378 Reports and Proceedings — 



Hebrides." By James Geikie, Esq., F.E.S.E., F.G.S., of H.M 

 Geological Survey of Scotland. — First paper. 



The author commenced by describing the physical features of 

 Lewis, which he stated to be broken and mountainous in the south, 

 whilst the north might be described as a great peat moss rising 

 gradually to a height of about 400 feet, but with the rock breaking 

 through here and there, and sometimes reaching a higher elevation. 

 The north-east and north-west coasts are comparatively unbroken, 

 but south of Aird Laimisheadar in the west and Stornoway in the 

 east, many inlets run far into the country. The island contains a 

 great number of lakes of various sizes, which are most abundant in 

 the southern mountain tract and in the undulating ground at its 

 base. The greater part of Lewis consists of gneiss, the only other 

 rocks met with being granite and red sandstone, and conglomerate 

 of Cambrian age. The stratification of the gneissic rocks is generally 

 well marked ; the prevalent strike is N.E. and S.W. with S.E. dip, 

 generally at a high angle. The author described in considerable 

 detail the traces of glaciation observed in the lower northern part of 

 Lewis, and inferred from his observations that the ice passed from 

 sea to sea across the whole breadth of this district, and that it not 

 only did not come from the mountainous tract to the south, but must 

 have been of sufficient thickness to keep on its course towards the 

 north-west undisturbed by the pressure of the glacier masses which 

 must at the same time have filled the glens and valleys of that 

 mountain region. After describing the characters presented by the 

 bottom-till in the northern part of Lewis, the author proceeded to 

 notice those of the lakes, some of which trend north-west and south- 

 east, others north-east and south-west, whilst those of the mountain 

 district follow no particular direction. The lake-basins of the first 

 series he regarded as formed at the same time and by the same 

 agency as the rocJies moutonnees and other marks of glacial action ; 

 they are true rock-basins or hollows between parallel banks formed 

 wholly of till, or of till and rock. The N.E. and S.W. lakes coin- 

 cide in direction precisely with the strike of the gneiss ; and the 

 author explained their origin by the deposition of till by the land-ice 

 in passing over the escarpments of the gneiss facing the north-west. 

 The lakes of the mountain district are regarded by the author as all 

 produced by glacial erosion. The author considered that the ice 

 which passed over the northern part of Lewis could only have come 

 from the main land. Eeferring to the glaciation of Eaasay, he 

 showed that the ice-sheet which effected it must have had in the 

 Inner Sound a depth of at least 2700 feet, and taking this as ap- 

 proximately the thickness of the mer de glace, which flowed into the 

 Minch, which is only between 60 and 60 fathoms in depth, no part 

 of this ice could have floated, and the mass must have pressed on 

 over the sea-bottom just as if it had been a land surface. Ice 

 coming from Sutherland must have prevented the flow of the Eoss- 

 shire ice through the Minch into the North Atlantic, and forced it 

 over the low northern part of Lewis ; and the height to which Lewis 

 has been glaciated seems to show that the great ice-sheet continued 



