Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/158

 mentioned by the granite, the line of division of the two rocks describing an irregular horseshoe form, while the southern side of the killas extends to the coast and joins the Plymouth limestone.

The surrounding granite mountains rise to an elevation of from 1400 to 1900 feet above the sea, while the hills of killas keep a much lower range; Morwel Down, through which the tunnel is passing, is one of the highest in the central part of the killas, and is about 700 feet above the tideway in the river Tamar, which washes its base.

The killas district, which is attempted to be here described, is every where intersected with veins, or as they are technically called, Lodes. Those which are worked for copper or tin have universally a direction from north—east to south-west, or nearly so; those which run in other courses have all the appearances of a newer formation, and are generally unproductive of metal, if we except two instances, one of which is the lode on which the Beeralstone mines are working to a considerable extent, and the other the lode in Wheal Betsey Mine, in the parish of Mary Tavy, both of which produce lead and silver.

In the last 20 years this district has been the scene of very active exertion in the pursuit of mining, and the most spirited efforts have been made for tracing the veins, and instituting trials upon them for the discovery of their contents. These effects, as in most similar cases, have been attended with very various success, though on the whole, the result has been a favourable one. On many lodes considerable sums of money have been expended, without discovering sufficient quantities of ore to repay the disbursements, and on many the loss has been heavy; in other instances, though the fewer in number, mines have been established which have produced very large quantities of ore, principally of copper, and have paid the adventurers very handsome profits.