IN THE NOETH-WEST HIGHLANDS OF SCOTLAND. 



423 



thrust-plane, dipping underneath the Archaean gneiss in inverted 

 order (see section), as previously described by Mr. Callaway. There 

 can be no doubt, however, that this strip of Cambrian strata, 

 extending nearly two miles down the valley, is merely a continuation 

 of the mass on Coinne-mheall, as shown in section. 



Pig. 17. — Horizontal Section from the OyJcel Valley 

 across Ben More. (About 2 miles in length.) 



Ben More. 



The slice of Archaean rocks bearing these Cambrian sandstones 

 and Silurian strata, driven westwards by the Ben-More Thrust, is 

 of large dimensions. The Archaean gneiss and basic dykes form a 

 grand cliff about 1500 feet high overlooking Dubh Loch More, and 

 they sweep across the lofty peaks separating the Oykel from the 

 Gorm Lochs. Though still recognizable as a part of the old Archaean 

 platform, the rocks have undergone important changes due to the 

 movements, as will be shown further on. 



The outcrop of this great thrust-plane can be followed southwards 

 from the Oykel valley, round the western slope of Sgonnan More, 

 by Strathsheaskich, thence to Allt an Loin Dubh, curving round 

 Cnoc na Glas Choille to the base of the Cromalt Hills, w^here it is 

 overlapped by the Moine Thrust-plane. 



Of special interest, however, are the numerous outliers of the 

 materials lying above this thrust-plane, left by denudation in the 

 most extraordinary situations. The most important occur on Cnoc 

 an Leathaid Bhuidhe west of Loch Awe, on the moor south of Loch 

 Urigill, and on the limestone plateau south-east of Stronechrubie. 

 The two isolated masses, resting on the limestones to the north and 

 south of Allt nan Uamh, are of great importance, as they show the 

 original folding of the Ben-More Thrust-plane over the quartzite 

 range of Braebag, and the overlap of the Glencoul Thrust-plane by 

 the former (see fig. 3 8). 



Descending the slopes of Canisp the double unconformability of 

 the quartzites on the Cambrian sandstones and Archaean rocks is 

 well exposed, the lower zone being overlain by the " pipe-rock " in 

 natural sequence. Crossing the Loanan, the " pipe-rock " is followed 

 by the " Pucoid-beds," Serpulite-grit, and basal limestone, the latter 

 three zones being repeated by reversed faults. Ascending the west 

 declivity of Beinn an Fhuarain, the two lowest groups of limestone 

 are repeated by minor thrusts till we reach the outcrop of the Ben- 

 More Thrust-plane. Just above the plane, on the north-west face 



