I 



of North America with those of Europe. 51 



this principle did not correspond, in certain countries, with the 

 divisions indicated by the mineralogical character of the rocks ; 

 thus the limit between the two stages of the Silurian system, 

 very well marked in the state of New York, is observed near the 

 Mississippi in consequence of the predominance of magnesian 

 limestone ; it is the same with the Silurian and Devonian sys- 

 tems, the limit between which is found in the upper part of the 

 great calcareous formation called cliff limestone ; as well also as 

 with the Carboniferous system, in parts of the state of Ohio, 

 where it is in contact with the Devonian psam mites of Portage. 



These mineralogical transitions, which one would expect in a 

 country free from disturbances, would not, however, obscure the 

 proofs of a parallel development of the animal kingdom in the 

 two continents : for if, leaving aside the difficulties of fixing the 

 limits between the systems, we compare the systems together, or 

 still better, one by one the groups of which they are composed, 

 we acquire the conviction that identical species have lived at 

 the same epoch in America and in Europe, that they have had 

 nearly the same duration, and that they succeeded each other in 

 the same order. We have endeavored to prove that the first traces 

 of organic life in countries the most remote, appear under forms 

 nearly alike, at the base of the Silurian system ; and that the 

 same types, often the same species, are successively and in parallel 

 °rder, developed throughout the entire series of the palaeozoic beds. 

 If we have not succeeded in lifting the veil which still hides from 

 us the cause of this grand phenomenon, perhaps at least our 

 observations demonstrate the insufficiency of those causes by 

 which certain authors seek to explain it. They prove in effect 

 that the phenomenon itself is independent of the influences 

 which the depths of seas* exercise upon the distribution of ani- 

 mals: for if, in certain countries, the Silurian deposits prove a 

 deep sea, they have on the contrary in the state of New York, a 

 littoral character. They prove in fine, that in its general char- 

 acter it is equally independent of the upheavings which have af- 

 fected the surface of the globe ; for from the eastern frontier of 

 Russia even to Missouri, "distant from or near the lines of dislo- 

 cation, in the horizontal beds as well as in those which are dis- 

 turbed, the law according to which it is accomplished appears 

 to be uniform. 



(To be continued.) 



We do not pretend to sav that the differences of depth in the seas had not al- 

 ready an influence upon the distribution of animals; it is to this circumstance, on 

 lh « contrary, that we attribute the more or less local fauna which we often dis- 

 cover in the paleozoic formation. But these local fauna; always afford some spe- 

 cies which connect them with the epoch to which they b. >ng. They are the ex- 

 ce ptions (horsd'ccuvre) which do not derange the general symmetry. 



