Silurian Rocks. 87 



Obolus (which has a great affinity to the Lingula), as to form 

 entire beds. Here we have a parallel to those beds in the 

 Silurian series of the British Isles, abounding so copiously 

 in the Lingula attenuata. It is also a parallel to beds occur- 

 ring at a far more distant point, on the opposite side of the 

 Atlantic. Mr Lyell, in describing the Potsdam sandstone, 

 the lowest member of the Silurian series in North America, 

 as it occurs on Lake Champlain, says, *' In many places this 

 most ancient of the fossiliferous rocks of New York is 

 divided into laminae by the remains of innumerable shells of 

 the genus Lingula, They are in such profusion as to form 

 black seams like mica, for which they were at first mistaken. 

 It is highly interesting, that in this lowest fossiliferous bed, 

 one of its commonest organic remains should belong to a liv- 

 ing genus, and that its form should come very near to species 

 now existing. Throughout so vast a series of ages has Na- 

 ture worked upon the same model in the organic world !" 



The Silurian system of the northern countries of Europe 

 is, as a whole, closely analogous to that of Great Britain ; 

 and it proves that wherever the sediments of the same age 

 in the two regions resemble each other in lithological tex- 

 ture, such similarity is accompanied by a close approximation 

 and frequent identity in the associated organic remains. 

 When the fossils from the Silurian beds of Northern Europe 

 were compared, Mr Lyell informs us, by M. de Verneuil 

 with those brought by him from America, there was a great 

 distinctness ; but the representation of generic forms, whe- 

 ther in the organic remains of the Upper or Lower Silurian 

 strata, was most clear and satisfactory. The geologists of 

 New York make three distinct groups in the Lower Silurian, 

 and four distinct groups in the Upper Silurian series of that 

 country, and Mr Lyell is of opinion that these divisions are 

 based on sound principles ; that is, on mixed geographical, 

 lithological, and palseontological considerations ; the analogy 

 of European geology teaching us that minor subdivisions, 

 however useful and important within certain limits, are never 

 applicable to countries extremely remote from each other or 

 to areas of indefinite extent. The Silurian rocks are deve- 

 loped in North America on a great scale, and, like those of 



