I. The _intrusive_ traps occur in the form of walls and veins, sometimes in that of flat parallel beds, and often as huge amorphous masses, to which no definite name can be given. But whatever shape they may assume, they generally agree in presenting well-marked features, whereby their origin can be readily ascertained. The rocks through which they pass are more or less hardened, often contorted, and sometimes traversed by innumerable cracks and rents, into some of which the trap has penetrated in the form of veins. A trap-dyke is a long wall of igneous matter, cutting more or less perpendicularly through the surrounding rocks. Sometimes these dykes attain a breadth of many yards, and may not unfrequently be traced for miles running in a nearly straight line over hill and valley, easily recognisable by a long smooth ridge, with the rock protruding here and there from below where the soil is thin. It is interesting to follow out one of these long ramparts from its beginning to its close, and mark how undeviatingly it cuts through the rocks. No matter what may be the nature of the stone encountered, hard conglomerate, friable shale, compact limestone, or jointed fissile sandstone, all are broken across, and the right line preserved throughout. Nay, I have seen a still more curious instance of this persistency, where the dyke ran for four miles through a set of mountain limestone and lower coal-measure strata, and several enormous sheets of greenstone and basalt. Even when passing through these traps the dyke remained perfectly distinct, its crystalline structure and external configuration presenting a well-marked contrast with those of the surrounding eminences. Of course it must have been formed after all the rocks through which it passed. The sandstones and shales must have settled down long previously on some estuary bed or sea-bottom; the corals and shells of the limestones, and the matted plants of the successive coal-seams must have lived and died, perhaps thousands or millions of years before, and their remains have hardened into stone, ere the continuity of the strata was broken across by the long deep wall of greenstone. Trap-dykes are accordingly appropriately termed _intrusive_. They have been intruded among and must always be later than the rocks in which they occur. In tracing out their character, more especially in a trappean district, such as that of Linlithgowshire, where they abound, we soon find other evidence of their intrusive nature. Where they pass through limestone, they sometimes convert it into a white saccharine marble; shales they bake into a sort of porcelain or burnt pottery; and sandstones become semi-fused into a hard homogeneous quartz-rock. Nor are the changes confined to the rocks traversed; the dykes themselves, along their sides, become fine grained and hardened; occasionally, too, the colour alters from the usual bluish or greenish-grey to black, or