On the Geological Structure of the Highlands of Scotland. S9 



gentle slope across the truncated horizontal beds of Cambrian 

 sandstone till the latter is cut out altogether, and the base of the 

 Lower Silurian series rests directly upon the Laurentian gneiss.* 

 The section (Plate XI., fig. 9 )taken across Quenaig, on the north shore 

 of Loch Assynt, shows a similar structure amongst the formations. 



This disappearance of the Cambrian beds as we proceed north- 

 wards is a remarkable feature in the physical geology of the 

 district. The abrupt truncation of the sandstone at the edge of the 

 Silurian quartzite, leaves no doubt that the formation was enor- 

 mously denuded, even before the Lower Silurian beds began to be 

 formed. At that period, however, the beds were tilted towards 

 the west. Their present horizontal position being in consequence 

 of the tilting towards the east of the Silurian beds. 



On approaching the deep inlet called Loch Dhu (or Dow) and 

 the ferry of Kylesku, we passed near the base of the grand 

 precipices along which Quenaig terminates towards the north. 

 The bold bluffs of Cambrian sandstone rise about 2,000 feet, and 

 form certainly the noblest cliffs I had seen (Plate IX., fig. 3.) Not- 

 withstanding the great thickness of the formation (about 2,000 feet) 

 in Quenaig, the whole had actually disappeared under our feet; and 

 we could trace the Silurian quartzite margin resting directly upon 

 the dark hornblendic schists of the Laurentian formation which 

 extends from the base of Quenaig to the coast, and to the shores of 

 Loch Dhu. 



From Kylesku to Scourie our course lay over a tract of 

 broken and comparatively featureless country, highly gla- 

 ciated by an ice-sheet which had moved towards the Atlantic. 

 The rocks belong to the Upper Laurentian series, are chiefly 

 hornblendic, highly crystalline in structure, schistose, and 

 gneissose; occasionally schist of bronze mica was observed. 

 Quartz veins are of frequent occurrence, and the beds are slightly 

 crumpled. These rocks we had a good opportunity of examining 

 during our last excursion northwards, on the following day, when 

 we drove from Scourie to Rhiconich, a distance of twelve miles, 

 the whole over Laurentian rocks. 



Our course lay over an exceedingly rugged country, with 



numerous little lochs, rock-basins, gullies, and deep ravines. 



Everywhere the rocks were exposed to view, presenting remarkably 



ice-worn surfaces, often strewn with boulders of quartzite and 



* The scenery at the head of Loch Assynt forms the Frontispiece to " Siluria " 4 Edit. 



