32 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



straight walls which, with a strangely artificial look, mount into the face of the cliffs on the 

 one side, and project in long black reefs into the sea on the other (fig. 1). Every visitor 

 to the islands of the Clyde will remember how conspicuous such features are there. But it 

 is among the Inner Hebrides that this kind of scenery is to be found in greatest perfection. 

 The soft dark Lias shales of the island of Pabba, for example, are ribbed across with 

 scores of dykes which strike boldly out to sea. Where, on the other hand, the material 

 of the dykes is coarse in grain, or is otherwise more susceptible to the disintegrating 

 influences of the weather, it has rotted away and left yawning clefts behind, the 

 vertical walls of which are those of the fissures up which the molten rock ascended. 

 Some good instances of this kind are well known to summer visitors on the eastern shores 

 of Arran. Others, on a large scale, may be seen in the interior of the same island along 

 the crests of the granite ridges, and still more conspicuously on the jagged summits of 

 Blath Beinn and the Cuillin Hills of Skye. 



§ 1. Geographical Distribution. 



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, 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 North Sea, including the Isle 

 of Man, and appear in the north of 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 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 

 ;i third of the total land-surface of the British Isles (Plate I.). 



