1 50 On some of the Strata in the Neighhourhood of Lvndoii, 



In the fossil animal bodies found in chalkj not the least di- 

 minution of the sharpness of their ridges or poinis is o!)- 

 servable, nor is the Ica.st duhiess of tlie delicate lines and 

 enil)nssment3 of the crusts, or of the spines of the echini, 

 to be detected. 



That the deposition of chalk and of flint was sometimes 

 alternate, and even, as it is expressed bv Messrs. Cnvier ancf 

 B'.ongniart, periodical', appears from the seams or strata of 

 l^iuty nodules, and particularly fron> the widely extended 

 flat or t.tbular flinty depositions interposed between the 

 chalk. 



B-it that the chalk. was permeated hv the sllex at some di- 

 stance of time after the deposition of the former, seems also 

 to be proved by tlie stateof the ibssils of tliis stratum. There 

 does not appear to be a single instance in which the animal 

 remains are impregnated witii silex. On the contrary, the 

 SLibstance of all these fos.^ils has become calcareous spar^ 

 and their cavities have been filled with flint; thus plainly 

 evincing that sufiicient time must have elapsed for the cry- 

 stallization of the calcareous spar, previous!) to the infiltra- 

 tion of the flint. 



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

 docs the flint, although in contact with the calcareous spar, 

 appear to have become rnixed with it. The reve^^e of this 

 is the case with the chalk, since this latter mav he seen in 

 almost every degree of union with the flnit; from being 

 blended with its substance, to being merely united with its 

 surface, and forming the white coat of the flint. It has 

 been, without doubt, from certain appearances resulting 

 from this union, that M. Carrosi and others have been led 

 to believe in the change of lime to flint. 



There can be hardly any hesitation in agreeing with Mr. 

 Jameson, that the most probable explanation of the forma- 

 tion of imbedded flint is that which was first, proposed by 

 Werner, " that during the deposition of chalk, air was 

 ovolved, which, in endeavouring to escape, formed irregular 

 cavities, that were afterwards filled up, bv infiltration, with 

 flint*." Tl'.e decomposition of the softer parts of the ani- 

 mals, which were thus entombed, may be considered as a 

 verv probable source of a part of those gaseous matters 

 which formed tliese cavities : and the counexion of tho 

 animal remains with these nodules of flint is easily explained 

 by supposmg the shells, crusts of the echini, &c. to have 

 projected into these cavities, or to have been adherent to 



Sy:'. cm. of Mli!cralog;y by Prcf. Jameson, vol. i. p. 1" 



their 



