observed in some Metalliferous Veins. 271 



confirmatory of M. Ampere's theory of the electric origin of ter- 

 restrial magnetism. 



The Mineral Veins of Cornwall traverse both the granite and 

 slate rocks (and often the Elvan courses, which rake through 

 both the granite and slate) without interruption ; they are high- 

 ly inclined tabular masses of great extent ; on a small scale, ex- 

 hibiting numerous curves and irregularities, both in direction 

 and dip, with very variable breadths ; but on a large view ha- 

 ving an approximation to rectilinearity. Their composition is 

 chiefly quartz, but, with other earthy minerals, in many places 

 mixed with metallic substances, viz. copper and iron pyrites, vi- 

 treous copper ore, oxide of tin, blende, galena, with admixtures of 

 small quantities of other minerals ; as native copper, red oxide, 

 and the carbonates of copper, salts of lead, &c. ; all frequently 

 so intimately and indiscriminately mixed (mechanically) with 

 the rock-contents of the vein, that their separation is among the 

 most difficult and expensive of our mining operations. 



These irregularly distributed masses, veins, granules, crys- 

 tals, and other forms of the ores, have usually a prevailing dip 

 longitudinally through the vein itself (shuts or shoots) ; and 

 this is almost universally from the granite and towards the slate, 

 whichever of them may be the containing rock. It is also most 

 commonly the case, that the vitreous copper-ore occurs in those 

 parts of veins which are in the granite, and in the massive slate 

 rocks (greenstones) contiguous to it ; whilst the copper pyrites 

 more usually occur in the lodes when in the shistose members 

 of the slate series ; not, however, without many exceptions to 

 both. 



Tin ores abound more in the granite than in the slate dis- 

 tricts, notwithstanding very large quantities of it have occurred 

 in the latter. It is a generally recognised fact, that although 

 the most perpendicular veins are not always the richest, yet the 

 most productive parts of a given lode are those nearest to per- 

 pendicularity. 



It is also true, but with many exceptions, that in parallel 

 lodes the ores generally occur near the same N. and S. line. 

 The same vein is, however, seldom productive in both granite 

 and slate ; notwithstanding, parallel veins are often rich in simi- 

 lar ores in different rocks ; thus the veins of Wheal Vor and of 



