Page:The parochial history of Cornwall.djvu/208

166 The granite immediately in contact with the slate, consists of compact felspar, containing particles or crystals of felspar, quartz, and mica, in variable proportions, but the whole generally increasing towards the centre of the mass. So that the granite is changed into a felspar rock or porphyry rock, scarcely ever resembling a well characterised granite; while the slate in contact has received the various names of greywacke, greenstone, clayslate, and killas; but it appears to be a rock sui generis, consisting almost entirely of compact felspar, coloured purple or blue by its intimate union with a dark-coloured micaceous mineral, sometimes seen distinct on the surface of the slate, and from which it appears to derive its lamellar structure. The bases of these two rocks are therefore the same, and at the point of contact it is often difficult to draw a line between them; for the slate passes into white compact felspar by the gradual disappearance of its colouring mineral; and this granite rock, by the more and more additions of felspar, quartz, and mica, reassumes its usual character.

On this view of the subject, it is easy to comprehend why the granite and the slate alternate and mutually pass into each other; and an explanation may also be given of the complicated phenomena of granite veins in slate, when it is assumed that both rocks are not only of contemporaneous origin, but likewise similar in their mineral composition.

Between this mining district and a line drawn east and west across the parish, a little north of the church town, the land is in most places very good; but north of this line, at the extremity of the parish, where it abuts on the sea, the ground is almost entirely uncultivated, affording nothing more than a slight pasturage for sheep. At Godrevy Point there is laid open an interesting section of diluvial deposits: one of the beds, composed of shelly sand and pebbles, is consolidated with sandstone and conglomerate.