﻿Charles Callaway — Migration of Species. 445 



V. — The Migration of Species as related to the Correlation 



OF GrEOLOGIOAL FORMATIONS. 

 By Charles Callaway, M.A., B.Sc, F.G.S. 



THE topic of this paper is suggested by a somewhat extended 

 practical study of the fossils of North America. A comparison 

 of these forms with our European faunas will, I think, throw some 

 light uj)on migration, and upon the correlation of strata. 



When comparing western with eastern formations, we are thrown 

 back chiefly upon the testimony of fossils. Mineral resemblance, such 

 as is observed between the Wenlock Limestone and Shale in the east, 

 and the Niagara Limestone and Shale in the west, is of little value 

 at great distances. The Wenlock Limestone, for instance, passes 

 into an arenaceous deposit towards Wales, and the Niagara Lime- 

 stone thins out towards the east in the State of New York ; so that 

 the continuity of the two formations is an extreme improbability, 

 Stratigraphical position, the third test of contemporaneity, is also of 

 little service, and is itself dependent upon fossil evidence. We 

 cannot, for example, justly argue that the Niagara group of New 

 York represents our Wenlock on the ground of its stratigraphical 

 position, unless we admit that the formations overlying and under- 

 lying it correspond respectively to our Ludlow and May Hill 

 Sandstone. But we can prove that correspondence only by strati- 

 graphical position, which would land us in absurdity ; or by fossils. 

 The uncertainty of mere stratigraphical evidence is well illustrated 

 by the Devonian and Old Eed Sandstone of South-western Britain ; 

 and, if this method of proof is so dubious in the case of formations 

 separated by the Bristol Channel, it is obviously much more un- 

 trustworthy when applied to groups on opposite sides of the At- 

 lantic Ocean. 



It has been maintained that identity of species, so far f-om proving 

 contemporaneity, is an evidence of non-contemporaneity. If, for ex- 

 ample, a species migrates from Britain to America, it is clear that 

 the beds containing it in America are uBwer than the strata which 

 contain it in Britain. This is undoubtedly true if we use the word 

 " contemporaneous " in a strictly historical sense. It is evident that 

 the whole of the formation in America characterized by a certain 

 fauna cannot be contemporaneous with the whole of the series in 

 Britain containing the same fauna; the former will be later than 

 the latter by the time occupied in the migration of the fauna a 

 distance of 3000 or 4000 miles. This is on the supposition that the 

 migration was from east to west. It is possible, however, that the 

 fauna may have originated in some intermediate centre, and spread 

 both east and west ; in which case eastern and western formations 

 characterized by the same (or a similar) fauna, may be strictly con- 

 temporaneous. Even on the extreme supposition of an east and 

 west migration, I shall attempt to show that the time occupied in 

 the migration is unimportant in comparison with the duration of 

 geological epochs. 



It must be remembered that, for purposes of correlation, the group 



