Xl PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



species in any one class of animals ; we ought even to be cautious in 

 accepting the general proposition, that change in organic forms and 

 lapse of time are at all necessarily correlatives*." 



So also if animals inhabiting shells are subject by revolutions in 

 climate, or other causes, to remove from one region to another, and 

 such we know to be the case, that removal may take place after the 

 shells of several generations of the species may have been imbedded 

 in sediment. The species may become extinct in the first region, and 

 prevail for a long time in that to which they had removed, and their 

 shells may, in hke manner, become imbedded in stone. Thus strata 

 in distant places, although characterized by shells of the same species, 

 may not be of synchronous formation. 



I make the foregoing observations however with great diffidence, 

 I throw them out as suggestions for consideration ; they relate to 

 matters of great complexity and difficulty, but which are of funda- 

 mental importance in our researches into the earth's history, and 

 they are in a very unsettled and uncertain state. The opinions I 

 have now hazarded are, in some degree, at variance with those of a 

 geologist of great authority, one from whose conclusions I very rarely 

 differ, and never without doubting the soundness of m.y own judgment. 

 Thus, Mr. Lyell, after comparing the tertiary formations of North 

 America and Europe, of which he has treated in his * Travels,' and 

 in papers published in our Journal, has come to the conclusion that 

 the Eocene and Miocene formation of the United States, as determined 

 by the relative proportions of recent and extinct species of fossil shells, 

 are truly contemporaneous in age with the deposits termed by him 

 Eocene and Miocene on this side of the Atlantic. With some species 

 identical with those of the neighbouring seas, they contain a great 

 number of forms which he regards as " representative." The syn- 

 chronism he considers to be established not only by agreement in 

 the relative position and the characters of the whole fossil faxma, 

 but by the same kind of evidence as that which induces, geologists to 

 consider the coal-fields of the United States and the cretaceous strata 

 as equivalents in time and position with the groups similarly desig- 

 nated in Europe. The numerous points of agreement in the palaeon- 

 tology of the successive tertiary and post-pliocene formations of Ame- 

 rica and Europe he beUeves to have been brought about by the pre- 

 dommant influence of climate controlling the minor effects of local 

 geographic revolutions, and causing a near approach to a uniform rate 

 of fluctuation in the organic world throughout the whole northern 

 hemisphere, from the Eocene to the recent periods. 



By whatever names we designate geological periods, there appear 

 to exist no clearly defined boundaries between them in reference to 

 the whole earth ; such a marked line may be seen in particular loca- 

 lities, but every year's experience, and our more intimate acquaint- 

 ance with the phsenomena exhibited in different countries, and with 

 the distribution, structure and habits of animals and vegetables, teach 

 us that there is a blending, a gradual and insensible passage from the 

 lowest to the highest sedimentary strata, particularly in respect of 

 * Geology of South America, p. 105. 



