Vol. 52.] BASALT-PLATEAUX OP NORTH-WESTERN EUROPE. 403 



To gain some idea of the amount of this waste we must, in the 

 first place, picture to our minds the extent of ground over which the 

 lavas were poured, and the depth to which they were piled upon it. 

 Whether the now isolated basalt-plateaux of Britain were once 

 united into a continuous plain of lava may never be ascertainable. 

 It is quite certain that every one of these plateaux was formerly 

 much more extensive than it is now, for each of them presents as 

 its terminal edge a line of wall formed by the truncated ends of 

 horizontal basalt-sheets. And there seems no improbability in the 

 assumption that the whole of the great hollow from the centro of 

 Antrim up to the Alinch was flooded with lavas which flowed from 

 many vents between the hills of ancient crystalline rocks forming 

 the line of the Outer Hebrides on the west, and those of the 

 mainland of Scotland on the east. 



The depth to which some parts of this long hollow were over- 

 flowed with lava exceeded 3000 feet. The original inequalities of 

 surface were buried under the volcanic materials which were spread 

 out in a vast plain or series of plains, like those that have been 

 deluged by modern eruptions in Iceland. Owing, however, to a 

 general but unequal movement of subsidence, the lava-fields sank 

 down here and there to, perhaps, an extent of several hundred feet. 

 so that the old land-surface on which they began to be poured out 

 now lies in those places below the level of the sea. 



I have shown that even during the volcanic period, while the 

 lavas were still flowing from time to time, erosion was in active 

 progress over the surface of the volcanic plain. The buried river- 

 channel of the Scuir of Eigg, and the records of water-action described 

 in the present paper, prove that rivers descending from the moun- 

 tains of the Western Highlands carried the detritus of these uplands 

 for many miles across the lava-fields, swept away the loose material 

 of volcanic cones, and cut channels for themselves out of the black 

 rugged floor of basalt. 



The erosion thus early begun has probably been carried on 

 continuously ever since. The present streams may be looked upon 

 as practically the same as those which were flowing in the Tertiary 

 period. There may have been slight changes of level, oscillations 

 both upward and downward in the relative positions of land and 

 sea, and shiftings of the watercourses to one side or other ; but 

 there seems no reason to doubt that the existing basalt- plateaux, 

 which were built up as terrestrial areas, have remained land-surfaces 

 with little intermission ever since, although their lower portions may 

 have been in large measure submerged. 



In the existing valleys, fjords, and sea-straits by which these 

 plateaux have been so deeply and abundantly trenched, we may 

 recognize some of the drainage-lines traced out by the rivers which 

 flowed across the volcanic plains. The results achieved by this 

 prolonged denudation are of the most stupendous kind. The 

 original lava-floor has been cut down into a fragmentary tableland. 

 Hundreds of feet of solid rock have been removed from its general 

 surface. Outliers of it may be seen scattered over the mountains 

 of Morven, whence they look into the heart of the Highlands. 

 Others cap the hills of Rum, where they face the open Atlantic. 

 Q.J.G.S. No. 206. 2e 



