The limits of the region within which the dykes occur cannot be very precisely fixed. There can be no doubt, however, that on their southern side they reach to the Cleveland Hills of Yorkshire and the southern borders of Lancashire, perhaps even as far as North Staffordshire (p. 106), and on the northern side to the farther shores of the island of Lewis--a direct distance of 360 miles. They stretch across the basin of the Irish Sea, including the Isle of Man, and appear in Ireland north of a line drawn from Dundalk Bay to the Bays of Sligo and Donegal. Dykes are of frequent occurrence over the north of England and south of Scotland, at least as far north as a line drawn from the coast of Kincardineshire along the southern flank of the Grampian Hills, by the head of Glen Shee and Loch Tay, to the north-western coast of Argyleshire. They abound all along the line of the Inner Hebrides and on parts of the adjacent coasts of the mainland, from the remoter headlands of Skye to the shores of County Louth. They traverse also the chain of the Long Island in the Outer Hebrides. So far as I am aware, they are either absent or extremely rare in the Highlands north of the line I have indicated. But a good many have been found by my colleagues in the course of the Geological Survey of the northern lowlands of Aberdeenshire and Banffshire. The longest of these has been traced by Mr. L. Hinxman for rather more than two miles running in a nearly east and west direction through the Old Red Sandstone of Strathbogie, with an average width of about 35 feet. Another in the same district has a width of from 45 to 90 feet, and has been followed for a third of a mile. But far beyond these northern examples, I have found a number of narrow basalt-veins traversing the Old Red flagstones of the Mainland of Orkney, which I have little doubt are also a prolongation of the same late series. Taking, however, only those western and southern districts in which the younger dykes form a notable feature in the geology, we find that the dyke-region embraces an area of upwards of 40,000 square miles--that is, a territory greater than either Scotland or Ireland, and equal to more than a third of the total land-surface of the British Isles (Map I.). Of this extensive region the greater portion has now been mapped in detail by the Geological Survey. Every known dyke has been traced, and the appearances it presents at the surface have been recorded. We are accordingly now in possession of a larger body of evidence than has ever before been available for the discussion of this remarkable feature in the geology of the British Isles. I have made use of this detailed information, and besides the data accumulated in my own note-books, I have availed myself of those of my colleagues in the Survey, for which due acknowledgment is made where they are cited.