532 Mr. Archibald Geikie [June 7, 



probability that such a connection with the surface was really esta- 

 blished. In some of the conglomerates of the next succeeding group 

 (Cambrian or Torridon sandstones), there occur fragments of highly 

 vesicular lavas, which show that at some time previous to the deposit 

 of these coarse sediments, active volcanic vents existed somewhere in 

 the region of the north-west of Scotland. As yet, however, no trace 

 has been discovered of any of the lava-streams which flowed out at 

 the surface. 



Although volcanic energy has long been quiescent over the 

 British Isles, probably no area in Europe exhibits within so 

 limited a space so long and varied a record of volcanic eruptions. 

 There is, therefore, a peculiar interest about these traces of the 

 ancient volcanoes which in Archaean time rose along the Atlantic 

 border in the north-west of Scotland, for they stand at the very be- 

 ginning of that long history. Moreover, so far as we can interpret 

 their remains, they seem in a curious way to have anticipated the 

 characteristics of the last great volcanic episode in Britain — that to 

 which we owe the Tertiary basaltic j)lateaux of Antrim and the Inner 

 Hebrides. In both cases, the distinguishing feature was the Assuring 

 of the terrestrial crust and the uprise of basic lava in the rents, with 

 the consequent production of innumerable j^arallel dykes trending in 

 a general north-westerly direction. 



In the third place, after the production of the basic dykes, there 

 came another prolonged interval, during which a series of remarkable 

 terrestrial disturbances affected the north-west of Scotland. The 

 crust of the earth in that part of Europe was once more dislocated by 

 innumerable fissures, produced probably at successive epochs of 

 paroxysm, for they can be grouped into three distinct series. Of 

 these, one runs approximately parallel with the north-west dykes, the 

 second trends east and west, and the third runs north-east and south- 

 west, or north and south. So far as yet discovered, no lava of any 

 kind welled upwards into these fissures. They are ruptures, but not 

 dykes. They were accompanied, however, by the manifestation of 

 another form of terrestrial energy, the geological efficacy of which 

 has only recently been recognised. The lines of vertical fracture 

 became also lines of horizontal or oblique movement during the vast 

 strain of terrestrial contraction. One side was driven past the other 

 side, and with such irresistible force that the rocks for some distance 

 on either side were dragged into the line of movement, crushed down, 

 and forced to assume a new crystalline arrangement of their materials. 

 The basalt dykes, reduced sometimes from a width of 50 or GO yards 

 to only four feet or less, were changed into diorites, and where the 

 shearing was greatest, into hornblende-schists. The gneiss, in like 

 manner, was thrown into sharp folds, and had a newer foliation 

 developed in it parallel with the new planes of movement. 



In the fourth place, during the prolonged succession of changes 

 which I have thus briefly summarised, there must have been in pro- 

 gress a continuous denudation of the surface of the Archaean laud in 



