Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/141

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131
Mr. William Phillips on the Veins of Cornwall.

which it joins the vein to the extremity in the valley. This may perhaps be the case with some, but the practice is not uniform. By making them on an inclined plane, a freer current would be given to the water, yet much of their benefit would be lost, even although their declination were not to exceed one inch in a fathom. For, in supposing this plan to have been pursued in the driving of the deep adit above mentioned, which, from its extremity to North Downs mine, is about four miles or 3520 fathoms long, it will be obvious that if a declination of one inch in a fathom be allowed, the elevation of the adit at North Downs mine would be about 300 feet above its extremity; an elevation nearly if not quite equal to that of the surface of the mine. If therefore in the driving of an adit any allowance be made for the current of water it may be presumed that the interest of the miner will induce him to make it as small as possible.

Country.

Veins containing copper, as well as those containing tin are found both in granite and in schist, though until within the last 50 years, it was esteemed in Cornwall a hopeless expectation to find a vein containing copper in the former of these rocks. Experience has however in many instances, in the parishes of Redruth and Gwennap, as well as in some others, proved that veins of copper ore are found in granite. In both of those parishes granite and schist have in some mines been found to alternate; this alternation has not, as I conceive arisen from their stratification, but from the casual unevenness of the first being supplied by a deposition of the second. It has, I believe, been but rarely noticed that the course of a vein has been along their junction; but some instances of this have certainly

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