56 



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 represented in the Plate. This well-known fact is another 

 proof of the east and west fractures having been filled subse- 

 quently to, or contemporaneously with, the formation of the 

 splits. 



These accumulations of minerals appear to have been pro- 

 duced by the splits having been filled with substances imper- 

 vious to water, the metallic solutions being thus retained entirely 

 on their respective sides; and when one side happens to be more 

 strongly impregnated than the other, the quantity of minerals 

 formed in the transverse fissures will be found in the same re- 

 lative proportion. The sketches in Plate XVII., illustrating 

 these kind of accumulations, have been taken from mines in 

 America and Europe ; therefore they may be considered as real 

 sections, to which we beg reference for a more clear idea of this 

 interesting part of our inquiry. 



The impermeable porphyritic channels have the same influence 

 on the deposits of minerals as the cross-courses, or meridional 

 splits, i. e. they dam up and retain the metalliferous solutions 

 on one side, and thus produce large accumulations. In South 

 Roskear, Tin Croft, and some other mines in Cornwall, the mi- 

 neral is found almost entirely confined to one side of their in- 

 tersecting cross-courses and hornblende veins. One of the 

 most noted examples of this fact, mentioned in the Geological 

 Report of Cornwall, is that of Wheal Alfred, near Guinear. The 

 elvan vein, Plate XVII. fig. 1, runs from south-west to north- 

 east ; the lode intersects it obliquely ; while in the slate, on the 

 eastern side, it contained mineral ; but on approaching the elvan 

 it became much richer, and yielded sufficient ore to afford a profit 

 of £140,000. After quitting the elvan on the western side the 

 lode became poor, and eventually the mine was abandoned as 

 unproductive. The arrows in the sketch represent the nature of 

 the accumulation on one side of the elvan, and the apparent 

 cause of the poverty on the other. Viewing the subject on a 



