256 J. D. Dana on Pseudomorphism. 



improbable that the siliceous solutions that have altered so 

 many calcareous fossils should have contained carbonic acid. 

 The reason that silica is so common in pseudomorphs, as 

 well as so common a material in the constitution of fossilized 

 wood and shells, consists in its ready solution in water at 

 high temperature under pressure, whenever an alkali is pre- 

 sent, as is seen in many volcanic regions ; and its ready de- 

 position again as the waters cool. Soluble minerals cannot, 

 of course, form pseudomorphs of this kind ; and lime, which 

 is slightly soluble, is a rare material for this purpose, because 

 it is not as readily deposited under the circumstances sup- 

 posed. Yet we have calcareous petrifactions, which are 

 formed from cold concentrated solutions of carbonate of lime 

 in carbonated waters ; the lime being deposited as the waters 

 evaporate, and the excess of carbonic acid passing off. 



Admitting the preceding views, we comprehend also the 

 formation of other common pseudomorphs in this division, 

 consisting of broivn iron ore. This mineral acts like silica 

 in depositing itself in the place of the successively removed 

 particles of the original mineral. The beds of this ore have 

 proceeded evidently from more or less perfect solutions, as 

 the stalactitic forms of the mineral prove. And should any 

 process remove the material of a crystal exposed to such a 

 solution, this iron ore might gradually take its place. It has 

 not the power, like silica, of decomposing carbonate of lime 

 under any circumstances ; and, consequently, it is inefficient, 

 except in making itself simply a substitute for the removed 

 mineral. 



The pseudomorphs of j^erozide of iron are often siliceous 

 within, and it seems that the waters containing the iron were 

 also siliceous. The ironstone petrifactions of wood, so com- 

 mon in some formations, have often the hai'dness of quartz. 

 These facts, in connection with this — that the iron is the 

 anhydrous peroxide instead of the hydrous brown iron ore — 

 afford evidence that the petrifying waters were siliceous, and 

 heated to a high temperature, the iron ore not being hydrous 

 on account of the latter condition. 



The change of dathoUte to calcedony (Haytorite), wc are 

 inclined to place in the same category with the above, al- 



