Observations on Pseudomorphism. 85 



psendomorphs, though so different in composition and character. 

 We may suppose carbonic acid to be present, to aid in carrying 

 otf the calc spar in order to produce the siliceous pseudomorph ; 

 but this explanation would not extend to sulphate of barytes, 

 fluor spar, &c. Besides, it appears 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 psendomorphs, as 

 well as so common a material in the constitution of fossilized 

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

 temperatures under pressure, whenever an alkali is present, as 

 is seen in many volcanic regions ; and its ready deposition 

 again as the waters cool. Soluble minerals cannot of course form 

 psendomorphs of this kind; and lime, which is slightly soluble, 

 is a rare material for this purpose, because it is not as readily de- 

 posited imder the circumstances supposed. Yet we have calca- 

 reous petrifactions, which are formed from cold concentrated so- 

 lutions of carbonate of lime, in carbonated waters ; the lime 

 being deposited as the waters evaporate, and the excess of car- 

 bonic acid passing oflF. 



Admitting the preceding views, we comprehend also the for- 

 mation of other common psendomorphs in this division, consist- 

 ing of brown iron ore. This mineral acts like silica in deposit- 

 ing itself in the place of the successively removed particles of 

 the original mineral. The beds of this ore have proceeded evi- 

 dently from more or less perfect solutions, as the stalactitic forms 

 of the mineral prove. And should any process remove the ma- 

 terial of a crystal exposed to such a solution, this iron ore might 

 gradually take its place. It has not the power, like silica, of de- 

 composing carbonate of lime under any circumstances, and con- 

 sequently it is inefficient, except in making itself simply a sub- 

 stitute for the removed mineral. 



The pseudomorphs o( peroxyd of iron are often siliceous with- 

 in, and it seems that the waters containing the iron, were also 

 siliceous. The ironstone petrifactions of wood, so common in 

 some formations, have often the hardness of quartz. These facts, 

 in connection with this — that the iron is the anhydrous peroxyd 

 instead of the hydrous brown iron ore — afford evidence that the 

 petrifying waters were siliceous, and heated to a high tempera- 

 ture, the iron ore not being hydrous on account of the latter con- 

 dition. 



