40 Scientific Proceedings, Royal Dublin Society. 



of other rocks from the interior mountains. There is no mistaking 

 the direction of the ice movement, which has here been west- 

 ward ; but it is only in protected spots that boulder clay, or 

 moraine matter, is to be found. In general the rocks are bare to 

 a remarkable degree, the ice having apparently swept the loose 

 materials out into the ocean. 



On this day we passed underBen Stack (Plate X.,fig. 4), a pyramid 

 of dark Laurentian hornblendic gneiss, rising 2,364 feet above the 

 sea, and the highest elevation (as I was informed by Professor 

 Geikie) to which the Laurentian rocks rise on the mainland. We 

 were also close under the escarpments of Ben Arkle (Plate XI., fig. 8) 

 and Foinaven, the former 2, 576 feet, and the latter 3,016 feet. Th ese 

 are formed of Laurentian beds, capped by Lower Silurian 

 quartzite, — the whiteness of the latter contrasting with the dark 

 tints of the former. As already stated, the Cambrian sandstones 

 have been entirely denuded away over this part of the country 

 previous to the deposition of the Lower Silurian beds. The 

 section (Plate XI., fig. 8) may be compared with that across Quenaig 

 (Plate XL, fig. 9), in order that the changes in the stratification 

 and physical features may be better understood. 



It should be observed that throughout the district we examined, 

 the lower quartzite at the base of the Silurian series generally 

 crops out as an elevated escarpment, towards the south, capping 

 unconformably the Cambrian sandstone ; towards the north, the 

 Laurentian gneiss. As this escarpment crosses transversly the 

 general drainage of the country, and the course of those rivers which 

 rise in the interior and flow westward towards the ocean, it is 

 intersected by deep and wide valleys, so that it is exceedingly 

 broken, and the lines of clifi" run far up the valleys towards the 

 east. Coulmore, Canisp, Quenaig, Ben Arkle, and Ben Foinaven 

 arc all situated on the margin of the lower quartzite. 



On the other hand, the upper quartzite rises into equal, or still 

 higher elevations such as Ben Dearig, Ben More in Assynt, Ben 

 Hee, and Meal Horn, and generally forms the watershed of the 

 North Highlands. The Cambrian beds generally form isolated, 

 or partly detached masses. In no district I have ever visited do 

 the physical features give more clear expression to the geological 

 structure than in the North Highlands of Scotland. 



The Laurentian rocks extend all the way from Scourie to 

 Uhiconich and Laxford Bay, and from thence to Cape Wrath. 



