A9 THE MEDALS OF CREATION. Cuap. III. 
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One of the most eminent naturalists and chemists of ; 
the United States, Mr. Dana,* suggests that the reason — 
why silica is so common a material in the constitution of — 
fossil wood and shells, as well as in pseudo-morphie erys- 
tals,t is the ready solution of silex in water at a high 
temperature (a fact affirmed by Bergman{) under great — 
pressure, whenever an alkali is present, as is seen at the 
present time in many volcanic regions, and its deposi- 
tion again when the water cools. A mere heated aqueous 
solution of silica, under high pressure, is sufficient to explain 
the phenomenon of the silicification of organic structures. 
Mr. Dana states that “a crystal of cale-spar in such a fluid 
being exposed to solution, from the action of the heated 
water alone, the silica deposits itself gradually on a reduc- 
tion of temperature, and takes the place of the lime, atom — 
by atom, as soon as set free. Every silicified fossil is an 
example of this pseudo-morphism ; but there seems to be 
no union of the silica with the lime, for silicate of lime is 
of rare occurrence. § 
I proceed to consider the various states in which the 
remains of animals and plants are preserved in the mineral 
kingdom, and shall occasionally offer suggestions for collect- 
ing and preparing specimens ; but particular instructions 
on this head will be given in the sequel, when the different 
kinds of fossils are respectively considered. 
* American Journal of Science for January 1845. 
t+ Pseudo-morphie crystals are crystals moulded in the cavities left 
by other crystals which they have replaced. See Dr. Blum on 
Pseudo-morphous minerals. 
+ Bergman first determined the solubility of silex in simple water, 
aided by heat, and demonstrated its existence in the Geysers and 
other boiling springs of Iceland.—Parkinson, Org. Rem. vol. i. p. 324. 
§ See my “ Notes on a Microscopical Examination of Chalk and 
Flint,” Annals of Natural History, August 1845. 
