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254 



MIXERALOGY. 



[Sect VIII. 



pected, both varieties are to be sometimes seen in the 



Ores of the useful metals, such as sulphuret 



same vein 



of lead, copper pyrites, and peroxide of tin, may, and do, 

 as well form the cementing matter of such fragments, as 



common quartz, carbonate of liiaej or other minerals. 

 In collecting some minerals which have covered others, it 

 may be frequently desirable to obtain enough of the first 

 to show how the latter may have occurred. Rock crystals 

 are thus often seen investing other minerals, the most 

 delicate threads of the latter being preserved in them. 

 By a little care we may take out enough of the crystals 

 to show completely how these threads may have radiated 

 from a centre or have been otherwise disposed. 



As with other mineral substances, we find that ores of 

 the useful metals have been sometimes thrown down in a 

 fissure at one time and not at another, the deposit of one 

 ore sometimes repeated, at others not. Thus there may 



have been a coating of a zinc ore at one time, of copper 



ore at another, and a covering of tin ore upon these, 



sor^^ 



ilXC 



times separated 



other mineral substances, at 



others in deposits one above the other. Again, we find, 

 in the successive dislocations which are sometimes seen 

 to have effected the lines of fissures, that while the 

 lines of least resistance to the applied force have been 

 chiefly through the contents of the original fissure, occa- 

 sionally a new fissure has been made through portions 

 of the adjoining rock ; so that the minerals which may 

 have been subsequently deposited in the new crack or 

 fissure will be partly in the old line, and partly amid the 

 newly "broken ^vid adjacent rocks. 



It would be out of place to attempt a general notice of 



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