272 Mr Kenwood on the Electric Currents 



the Great Work mines, are worked in both granite and slate ; but 

 the former is productive in the last mentioned rock, and the lat- 

 ter mine in the first named. Again, the copper-ore at Tresa- 

 vean is all in granite, whilst in the parallel veins of the adjacent 

 United, Wheal Squire, and Ting Tang mines, it is entirely in 

 slate. 



The general direction of the principal metalliferous veins is 

 about E. and W. (magnetic), but there is second series, the 

 contra lodes, bearing about N. W. and S. E. Both these sys- 

 tems are usually intersected, and often dislocated (heaved) by a 

 third series, the cross courses, which strike about N. and S. 

 The dip will, of course, in either case, be at right angles to 

 the direction, but sometimes to one sideband at others to the 

 other ; the lodes, for example, sometimes dipping to the N. some- 

 times to the S. The directions more or less coincide with the 

 lines of symmetrical structure, by which both series of rocks are 

 divided. 



There is, however, one rather extensive district (that of Saint 

 Just) in Cornwall, in which the metalliferous veins bear about 

 N. and S., and the cross courses (guides) about N.E. and 

 S.W. 



The extensive mining operations in this county afford excel- 

 lent opportunities for examining the subterranean temperature. 

 From an extensive series of observations, I have satisfied myself, 

 that, at all depths yet attained here, the slate rocks are on an 

 average from two to three degrees warmer than the granite at 

 the same level. Records of General Science, Sept. 18.36 ; and 

 Edinburgh New Philosophical Journal, vol. xxi. p. 376. 



Of the mineral contents of the water from some of the mines, 

 Mr Fox says (Cornwall Geol. Trans, vol. iii. p. 323), " I have 

 examined the water from the bottom of several deep mines, and 

 find it generally to contain very little foreign matter, not ex- 

 ceeding one to five or six grains in a pint. Its relative purity 

 seems to have no reference to the temperature, nor to the depth 

 of the mines ; for instance, the deposit from the water, taken 

 from the two deepest in this county (each nearly 250 fathoms 

 deep), Dolcoath and Huel Abraham, after evaporation, did not, 

 in either case, exceed two grains from a pint. On the other 

 hand, the water from the Consolidated mines, when evaporated, 



