STRATA IH THE NEIGHBOURHOOD OP LONDON. 1 1^ 



•urroiinding chalk ; and the rarity of the specimens of echi" 

 nites with their attached spines depends in a great measure 

 on the mode, in which these specimens are obtained. The 

 sp«cihiens seen in cabinets are seldom found by the naturalist 

 himself, but are preserved by the work people, who break 

 the chalk, when any uncommon appearances catch their eye. 

 But it frequently happens, that these marks are not seen 

 until the piece is broken by their tool, and with it perhaps, 

 the entire animal. 



The perfect state of the surfaces of the chalk fossils proves Farther proof, 

 also, that this deposition proceeded from the surrounding 

 fluid ; and that it was not derived from the immediate action 

 of any chemical agent on the shells and other calcareous 

 coverings of the animals living at the bottom of the sea. 

 In the fossil animal bodies found in chalk, not the least 

 diminution of the sharpness of their ridges or points is ob- 

 servable, nor is the least dulness of the delicate lines and 

 embossments of the crusts, or of the spines of the echini^ to 

 be detected. 



That the deposition af chalk and of flint was sometimes Chalkand flint 

 alternate, and even, as it is expressed by Messrs, Cuvierand deposited aU 

 Brongniart, periodical^ appears from tr.e seams or strata of ^"^ 

 flinty nodules, and particularly from the widely extended 

 flat or tabular flinty despositions interposed between the 

 chalk. 



But that the chalk was permeated by the silex at some gut tjjg ^.^^^^^ 

 distance of time after the deposition of the former, seems permeated by 

 also to be proved by the state of the fossils of this stratum. Jfmeafte/iS^* 

 There does not appear to be a single instance, in which the deposUiqn, 

 animal remains are impregnated with silex. On the con- 

 trary, the substance of all these fossils has become calcareous 

 spar, and their cavities have been filled with flint ; thus 

 plainly evincing, that sufficient time must have elapsed 

 for the crystallization of the calcareous spar, previously lo 

 the infiltration of the flint. 



It may not be improper to remark, that in no instance The fiint has 

 does the flint, although in contact with the calcareous spar, w^Ih^thech^fc 

 appear to have become mixed with it. The reverse of this ^ut the chalk 

 is the case with the chalk, since this latter may be seen in gSJ^^**^*^* 

 almost every degree pf union with the flint; from being 



blended 



