12 



Mineralogical characters. These beds are often a slightly 

 argillaceous yellowish sand, in which the fossils are imbedded 

 without cohesion. In other instances the matrix is hard and 

 siliceous, obviously composed of fine sand. In others, again, 

 a yellowish or gray clay forms the strata, and sometimes all 

 these substances are variously mixed, but the lower stratum is 

 invariably a lead coloured clay or argillaceous marl. Thefe is 

 mostly some intermixture of comminuted shells, . but that the 

 fossils of these beds have been deposited in a tranquil -sea, is 

 obvious from the extreme perfection of the shells, and the con- 

 stant occurrence of the most delicate bivalves, with both valves 

 in their natural connexion. 



Organic characters. Speaking of this formation, in Europe, 

 Conybeare and Phillips remark, that " the shells are found in an 

 excellent state of preservation, and though generally in a con- 

 fused mixture, are sometimes, so disposed, that patches of par- 

 ticular genera and species appear." 



" Like fossils of most other strata, this assemblage of shells 

 manifests a peculiar distinctive character. A few shells only, 

 which may be placed among those which are supposed to be 

 lost, or among those which are the 'inhabitants of the distant 

 seas, are here discoverable, the greater part not appearing to 

 differ specifically, as far as their altered state will allow of de- 

 termining, from the recent shells of neighbouring seas." 



The above description is strikingly applicable to the American 

 Upper Marine beds, in certain districts, but in others a large pro- 

 portion of extinct species, and such as inhabit distant seas, are 

 of common occurrauce, and manifest a difference in the relative 

 ages of the deposits, yet none of those which I am now consid- 

 ering, exhibit any of the characters peculiar to the London clay 

 and Calcaire grassier. For the present, it may suffice to re- 

 mark, that of all the species which these strata have hitherto af- 

 forded, about forty are specifically identical with the living 

 Testacea now inhabiting the coasts of the United States and 

 the islands of the West Indies. A considerable number of spe- 

 cies are common to the strata of Europe and America; while 

 some of these very shells are also found recent on both sides 

 of the Atlantic, shewing how extensive may be the distribution, 

 and how long the 'duration of a single species. 



Geographical distribution. This formation first appears in 



[26] 



