Plastic Clay only a Suboidlnate Bed, 163 



that " son terrain marno-charboneux est quelquefois tout fait 

 au-dessous des calcaire tritonien ?"" — P. 177. But this is the sole 

 place assigned to the plastic clay, so that the conclusion is, that 

 both clays are hut one single deposit. 



On the other hand, if M. Brongniart had not thought proper 

 to give to the plastic clay the rank of a formation, who ever, in 

 England, would have thought of separating the London clay 

 from the plastic clay ? An argillaceous deposit from top to bot- 

 tom, but more plastic or sandy in the inferior than in the supe- 

 rior part, has it ever been divided elsewhere into two distinct 

 formations ? Besides, what is artificially called the plastic clay 

 in the London basin, contains marine or even fresh- water shells, 

 so that the parallelism of that clay with the clay of Paris, could 

 only be maintained, when M. Brongniart erroneously united in- 

 to one mass all the known tertiary lignites. 



Lastly, If the English plastic clay is acknowledged to be 

 identical with that of Paris, we dare defy M. Brongniart to 

 name any place in Europe, beyond the Paris basin, where that 

 clay undoubtedly exists. If he had known other localities be- 

 sides the two formerly mentioned, and which we proved to be 

 erroneous, certainly he would have done so. We then consider 

 ourselves entitled, and without the risk of being considered in- 

 novators, to infer that the plastic clay, as well as the marno- 

 charboneux group of M. Brongniart, does not exist as ajbrnia^ 

 tion or terrain, but that it is only in the Paris basin, the first 

 member (but not always present) of a marine principally calca- 

 reous deposit, which contains subordinate beds of a clay not very 

 unlike the preceding, and not unfrequently lignites or fresh- water 

 shells, or even mixtures of marine and fresh-water shells. In 

 classifying the Parisian clays in this manner, I follow the clas- 

 sical geological doctrines taught me by my old master Professor 

 Jameson. A grand merit of the Wernerian school is teaching 

 the true principles of the science, under which are included, 

 amongst other considerations of the first importance, the dis- 

 tinction of what is termed a formation, a deposit, a bed, &c. 

 The zoological school of geology can adorn the science, but 

 never act in opposition to geognostical axioms, without falling 

 into errors. 



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