Page:Cornwall; Cambridge county geographies.djvu/41

 GEOLOGY AND SOIL 25 contorted the superposed beds. There are as well in- trusive veins of igneous rocks. In the Lizard district is serpentine, a compact, tough rock often of a green colour, lending itself to a high polish, and forming magnificent cliffs with a special gloss and colour, as well as maintaining on the surface a special flora. The prime feature in Cornish geology is the upheaval of the granite, distorting, folding back, and altering the superincumbent beds. In the north-east of Cornwall from a line drawn from below Launceston, on the Tamar, to Boscastle the rocks belong to the culm measures of North Devon. All the rest of the peninsula, except the protruding granite and the serpentine of the Lizard, pertains to the Devonian series of sedimentary rocks, in which the first signs of life appear; consisting largely of clay-slate, locally known as Killas, alternating with beds of red or grey grit and sandstone. Although these slaty rocks must be some thousand feet in thickness, they have been so broken up and turned over by the convulsions of the earth that their chronological sequence cannot easily be determined. In these con- vulsions they have been rent, and through the rents have been driven hot blasts that have deposited crystalline veins, or injections of trap and other volcanic matter, altering the character of the rock through which they have been driven. By the Menheniot Station on the G.W.R. is a hill of serpentine thrown up at one jet, and now largely quarried for the sake of the roads. The culm measures already alluded to consist of black shales and slates with seams of grit and chert, much un-