UPPER, OR FLINTY CHALK. 145 



is a high temperature absolutely essential to the solution of silex in water 

 since this earth occurs in a large proportion in the mineral waters of our 

 own island *, and also enters into the composition of the epidermis of 

 various plants of the cane tribe, and of the English reeds, and grasses. 

 The epidermis of the Equisetum hyemale, or Dutch rush, consists almost 

 entirely of silex f. 



There is scarcely a single fact in geological science, that has so much 

 excited the attention of philosophers, or given rise to so many unsatis- 

 factory conjectures, as the formation of the siliceous nodules of the chalk. 

 Upon this interesting subject, which is still involved in much obscurity, 

 I shall not presume to hazard an opinion, but content myself with offer- 

 ing a condensed view of those theories, that have received the sanction of 

 some of our most eminent geologists. 



The celebrated Werner, whose opinions are embraced by Professor 

 Jameson, Mr. Parkinson, and others, oflfersthe following explanation of the 

 phenomenon in question. He supposes, that " during the deposition of 

 the chalk, air was evolved, which, in endeavouring to escape, formed irre- 

 gular cavities that were afterwards filled up by infiltration with flint. The 

 decomposition of the softer parts of the animals thus entombed, may 

 be considered as a probable source of part of the gaseous matter that 

 formed these cavities ; and the connexion of the animal remains with 

 those nodules of flint, is easily explained by supposing the shells, crusts 

 of echini, &c. to have projected into the hollows, or to have been ad- 

 herent to their sides, at the period at which this infiltration took place. 

 That the separation and disposition of the matter forming the nodules, 

 have been the effect of crystallization, is rendered evident by the cavities 

 left either in these nodules, or in the fossils, being generally Uned with 

 quartz crystals :j:." 



Upon this subject Dr. Berger § remarks, that with regard ^' to the forma- 

 tion of flints in chalk, if we adopt the theory of Werner, 1 should be as 



• The mineral waters of Bath contain twenty grains of silex in ten pints and a half. Nichol- 

 son's Journal, Vol. iii. p. 403. 



•f Organic Remains, Vol. i. p. 328. 

 + Geological Transactio7is, Vol. i. p. 350, 

 ] Ibid. p. 96. 



U 



