192 OUT I. INKS 01 III LD-GEOLo PARTI 



that part of the ground, and unless the rocks should 

 be too much concealed, he may hope to meet with an 

 indication of the actual outcrop of the vein. It is n. t 

 always, nor perhaps often, safe- to pronounce as to the 

 commercial value of such a vein from surface evidence 

 of this kind. The rock may need to be opened up, 

 and boring or mining may be required for some 

 way below the surface before a reliable opinion can 

 be expressed as to whether or not the vein may be 

 workable to profit. 



Mineral veins commonly run in straight or slightly 

 bent lines, and often may be grouped in two or more 

 series, one of which is usually cut by the others, forming 

 thus a network of main-veins and cross-veins. The 

 disposition of these veins may be inferred from the 

 accompanying map of a portion of one of the mineral 

 tracts of Cornwall (Fig. 76). The metalliferous char- 

 acter of a vein is apt to vary with the nature of the 

 rock; plenty of ore may be obtained so long as 

 the vein runs in one rock, but the supply is apt 

 to diminish, or even to die out altogether in another 

 rock. 



As mineral veins have been filled in by the deposition 

 of different minerals on the walls of fissures, a succession 

 of deposits may be observed in their component 

 materials. It not unfrequently happens that these 

 deposits are in duplicate, the one half of the vein being 

 a repetition of the zones of the other half. In Fig 77 

 for instance, the original walls (WW) of the open fissure 

 are each coated with a layer of quartz (i, i). Then comes 

 one of blende (2, 2), another of galena (3, 3), a fourth of 



