In the first place, the dykes intersect nearly the whole range of the geological formations of the British Islands. In the Outer Hebrides and north-west Highlands, they rise through the most ancient (Lewisian) gneisses, through the red pre-Cambrian (Torridon) sandstones, and through the oldest members of the Cambrian system. In the southern Highlands, they pursue their course across the gnarled and twisted schists of the younger crystalline (Dalradian) series. In the South of Scotland and North of England, they traverse the various subdivisions of the Lower and Upper Silurian rocks. In the basins of the Tay, Forth, and Clyde they cross the plains and ridges of the Old Red Sandstone, with its deep pile of intercalated volcanic material. In Central Scotland, and the northern English counties, they occur abundantly in the Carboniferous system, and have destroyed the seams of coal. In Cumberland and Durham, they traverse the Permian and Trias groups. In Yorkshire, and along the West of Scotland, they are found running through Jurassic strata. In Antrim, they intersect the Chalk. Both in the North of Ireland, and all through the chain of the Inner Hebrides, they abound in the great sheets and bosses of Tertiary volcanic rocks. These are the youngest formations through which they rise. But it is deserving of note, that they intersect every great group of these Tertiary volcanic products, so that they include in their number the latest known manifestations of eruptive action in the geological history of Britain.[204] [Footnote 204: They have not been found cutting the pitchstone-lava of the Scuir of Eigg.]