Mr. A i kin on the Gravel at Litchfield. 429 



been subjected to the solvent action of water under some particular 

 modification, by which nearly the whole interstitial matter (with 

 the exception of a few flakes here and there of quartzy chalcedony) 

 has been removed, while the quartz moulded within the tubes of 

 madrepore and representing most perfectly the external form of the 

 zoophyte, alone remains. 



The only difference that chemical analysis has detected between 

 quartz and chalcedony, is that the former is silex with perhaps one 

 or two per cent, of water, while the latter contains, besides, about 

 two per cent, of alumine and lime ; but this difference appears by 

 no means sufficient to account for the absolute permanence of the 

 one, and the readiness with which the other suffers decomposition 

 under the same circumstances. The moisture contained in the 

 bed is primarily rain water, and it is not easy to see what active 

 agent it can become charged with in draining into the interior of 

 the mass, except carbonic acid or carbonate of lime : any of the 

 stronger acids, such as the sulphuric, resulting from the decompo- 

 sition of pyrites, would immediately be neutralized by the calcareous 

 matter in which the whole bed abounds. Alternations of moisture 

 and dryness, of heat and cold, to which the decomposition of the 

 exposed surfaces of rocks is so generally attributed, could have 

 little or no influence in this case; since the state of moisture and 

 of temperature at the depth of twenty feet or more in a bed of 

 gravel cannot be supposed to undergo much change. 



May I take the liberty of recommending to the well known ac- 

 tivity of the members of this Society an examination into the cir- 

 cumstances on which the phenomena mentioned above depend, 

 and which appear to be not a little important both in a chemical 

 and geological point of view ? 



Vol. iv. 3 i 



