About a mile from St. Mary Tavy, near the place where the roads join which lead from Plymouth to Oakhampton, and from Tavistock to Two Bridges, there is a bed of greenstone of some feet in thickness, in the grauwacke slate; it decomposes into a green earth. With regard to the grauwacke itself, the direction and inclination of its strata continue the same, it only contains more quartz as we approach St. Mary Tavy, and becomes at the same time less slaty.
St. Mary Tavy is six hundred and forty-eight feet above the level of the sea. A copper mine is worked here at a great depth, and amongst the rubbish I found grauwacke and schistose limestone,[1] heaped one above the other; which shews that the epochs of formation of these two rocks are nearly coeval, since we find beds of the one included in those of the other.
From St. Mary Tavy to Launceston by Brentor and Lifton, we cross successively the Lyd and the Tamar, continuing in the grauwacke slate formation to within a mile of Launceston. The strata of the grauwacke slate are very distinctly seen at the ferries of these two rivers. It is succeeded by a schistose limestone having a very fine paste of a dark blue colour and dull lustre, dividing into large flags, which are put to the same use as slate, and which I should have taken for such, if I had not found that it effervesced with acid.[2] Besides, it is here only in subordinate beds, and I do not believe that it extends very far.
I saw at the house of the Rev. William Gregor of Creed, two rolled pieces, one of which appeared to me to be idocrase (vesuvian)
