§8 Observations on Pseudomorphism. 



It is not impossible that the change of feldspar takes place 

 without the agency of carbonic acid, by means of moisture alone. 

 (See page 70.) 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 removed ; 

 this silicate would however be soon decomposed by the carbonic 

 acid of ihe atmosphere. 



We cannot affirm but that some alumina also passes off with 

 the hydrous silicate of potash, and that a zeolite as well as free 

 silica is liberated, for the result miglit still be the same, proceed- 

 ing as it does from the chemical affinities between the remaining 

 elements and water. The possibility of such an effect shows 

 how very unsatisfactory, as an explanation, is the mere statement 



that if 3K, 8S be removed from 3 atoms of feldspar, and 6H be 

 added, kaolin will result ; (see page 70.) This is true; but we 

 have no evidence that such is the " removal" and "addition." 

 It is equally true that kaolin will proceed from 4 atoms of feld- 

 spar, if Al, Ka", Si ' 2j be removed and 6H be added. 



Without dwelling longer on this branch of the subject, we 

 proceed to the next subdivision, mentioning a few examples 

 only. 



Carbonate of Barytes to Sulphate of Barytes. — The change of 

 carbonates to sulphates, when within reaclv of a decomposing 

 metallic sulphuret, or by the action of waters containing sulphu- 

 retted hydrogen in solution, is well understood. 



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

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

 chemistry has shown, any of the insoluble salts of lead may be 

 decomposed by soluble carbonates. Carbonate of soda effects the 

 change readily in sulphate of lead, when the two are heated to- 

 gether. Is it not possible that carbonate of lime, in the slow pro- 

 cesses of nature, may occasion the same change, although not 

 aided by heat? 



Tnngstate of Lime to Tungstate of Iron, ( Wolfram. ) — Proba- 

 bly due, by a double exchange, to the slow action of nascent sul- 

 phate of iron, proceeding from the decomposition of pyrites in 

 the same rock ; but whether heat be required or not, as in the 

 process of the laboratory, we cannot say. 



/ii* 



