﻿1861.] 



MURCHISOX AND GEIKIE — HIGHLANDS. 



207 



through Ross-shire into the Isle of Skye,— that they are covered by 

 a vast series of micaceous flaggy or gneissose schists, — that these 

 are disposed as a great synclinal trough, the centre of which tra- 

 verses the head of Glen Shiel, the middle of Loch Quoich, and the 

 watershed at Glen Finnan, — and that, by the curving of this trough, 

 the quartzose beds which form its outer or lower edge along the 

 western coast at Arisaig are brought up again along the line of the 

 Great Glen. 



"We shall now endeavour to describe the anticlinal arch of the 

 Great Glen, and to point out how the same strata undulate to the 

 eastward of that arch to form the remainder, or southern portion, 

 of the Scottish Highlands. 



The remarkable chain of lakes which extends from Inverness to 

 Fort William presents, even on the roughest map, unmistakeable 

 evidence of a line of fracture in the earth's crust. Loch Ness, 

 deeper by many fathoms than the German Ocean, showed, in the 

 sympathetic movement of its waters during the Lisbon earthquake, 

 the depth and extent of the fissure which it occupies. So marked a 

 line might justly be supposed to indicate a great displacement of the 

 rocks. And yet, though the fracture is probably more extensive than 

 any other in the country, it has not been attended, so far at least 

 as we have yet been able to ascertain, with any marked upheaval 

 or depression of the rocks on either side. We at present regard it 

 as a fracture without a throw, or, at least, with such a throw as not 

 at all to interfere with the regularity and perspicuity of the section. 



The line of this chain of lakes, or the Great Glen, as it is properly 

 called, runs along the line of an anticlinal axis. West of the lakes 

 the strata (as we have seen along the banks of Loch Eil) have a 

 north-westerly dip ; east of this line they incline to the south-east. 

 Moreover, it would seem that this axis lessens in intensity towards 

 the north and increases towards the south ; in other words, the 

 southern prolongation of the arch brings up lower and lower beds, 

 while in its northern extension it appears to be dying away. As 

 we trace it southward from the grey quartzose beds last described at 

 Loch Eil, we gradually re- encounter the whole of the Sutherland 

 and Ross-shire succession of quartz-rocks and limestones, and ob- 

 tain thus additional aid in correlating the crystalline rocks of the 

 North-western Highlands with those of the central counties. 



Islay and Jura. — By much the clearest and most complete series 

 of these repeated strata occurs in the Islands of Islay and Jura, 

 whence they range north-eastward up the Linnhe Loch, and south- 

 westward into Ireland. The annexed figure represents the struc- 

 ture of Islay (fig. 20). 



The rocks which form the N.W. promontory of the island at 

 Sanaig were examined by us under the disadvantage of a storm of 

 wind and rain. We ascertained, however, that they consist of clay- 

 slate, which we believed to dip below the mica-schists and grit- 

 bands of Sanaig-farm. They are well exposed on the shore, first 

 with a south-east dip, which soon turns to the north-west, with 

 which inclination they appear to plunge below the waters of the 



