36 Scientific Proceedings, Royal Dublin Society. 



Lawrentian Beds. 



(a.) Upper part; hoi'nblende schist, hornblende gneiss, and 

 hornblende rock, sometimes micaceous, and pene- 

 trated by quartz veins, ..... 



(h.) Lower part ; reddish gneiss, penetrated by veins of 

 granite or " pegmatite," ..... 



Of great but 

 uncertain 

 thickness 



(over 20,000 

 feet.) 



The above may be regarded as the general succession of the 

 rocks about Loch Assynt, and as far as Loch Broom towards 

 the south, and Loch Laxford towards the north. 



It need hardly be stated that within this compass the beds 

 vary much in thickness ; — but not in character. 



Throughout this region, and as far as the shores of Loch Dow, 

 the Cambrian beds often rise into isolated precipitous hills 

 or break off in grand escarpments and precipices, formed of nearly 

 horizontal beds, rising tier above tier to elevations of 3,000 feet 

 or upwards. Amongst the most remarkable is the north shoulder 

 of Quenaig (Plate IX., fig. 3), which rises in the form of a great 

 terraced buttress of red sandstone, its sides cut into deep gullies 

 by mountain torrents, and accessible only by stiff climbing. The 

 western shoulder of Suilven, as seen from the banks of the ferry at 

 Kylesku, the remarkable escarpment of Benmore Coygeagh, (Plate 

 IX., fig. 2), and the isolated truncated pyramid of The Stack (Plate 

 IX., fig. 1), are all instances of the results of denudation, acting 

 upon masses of horizontally bedded sandstone, in producing bold 

 and massive scenery — differing in character from that of any of 

 the formations either above or below. 



On reaching the Inn of Aultguish we came in sight of the 

 mountains of Cambrian sandstone, at a distance of about ten or 

 twelve miles to the westward, seen through a gap in the schis- 

 tose rocks, and rising above Loch Broom in massive terraces ; the 

 horizontal stratification being clearly discernible by the aid of 

 the binocular glass. The winter's snows still lingered on the 

 summits and on the surfaces of the upper terraces. In front of 

 them were the quartzite mountains of Ben Dearig and the neigh- 

 bouring heights, generally capped with snow, which rise in bold 

 rounded masses to an elevation of 3,551 feet. 



On descending towards the head of Loch Broom, along 

 the valley of Braemore, we visited one of the most re- 

 markable river gorges in the British Isles — a veritable 



