254 J. D. Dana on Pseudomorphism. 



immediately replaced by the petrifying material, and so neatly 

 as to leave every fibre still distinct after the change to stone. 

 There is only this difference in the process — gradual decom- 

 position removes the wood ; while, in most of these pseudo- 

 morphs, some solvent or chemical agent is required to carry 

 off the original minei-al ; and this distinction is more appa- 

 rent than real. 



The nature of the solvent or chemical agent is the unex- 

 plained mystery with regard to these pseudomorphs. 



It is a fact worthy of special consideration, that a large 

 majority of these altered ci'ystals are siliceous. The silica 

 of which they are constituted, we may safely conclude to have 

 been in solution, and some siliceous fluid, therefore, must 

 have been operating upon them during the change. Was this 

 fluid hot or cold ] Was it a simple solution of silica, or were 

 other salts pi'esent 1 A solution of silica, whether resulting 

 from the decomposition of felspar at the ordinary tempera- 

 ture, or whether, proceeding from submarine volcanic action, 

 will, in either case, contain other substances. The alkali of 

 the felspar, potash, or soda, passes off with the liberated 

 silica ; and, in the latter case, the heated waters, if marine, 

 will include both soda and magnesian salts. Such are some 

 of the elements that may have been active in producing these 

 changes. If we may suppose the solution to have been heated, 

 still more efficiency is given to the chemical agents it may 

 contain, through the dissolving power of hot water itself. 



But do we need other decomposing chemical agents besides 

 a heated solution of silica in water ■? Is not the process one 

 of removal by simple solution, and a contemporaneous sub- 

 stitution of silica 1 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 

 pseudomorphous process ; and, instead of its being confined 

 to the few silicified crystals, instances are distributed through 

 a large part of the sedimentary formations of all ages. 



There appears to be no union of the silica with the liber- 



