Page:Transactions of the Provincial Medical and Surgical Association, volume 2.djvu/79

 in different districts, but consist principally of hornblende and felspar rocks, and sparingly of limestone and serpentine.

In the Hundred of Penwith, the especial object of this memoir, the proportion of the granite and slate formations, (which alone exist there) at the surface, is nearly equal, the former, perhaps, being rather the greatest. In the more immediate district of the Landsend, bounded on the eastward by a line drawn from Saint Michael's Mount to the Estuary of Hayle, the whole substratum is granite, with the exception of a narrow and interrupted border of slaty rocks on some part of the shores. In the remaining part of the Hundred, there is some granite, also, but the greater part consists of slate formation. This formation, in this district, exhibits two varieties; the one consisting exclusively of clay slate, and the other principally of a series of rocks composed of hornblende and felspar. The latter variety appears to exist in the more immediate vicinity of the granite; the former at some distance from it. The clay slate exhibits a considerable variety of structure, as do the several rocks composing the other variety of the formation. These latter appear in the form of alternating strata of hornblende rock, hornblende slate, greenstone, slaty felspar and clay slate. These rocks invariably incline from the main body of the granite under a varying angle of from 20° to 40°. At the immediate junction of this with the superincumbent slaty and hornblende rocks, there is often a close inter texture of parts, but never any thing like an intermixture on the great scale, much less any alternation of strata. They are essentially and invariably