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be perfectly limpid and not viscous. It gelatinizes under the 

 action of heat, unless miich diluted it is not easily preserved; it 

 becomes opalescent, and the jelly separates, which cannot be 

 again dissolved in water. Coagulation is quickly effected by a 

 solution containing a small quantity of any alkaline carbonate. 

 This soluble form of silica unites -ndth various organic matters. 

 There is reason to believe that this solution may play an import- 

 ant part in the phenomena of nature ; there is no difficulty in 

 explaining how such a solution can be obtained. Bischoff, in 

 his " Chemical Geology," states, " It is very deserving of notice 

 that carbonate of lime may be displaced by almost all silicious 

 Bubstances, and consequently'' it is possible that entu-e layers of 

 limestone may be displaced by silica, and this may serve to 

 account for some remarkable instances of the occui-ence of quartz 

 layers in the sea." "It may be iaferred that when water 

 exercises a decomposing and solvent action on minerals, silicates 

 with excess of acid, and others with excess of lime are formed, 

 the more soluble product being removed and the less soluble left. 

 Hence it is intelligible that the alteration of the composition of 

 minerals exposed to such influences should be very gradual, and 

 in the first stage scarcely recognizable by either mineralogical or 

 chemical means." It has been shown that water coutaining 

 carbonic acid, with alkaline carbonates and silicates may deposit 

 silicious substances with very minute admixtm-e of bases. Such 

 is the case with flint and opal, which contain small quantities of 

 alumiaa, lime, soda, and oxide of ii'on. 



Bearing in mind then this capability of carbonate of lime, by 

 ceiiain means, beiug displaced by sdica or flint, a true explana- 

 tion of the nature and peculiarities of flints may be arrived at. 

 It is by no means intended to invalidate the observations of 

 Bowerbank and Johnson, who have foimd organic substances 

 converted into flint. But too much reliance sliould not be placed 

 on these supposed organic structures, for a section of chalk itself 

 after certaia preparation will exhibit under the microscope struc- 

 ture almost identical with that possessed by flint ; a large num- 

 ber of sponge spicules are always present in chalk, and these will 

 also become apparent. Because the most delicate structures of 

 organisms have been preserved in these flints, it has been 

 assumed that the sdicification was instantaneous, or at any rate 

 immediately after the death of the animal, but animal bodies 

 may have been entombed in the soft mud at the bottom of the 

 ocean, have been preserved for a lengthened period until so far 

 mineralized by the surrounding elements that a further meta- 

 morphism may have completed their preservation. Casts too in 

 the soft mud of the chalk-sea may have preserved so true a like- 

 ness of the individual that when eilicified they would give the 



