Geology and Magnetism. 531 



The feeders or roots of the split veins may be seen in the mines 

 of St. Just, the bunches of mineral depending entirely upon them ; 

 the principal being those coming in from the granite in a S.S.E. 

 direction, and forming diagonal shoots of ore from their junction 

 northward, as represented in Plate XV. The tin formation in the 

 St. Ives consols may be described as a number of large roots com- 

 ing in from the South-east, converging into one grand trunk, and 

 growing northward at an angle of about eight degrees from the 

 horizon, surrounded by the granite. A similar split vein may be 

 also seen on the banks of the Tarmar, having the principal feeding 

 veins or roots on the south-east side, in tender ground under the 

 bed of the river. Split veins, cross courses, &c. are not productive 

 without the feeders, the latter being the only means by which the 

 contents of the bordering rocks can be brought into them. 



The split veins of all mining districts are of the same nature ; 

 therefore as these effects are matters of fact, and easily referred 

 to, we shall not enter into further details. All split veins, be they 

 quartz, carbonate of lime, hornblende, or any other, have been 

 formed in the same manner, and consequently the contents are ar- 

 ranged in longitudinal plates. 



The influence of the impermeable Splits on the accumulation of the 

 Minerals in the Transverse Fractures. 



Independent of the " bunches" of minerals being found corres- 

 ponding to certain channels, the amount of the deposit in the 

 fissures is considerably influenced according to the position of the 

 intersecting splits. (Plate XVII.) 



That veins of fractures are enriched near their intersection by 

 cross-courses, fluccans, faults, &c, is a fact well known in Corn- 

 wall, North Wales, and the North of England ; the same fact is 

 also observed in Germany, in Mexico and South America : the 

 evidence for which is very clear, inasmuch as the bunches of mineral 

 are sometimes found confined to one side of the splits, as represent- 

 ed in the Plate. This well-known fact is another proof of the east 

 and west fractures having been filled subsequently to, or contem- 

 poraneously with, the formation of the splits. 



These accumulations of minerals appear to have been produced 

 by the splits having been filled with substances impervious to water, 



