Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/178

 166 to the metallic veins, since they cross them, as I had an opportunity of observing at different places in the mine of Dolcoath.

I do not know if there are any other true veins posterior to the cross courses, that is to say which cross them, and the direction of which would in that case be from east to west. Did these occur, it would point out three distinct periods at least, in the formation of veins in Cornwall. Ought the small metalliferous veins which run in the same direction with the cross courses, i. e. from north to south, to be considered of contemporaneous formation with them? It is in a vein of this sort that the arseniate of lead is found. This mineral has been lately discovered in the mine of Huel Unity, and has been analysed by the Rev. Wm. Gregor.

Although the cross courses are barren veins; they are nevertheless, always dug into on each side, to ascertain whether they contain any ore left behind by the older vein in the act of its being rent asunder. A metallic vein interrupted by a cross course is more generally impoverished than enriched.