and Fossil Remains near London. 351 



" infiltration, with flint."* The decomposition of the softer parts of 

 the animals, which were thus entombed, may be considered as a very- 

 probable source of a part of those gaseous matters which formed these 

 cavities: and the connection of the animal remains with these nodules 

 of flint is easily explained by supposing the shells, crusts of the echinii 

 &c. to have projected into these cavities, or to have been adherent 

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



That the separation and deposition of the matter forming these 

 siliceous nodules have been the work of crystallization, is rendered 

 evident by the cavities left either in these nodules, or in the fossils, 

 being generally lined with quartz crystals. 



Whilst endeavouring thus to explain the formation of these flinty 

 nodules, and the filling up of the cavities of the fossils with flint, a 

 difficulty arises from observing these bodies, insulated as it were in 

 their bed of chalk : it not being easy to conceive, how so copious an 

 infiltration should have taken place into these cavities, whilst the 

 surrounding chalk should only have received a slight intermixture of 

 siliceous grains. 



Something analogous is however observable in the formation of 

 the calcareous stalactite ; since in those caverns in which these 

 concretions have been forming for a very long period, the infil- 

 tration by which they are formed is found to continue to the present 

 day ; proving that the interstices of the superincumbent stone, 

 have not yet been filled by the concreting of the earthy particles 

 held in solution in the percolating fluid, by the crystallization of 

 which these bodies have been formed, and are now augmenting. 

 ' The Oberstein nodules of agate appear to have been formed under 

 somewhat similar circumstances ; since it is in general evident from^ 

 their external surfaces, that they also have had very little adherence to 

 * System of Mineralogy by Prof, Jauicson, toI. I, p. 172. 



