532 Geology and Magnetism. 



the metallic solutions being thus retained entirely on their respec- 

 tive sides ; and when one side happens to be more strongly impreg- 

 nated than the other, the quantity of minerals formed in the trans- 

 verse fissures will be found in the same relative proportion. The 

 sketches in Plate XVII., illustrating these kinds 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 influences 

 on the deposits of minerals as the cross-course, or meridional split, 

 i. e. they dam of 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 mineral is found 

 almost entirely confined to one side of their intersecting 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 Alfread, near Guinear. The el van 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 large scale, the accumulations pro- 

 duced by impermeable veins will be found according to the following 

 order : — 



When the splits or veins run from south-west to north-east the 

 " bunches" will be found principally on the eastern side: if the 

 splits run from south-east to north-west the deposits will be found 

 on the western-side, being the natural consequences of the oblique 

 mechanical interruptions of the solvents transmitted through the 

 grain of the rocks by means of the magnetic currents. Numerous 

 minor variations must necessarily occur from the effects of local 

 causes ; but we need not describe them, as we hope that the prin- 

 ciple here laid down will be found sufficient for the guidance of 

 the practical miner." 



