256 



MINERALOGY 



[Sect. VIII. 



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vein than tlie mere upper portion may become decomposed 

 in the same manner. In fact, the changes which have been 

 eiFected in the fissures containing mineral veins, the mode 



of throwing down a mineral substance, its subsequent re- 



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moval^ its reappearance, or apparent transport elsewhere, 

 the pseudoraorphous filling up of crystalline cavities, the 

 substitution of particles of one substance for another, the 

 evident alterations produced by new fissures, particularly 

 W'hen these have traversed the original fissures at right 

 angles, the differences of contents of fissures when they 

 take different directions traversing the same country and 

 association of rocks, are objects of high interest ; and 

 though nc doubt best studied in mining countries, where 

 opportunities are so numerous, and veins are so exten- 

 sively laid open, a voyager, with some little time on a 

 favourable portion of coast, may often nevertheless ac- 

 quire mucli inform-ation on these heads. To do so, and 

 procure illustrative specimens and a highly valuable col- 

 lection, interesting in many respects, it is not necessary 

 that the vein should be one containing the ores of the 



useful metals 



disloca 



tions, termed common faults, are often in a scientific 

 point of view equally important- 



The cavities of many igneous rocks, and indeed holes 

 and cavities in all, afford good places wherein to search 



found 



for minerals. They 

 well crystallized and in good condition, from not having 

 been exposed to destructive influences until the containing 

 rock be broken — always, it being understood, at distances 

 or depths from the surface where the atmospheric action 



may not have been much felt, or matters have entered 



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