Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/164

 support they may afford to any hypothesis on the formation of the rocks or the veins, though they may offer some hints on the subject.

Imperfect as this sketch is, it may serve to lead the attention of some more able member of the Society to the consideration of the appearances of this district, which I thought sufficiently curious to encourage an attempt at their description.

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POSTSCRIPT

December, 1816.

I have lately visited the tunnel in consequence of its completion, and therefore am enabled to complete the section of the hill, shewing that no new strata have been discovered since the preceding paper was written.

I have likewise ascertained more satisfactorily the dip or underlay of the lodes near the centre of the hill, and inserted them in the section with two cross lodes or cross courses, which traverse the lodes near that place. A remarkable alteration in the texture of the killas occurs on each side of one of these cross courses, it is found in such a decomposed state that it is converted into a soft clayey matter, so as to be very difficult to preserve a passage through until it can be securely arched.

A period of thirteen years has been occupied in bringing this great work to a conclusion, and it has not been done without the anxieties consequent on such an undertaking.

Two things of great importance in the practice of mining may be remarked of this work. First, the extreme accuracy of the line