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Sect VIIL] 



MINERALOGY 



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The vein a is represented as 



having brought different rocks into contact ; and we may 



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suppose, for illustration, that c is porphyry, and d some 

 schistose rock. The fissure b is intended to he a mere crack. 

 Often when dissimilar rocks are brought into contact, mine- 

 ral substances are found in the fissures, and this is a point 



which the voyager should not neglect. In certain coun- 

 tries the occurrence of the ores of the useful metals is 

 not unfrequent under such conditions. On tidal coasts, 

 should a vein of this kind be found productive, it may be 

 desirable to wait for low water to trace the direction of 

 the vein among any ledges or rocks which may be then 



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laid bare. Tliis 



the run of the yein inland 



but not with certainty ; for though fissures or faults may 

 take general lines on the large scale, they, as would be 

 expected, are very irregular for minor distances. 



Should crystals be found in any such vein, it is often 

 desirable to ascertain how they occur relatiyely to other 

 bodies, crystalline or otherwise, ^liole groups of 

 crystals are thus frequently seen placed on certain pro- 

 jecting surfaces, facing one direction, and this as well 



as 



sides of the vein, or tvalls^ as they are technically termed. 



as 



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