160 Dr Boue 071 the Ongin of Tertiary Rocks. 



The plastic clay not a distinct formation, only a subordinate bed.. 



One of the most important points in which our author has 

 given the victory to his opponents is, the separation of theplastic 

 clay, or his terrain argilo-sableux, from the other numerous 

 tertiary deposits of clay with lignites. He does not enumerate 

 any organic bodies as occurring in his plastic clay. — (Vide Ta- 

 bleau des Terrains, p. 182.) " Les debris organiques qu'on a 

 attribues a Pargile plastique, appartiennent ordinairement aux 

 argiles figulines ;" nor even any trace of lignite, so that if the 

 last mentioned mineral occur in it, it is only in so small a quan- 

 tity as to be omitted in a general view. He further acknow- 

 ledges, that the plastic clay is only a small deposit, " d'amas 

 conches sur des surfaces de roches ou dans les cavites qui peu- 

 vent presenter ces surfaces," — p. 181. He still admits, in op- 

 position to his former opinion, not only that there is true plas- 

 tic clay under the chalk (p. 183.), but also that the potter''s 

 clay (argile figuline) associated with tertiary lignite differs but 

 little from the plastic clay, " ne differ que tres peu de Targile 

 plastique,"** — p. 177. After this distinct statement and candid 

 agreement with opinions expressed by others long ago, we can- 

 not help thinking that our learned author has inserted, merely 

 by mistake in the same work, the remark, " qu'il y a entre le 

 calcaire grossier et la craie un terrain compose d'argile pure et 

 de bois bitumineux, accompagne de corps organises terrestres 

 ou fluviatiles,"'' — p. 185 ; and that, at page 194, he speaks of 

 " argile plastique et ses debris organiques d'eau douce." This 

 theoretical part of his work is probably of older date than the 

 rest ; besides, we only mention this apparent contradiction to 

 put in a stronger point of view M. Brongniart''s latest and most 

 correct classification. 



On the other hand, if he separates the plastic clay from the 

 deposits of tertiary clay with lignite, we may ask him the proofs 

 of the lignite clay of Cologne, and the plastic clay of Gross Al- 

 merode, in Hessia, being still parts of his argillo-arenacious 

 group. Indeed both localities present characters in opposition 

 to those assigned to that group; for at Cologne there are fossil- 

 shells, the fishes of the Dusodile are well known there, and even 



teeth of the mastodon have been described by Noggerath as im- 



2 



