J. D. Dana on Pseudomorphism. 259 



This example affords the general theory for many pseudo- 

 morphs of the more complex mineral species. Sotne one ele- 

 ment or more, hy the influence of external agents, enters into 

 new combinations, and is removed ; this disorganises the origi- 

 nal compound, and leaves the remaining elements free to com- 

 ine anew ; such as are capable, consequently, unite by their 

 affinities, either alone or with water (or other chemical agents 

 present) ; the excess, if any, and soluble, passes off in solu- 

 tion. 



It is not impossible that the change of felspar takes place 

 without the agency of carbonic acid, by means of moisture 

 alone. If so, we would suggest whether the moving cause 

 in the change rests not in the tendency of silica, potash, and 

 water to combine, instead of the attraction between potash 

 and carbonic acid ? The silicate of potash may thus be re- 

 moved : this silicate would, however, be soon decomposed 

 by the carbonic acid of the atmosphere. 



We cannot affirm but that some alumina also passes oflF 

 with the hydrous silica of potash, and that a zeolite as well 

 as free silica is liberated ; for the result might still be the 

 same, proceeding, as it does, from the chemical affinities 

 between the remaining elements and water. 



Without dwelling longer on this branch of the subject, 

 we proceed to the next subdivision, mentioning a few exam- 

 ples only : — 



Carbonate of Barytes to Sulphate of Barytes. — The change 

 of carbonates to sulphates, when within reach of a decom- 

 posing metallic sulphuret, or by the action of waters con- 

 taining sulphuretted hydrogen in solution, is well under- 

 stood. 



Sulphate of Lead to Carbonate of Lead. — Among the salts 

 of lead, the carbonate is the least soluble, and consequently, 

 as chemistry has shewn, any of the insoluble salts of lead 

 may be decomposed by soluble carbonates. Carbonate ot 

 soda effects the change readily in sulphate of lead, when the 

 two ai-e heated together. Is it not possible that carbonate 

 of lime, in the slow process of nature, may occasion the same 

 change, although not aided by heat ? 



Tungstate of Lime to Tungstate of Iron (Wolfram). — Pro- 



