 

 now so clearly exhibited by Mr. White, belongs to a class of 

 phenomena of which not a few examples have been brought to 

 light in other parts of the geological series, and of which many 

 more will doubtless be discovered in the progress of a criti- 

 cal and philosophical survey of fossiliferous deposits. Prof. 

 Kogers considered such a gradational change, or such a mingling 

 of races in successive formations, as but the natural result of the 

 accumulation of the strata during a long period of comparative 

 repose. He believed that the abrupt transitions so often observed 

 in passing from one geological formation to another were not, as 

 some maintain, an essential feature in the life-history of our 

 earth, but were the memorials of the disturbing and destroying 

 agencies to which its living races had been successively exposed. 

 These hostile influences have at no time been of equal intensity 

 over widely extended areas, but varying from region to region 

 have in some places arrested only in part the stream of living 

 descent, thus substituting for the abrupt transition which marks 

 the successive faunae of one district, the gentle gradations and 

 intermingling of forms presented by the corresponding deposits of 

 another. 



Referring even to the limits of the great paleozoic divisions, 

 so often defined by sharp lines of separation, observation has 

 shown that in some localities the transition is so gradual as to 

 present no greater amount of change in fossil forms than occurs 

 in passing from one subordinate formation to the next. Hence 

 we find that the ablest European geologists are not agreed as to 

 the line of separation between the Silurian and Devonian, or be- 

 tween the latter and the carboniferous deposits of some of their 

 best-known districts, while recent observations in this country 

 and abroad have tended to obliterate the presumed line of demar- 

 cation between the carboniferous and overlying Permian strata, 

 wherever the transition beds are most completely developed. 



