Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/133

 Rh which appears to me to rise above this mountain plain, and which is, without doubt, the highest point of all that part of the country, is Craw-Mere rock, where the two rivers Oakment and Dart have their source.

I bent my course from Two Bridges to Launceston, by St. Mary Tavy, Brentor, and Lifton, making a circuit of the exterior boundary of that part of Dartmoor forest. As long as the mountain plain continues, the country preserves the same appearance, and all along the road between Two Bridges and Tavistock, for the first six or seven miles, we find on the surface of the ground great numbers of granite-blocks; these probably come either from the tors, or are produced by the rock on the surface splitting in that manner, in consequence of the continued action of external agents. Several of these blocks are so firmly fixed in the ground from which they project, and are besides so uniformly spread over the surface in every direction, that they cannot be supposed to have been transported by a current to the place which they now occupy.

At the distance of three miles and a quarter from Tavistock the grauwacke begins to re-appear in a very distinct manner, and at the height of one thousand one hundred and twenty-nine feet above the level of the sea, which is rather considerable for this formation.

From this place, the country lowers with a pretty quick descent towards Tavistock, and this change of rock is accompanied by so complete a change in the vegetation, that it is impossible not to be struck by it. Nothing can be more remarkable than to see on the skirt of this mountain plain, towards St. Mary Tavy and Brentor, highly cultivated vallies, succeeded by rich pastures, which rise as high as the line of superposition of the secondary rocks, above which there is nothing but bare and naked rock.