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84 Observations on Pseudomorphism. 



silica? A crystal of calc spar in a hot siliceous fluid, being 

 exposed to solution from the action of the water alone, the 

 silica, depositing itself gradually on a reduction of temperature, 

 takes the place of the lime as soon as set free. This appears 

 to be the process by which such changes have taken place. 

 Every silicified fossil is an example of this psendomorphous pro- 

 cess ; and instead of its being confined to the few silicified crys- 

 tals, instances are distributed through a large part of the sedi- 

 mentary formations of all ages. 



There appears to be no union of the silica with the liberated 

 lime, since, although silicified fossils are so common, we find 

 nothing of the silicate of lime, which would thus result and show 

 itself, either in the adjoining rocks or the fossil itself. 



The process then, is probably a mere solution, and an attend- 



ing substitution. 



th 



forces excited among the molecules, by the process of solution, 

 when very slow and gradual, which leads the molecules of any 

 body that may be passing at the time from a liquid state, to take 

 the place successively of each molecule that is removed; and 

 thus it is that the original form to the minutest stria, is so exactly 

 assumed by the substituting mineral. 



Moreover, as a liquid, saturated with one salt, will not take up as 

 much of another, the solution of the lime in the case under consid- 

 eration may be the occasion of the deposition of silica in its place. 



Fluorspar and other minerals, may undergo this change in the 

 same manner. Heavy spar is stated in our chemistries, to be en- 

 tirely insoluble; yet there is no doubt that many of its crystalli- 

 zations in nature have taken place from solution. Cavities in 

 crystals of this mineral have contained a fluid which on evapora- 

 tion, afforded crystals of barytes. We are not therefore author- 

 ized to assert the entire insolubility of heavy spar under all cir- 

 cumstances of heat and pressure. This mineral is found occu- 

 pying cavities and filling veins in sedimentary limestones, that 

 do not show the least trace of the effects of heat. We must 

 suppose heated waters acting under pressure to have been the 

 solvent in these cases, but not heated so highly as to crystallize 

 the sedimentary limestones. 



Simple solution without decomposition appears to be the most 

 probable view of the change, as in the case of the silicified fos- 

 sils. The same explanation embraces all the various siliceous 



