RocJcs ofNortJi Devon, 8fc. 115 



limestones really belonged to the transition system*. To 

 these doubts I adverted in a note to the 33 §. of my Memoir 

 on the South of Ireland. Analogous relations appear also 

 to exist in the transition countries adjacent to the Rhine, and 

 the works now in progress to illustrate the organic exuvias 

 found in those tracts may assist in throwing additional light 

 on this subject f. 



This case, however, is not a singular one in geology, it be- 

 ing well known that in the instance of any two systems fol- 

 lowing one the other in the geological progression, although 

 distinguished respectively by peculiar organic remains, yet 

 certain species occur common to both systems. The partial 

 appearance therefore of certain species among the fossils of 

 transition strata which are common in the carboniferous series 

 also, does not appear a sufficient reason for including such 

 transition strata in the formation of the old red sandstone J. 

 Still less could any such inference be drawn from a considera- 

 tion of the component rocks, for, taken as a whole, none can 

 be more dissimilar than the Older Stratified Rocks of North 



* Encyclopcedia Metropolitana, art. Geology, pp. 577. 578. 



For further details of the fossils found in the Older Stratified Rocks of 

 North and South Devon and Cornwall, see Mr. De la Beche's Report on 

 Cornwall, &c. ; namely, pp. 50, 51, on North Devon; pp. 57, 60, 64, 75, 

 76, on South Devon ; and pp. 79, 81, 82, 83, 86, 87, 89, 90, 91, on Corn- 

 wall ; the authorities referred to being Professor Phillips and Mr. J. de C. 

 Sowerby. And compare with these details those given by Prof. Sedgwick 

 and Mr. Murchison in Lond. and Edin. Phil. Mag. for April 1839. 



't' See Dr. Beyrich's Bettr'dge zur Kenntniss der Versteiiierungen des 

 Rheim,iche7i Uebergansgebirges ; and also De Goniatites in MontUms 

 Rhcnanis occurrentibus ; works published at Berlin, April 1837. [An 

 English translation of this memoir appeared in the Annals of Natural His- 

 tory, No. XIV. for March 1839. Ed.] 



\ This doctrine appears to me to be supported by Mr. Murchison himself, 

 when judiciously remarking: 



" If, therefore, it should prove after all that nfew species of co7ic/iifers 

 continued in existence, from the formation of the Silurian Rocks to the 

 accumulation of the carboniferous limestone, how can their presence break 

 down the individuality and separation of systems, established upon such 

 a vast preponderance of direct zoological evidence in the other natural 

 classes? Even should a few other mollusca in the two systems be consi- 

 dered identical, there is no doubt, that by far the greater number of them 

 which truly belong to rocks rising from beneath the Old Red Sandstone, 

 are distinct from those which inhabit the strata above that system. 



" Such evidences are therefore nothing more than additional supports 

 of the important truth which geology has already established, that each 

 great period of change, during which the surface of the planet was essen- 

 tially modified, was also marked by the successive production and oblitera- 

 tion of certain races. 



" Let it not, however, be imagined that I wish to inculcate the doctrine, 

 of every ancient formation having been tenanted by creatures absolutely 

 peculiar to it. The large natural groups of strata only, or, so to speak, 

 systems, can be thus distinguished.'' {Silurian System, p. 582.) 



In this general view I quite agree. 



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