236 Proceedings of the Royal Physical Society. 



of the black mass of these summits (which is in great 

 measure crystalline dolerite), and, instead of lying upon the 

 ends of the greywacke beds, rises through them as a massive 

 dyke. This dyke has been carefully traced and mapped by 

 my colleagues in the Geological Survey, Mr B. N. Peach and 

 Mr H. M. Skae, from the Leadhills in Lanarkshire all the 

 way to Liddesdale (a distance of fully forty-five miles), and 

 Mr J. G. G. Wilson has followed it for some miles farther into 

 England. It forms one of the vast series of basalt dykes 

 which traverse the north of England and a great part of 

 Scotland, in a general east and west or south-east and north- 

 west direction. Many years ago I pointed out that these 

 dykes may be traced westwards in ever-increasing numbers, 

 till they reach the Miocene volcanic plateaux of the north 

 of Ireland and the Inner Hebrides ; that they cross rocks of 

 different ages, including the chalk and the older parts of the 

 Tertiary volcanic sheets ; that they cross large faults without 

 deflection or interruption* From these facts I afterwards 

 inferred that the dykes must be on the whole of Tertiary age, 

 and that they form part of that remarkable series of volcanic 

 eruptions which produced the terraced hills of Antrim, Mull, 

 Morven, Eigg, and Skye.i" Subsequent investigation has fully 

 borne out this inference. 



When the extraordinary number and the remarkable pre- 

 valence of these dykes are considered, together with the 

 extensive area over which they may be traced, they are found 

 to present problems of great difficulty. I have observed 

 them among the Orkney Islands and in Caithness, and have 

 followed them southwards along the whole length of Scotland 

 into the north of England. In some districts, particularly as 

 they approach the great basaltic plateaux of the west, they 

 become so numerous as to form one of the most prominent 

 features in the scenery as well as the geology of the districts. 

 The remarkable dykes of the coast lines of Cumbrae, Ayrshire, 

 Arran, Jura, Islay, Pabba, and Skye are familiar to all passing 

 travellers. It seems hardly credible that such prodigious 



* Trans. Roy. Soc. Eclin., 1860, xxii., p. 650, 



+ Trans. Roy. Soc. Eclin., 1866-67. Britisli Association Eeport for 1867, 

 Sect., p. 52. 



