T he more striking geological features of the mining districts of Cornwall are so generally known as scarcely to require description. The most commonly occurring rock is “killas,” or clay-slate, through which four large and several smaller protrusions of granite have taken place*. The cleavage-planes of the slates almost invariably dip from the intruding masses of granite, but usually at a less angle than the line of contact of the two rocks. Near the point of junction the granite often becomes fine-grained, and not unfrequently sends off veins into the adjoining slates. Masses of granite are also sometimes found imbedded in slate; and fragments of slate enclosed in granite are occasionally met with. At Herland, in the Crowan district, at a considerable distance from any known body of granite, isolated masses of this rock have been found at a depth of 110 fathoms†; and somewhat similar disconnected granitic blocks are said to occur, 49 fathoms deep, at Huel Buller, near Redruth‡. The granite constituting the larger areas is usually divided into floors resembling beds, which form sheets in the central portions of the several masses, while the edges bend beneath the surrounding sedimentary rocks, and approximate in conformation to the surface of junction between the two. The granites of Cornwall and of Dartmoor probably belong to the same geological age; and there is evidence that the great upheaval of the granite of the latter locality occurred in Post-Carboniferous times. The slates in the vicinity of granite are usually of a green,