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

 152 appears in the form of rounded nodules, giving the rock a porphyritic appearance. It is very common to find cavities in it, lined with crystallized tourmaline. I did not see this rock in situ, but it is so abundant in this district, that I suspect it forms a subordinate bed in the granite, perhaps as the matrix of an ore: in the vicinity of the Land's-end, the granite is often accompanied with masses of black schorl, both amorphous and crystallized.

From St. Just, which is four hundred and four feet above the level of the sea, the granite continues as far as the neighbourhood of St. Ives by Botallach, Morvah, and Zennor, though only at a short distance from the sea. It must however be observed, that the height of the granitic plain continues, and even increases as we advance. Thus, the highest point of that part of the road, in the neighbourhood of Mean Screefis, is six hundred and twenty-three feet above the level of the sea. The tors in that part of the country rise in succession, and form an almost uninterrupted boundary to the south of the road. Near St. Ives, the country suddenly lowers, and we enter the grauwacke, but at what place, or at what height, I cannot say.

Leaving St. Ives for Redruth, the road crosses a hill, the top of which lies to the right: it is granitic, but the common grauwacke having a very fine and close texture, appears on its side. The termination of this rock is pretty near the highest point of the road. Being in a hurry to pass the river Heyl, which can only be forded at low water, I had not time to determine the exact height.

The mouth of the Heyl lies in a small inclosed bay between St. Ives and Godrevy, situated N.N.E. of Mount's Bay. By these two bays this part of Cornwall is formed into a kind of isthmus. A great deal of sand is heaped up in St. Ives' Bay, particularly on the east side, forming small sand hills between Gwithian and Phillack.