Observations on Pseudomorphism. 88 



to require farther remark. The character of these crystals will 

 be gathered from our citations from the work of Dr. Blum. We 

 shall allude, on a following page, to the causes that may have 

 been instrumental in removing the crystals. 



PSEUDOMORPHS BY REPLACEMENT. 



The pseiidomorphs by replacement, are considered the most 

 singular of altered crystals. In some way, the substance of a 

 crystal is entirely removed, and at the same time is replaced by 

 some other material, without changing an angle or plane. Yet 

 these altered crystals appear to be little more singular in their 

 mode of formation than the common petrifaction of wood. The 

 particles of wood as they pass off are 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 decomposition removes the wood; while 

 in most of these pseudomorphs some solvent or chemical agent 

 is required to carry off the original mineral : and this distinction, 

 is more apparent than real. 



The nature of the solvent or chemical agent is the unexplained 

 mystery with regard to these pseudomorphs. 



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

 ity of these altered crystals are siliceous. The silica of which 

 they are constituted we may safely conclude to have been in so- 

 lution, and some siliceous fluid, therefore, must have been opera- 

 ting upon them during the change. Was this fluid hot or cold ? 

 Was it a simple solution of silica, or were other salts present ? 

 A solution of silica, whether resulting from the decomposition of 

 feldspar at the ordinary temperature, or whether proceeding from 

 submarine volcanic action, will in either case contain other sub- 

 stances. The alkali of the feldspar, 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 re- 

 moval by simple solution, and a cotemporaneous substitution of 



