Plastic Clay only a Subordinate Bed. 161 



bedded in that clay. We have then, on the contrary, the zoo- 

 logical characters of his proteique group, or of the upper ter- 

 tiary soil, as given by M. Brongniart himself. In regard to 

 the situation of the Gros Almeroda deposit, the classification 

 rests on a purely mineralogical resemblance, which may deceive 

 M. Brongniart in this case, as it has done in that of the plastic 

 clay in the greensand. No certain indication proves the just- 

 ness of this parallelism ; in the mean time, the association of this 

 clay with lignite, as well as in the Meissner, is contrary to the 

 characters assigned to plastic clay by M. Brongniart. Further, 

 as he admits in his plastic clay only an extremely fine mechani- 

 cal deposit, upon which chemical action has had some influence 

 (p. 181.) ; it results, from our limited observations, that plastic 

 clay, or potter's clay, differing but little from the first, occurs 

 in other formations besides those in which M. Brongniart is 

 now willing to admit its presence. Thus, the lignite of the up- 

 permost tertiary soil, his proteique group, sometimes contains 

 true plastic clay, as is the case at Wolfsegg, in Upper Austria, 

 in Bohemia, and Hungary, and it forms also beds in the sand- 

 stone of the lias marls at Kyfendorf, in Southern Coburg. We 

 may even inquire whether or not plastic clay does not exist in 

 more than one other formation, and especially in the old allu- 

 vial ones at Hamburgh ? Plastic clay, according to our view 

 of its mode of formation seems to be a deposit of very minute 

 fragments derived from granitic, syenitic, and felspathic districts. 

 When these rocks were not in the neighbourhood, the clay v;as 

 not deposited, but in its place other mineralogical compounds, as 

 sand, loam, &c. were deposited. Would it be rational to call 

 in the aid of mineral waters in a deposit of this kind, notwith- 

 standing its arenaceous characters ? 



The terrain marno-charhoiieiix of M. Brongniart is merely 

 a subordinate deposit at various heights in the tertiary soil, or, 

 if we choose, only in the Paris coarse marine limestone. This 

 fact no one denies, so that we regret M. Brongniart calls it a ter- 

 rain (p. 176.), and the more so as he puts this denomination as 

 parallel with great formation (grand formation, p. 4.). Farther, 

 he acknowledges that he places in that deposit various beds 

 (assises), for he tells us that his terrain is sometimes subordi- 



OCTOBER DECEMBER 1831. L 



