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242 



MINERALOGY. 



[Sect. Vlil. 



at one time the elements of some mineral substances have 

 succeeded to others in percolating into, and being depo- 

 sited in these hollows or cavities. If a crystal of any sub- 

 stance, such as felspar, disappears in the body of a rock, 

 leaving its mould, any mineral substance entering the 

 hollow may take its form, as is the case with the well- 

 known pseudomorphous crystals of peroxide of tin from 



jies, Cornwall, the peroxide of tin having thus filled 

 cavities left by felspar crystals. The other class of pseu- 

 domorphous minerals would appear to have been formed 

 in a different manner, there being little reason to suppose 

 that like the pseudomorphous minerals before mentioned 

 they have merely filled up moulds left by the disappear- 

 ance of the original minerals. On the contrary, the 



elements of the new mineral seem gradually, molecule by 



molecule, to have replaced the old mineral, so that the 

 original form is always retained. At first sight, perhaps, 

 this kind of replacement may be difficult to conceive, but 

 when we learn that a plate of steel was found in part 

 replaced by silver, having been left eight years in a case 

 at the mint in Paris, in contact, by one of its ends, with a 

 solution of nitrate of silver, which reached it slowly from 

 a fissure in the yessel containing the solution, this difficulty 

 vanishes. Now and then specimens are found wherein 

 the parts of the original crystals still occur, ^he remainder 

 replaced by another substance. 



By a careful consideration of these exceptions to the 

 agreement between the external forms of minerals with 

 their composition, v.hloh are by no means so formidable 

 when we regard the subject as a whole, the voyager may 

 derive most important aid in the determination of the 



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