Page:Cornwall (Salmon).djvu/21

 GENERAL PHYSICAL FEATURES l the Cornwall district shows at once that the main portion of the surface consists of what in the earlier days of scientific observation were known rather vaguely as the Grauwacke Group. A modern geologist would describe the beds as Devonian and as practically equivalent to the "Old Red Sandstone" of Monmouth, Here- ford, and other counties. The Devonian rocks of Cornwall comprise slates and limestones~aa. welT as red sandstones. They are conformable to the Silurian rocks which lie below, but not to the Carboniferous rocks which in some other localities overlie them, the Culm Measures in some parts overlapping the Middle Devonian Limestone. For our present purpose, however, it is un- necessary to treat very minutely of questions which are of far greater interest to geologists than to ordinary readers. Running in a direc- tion roughly N.E. and S.W. there are four large and more or less rounded patches occupied by granite. Indeed, it may be said that these four masses of granite are part of a group of six masses, Dartmoor being the largest and the most north-easterly of the series, and the Scilly Islands being the smallest and most south-westerly. In the northern part of Cornwall occurs a bed of Carboniferous rocks, known as the Culm Measures. The other less important deposits of Cornwall are a mass of Serpentine to the N. of Lizard Head, a bed of Gabbro to the E. of it, several long narrow beds of intrusive igneous rocks in the north-eastern and south-western