J. D. Dana oft Pseudomorphism. 255 



ated lime, since, although silicified fossils ai'e so common, we 

 find nothing of the silicate of lime, which would thus result 

 and shew itself, either in the adjoining rocks, or the fossil 

 itself. 



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

 attending substitution. There appears to be something in 

 the chemical forces excited among the molecules, by the pro- 

 cess 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 sub- 

 stituting 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 consideration may be the occasion of the deposi- 

 tion of silica in its place. 



Fluor-spar, and other minerals, may undei'go this change 

 in the same manner. Heavy-spar is stated in our chemis- 

 tries to be entirely insoluble ; yet there is no doubt that 

 many of its crystallizations in nature have taken place from 

 solution, — cavities in crystals of this mineral have contained 

 a fluid which, on evaporation, afforded crystals of barytes. 

 We are not, therefore, authorised to assert the entire insolu- 

 bility of heavy-spar under all circumstances of heat and pres- 

 sure. This mineral is found occupying cavities, and filling 

 veins in sedimentary limestones, that do not shew the least 

 trace of the effects of heat. We must suppose heated waters 

 acting under pressure to have been 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 silici- 

 fied fossils. The same explanation embraces all the various 

 siliceous pseudomorphs, though so different in composition 

 and character. We may suppose carbonic acid to be present 

 to aid in carrying off the calc-spar, in order to produce the 

 siliceous pseudomorph ; but this explanation would extend 

 to sulphate of barytes, fluor-spar, &c. Besides, it appears 



