416 EECENT WOEK OF THE GEOLOGICAL SrKVEY 



Archaean gneiss at the hase and in the heart of the mass, is made to 

 overlie these displaced materials. Within a short distance this belt 

 of green schist and sheared gneiss is succeeded by the Moine Thrust, 

 ushering in the micaceous and quartzose flagstones belonging to that 

 horizon. 



Section south of Loch More (fig. 10). — On the ridge south of Loch 

 More Lodge the Archaean gneiss with the basic clyhes is covered by 

 the basal quartzites and the " pipe-rock" in natural order, the latter 

 being traversed by a powerful thrust. The " pipe-rock " and " Tu- 

 coid-beds " are eventually truncated by a major thrust-plane, along 

 which the Fucoid-beds. Serpulite-grit and limestones, piled on each 

 other by reversed faults, have been driven, till they are overlapped 

 by a maximum thrust bringing up the Archaean gneiss of a red, 

 massive, granitoid type. These recognizable Archaean rocks are 

 abruptly truncated by a second maximum thrust ushering in recon- 

 structed gneiss and schists, the prominent planes, dipping E.S.E., 

 having been determined by the Post-Lower-Silurian movements. 

 The general colour of the rock is light or dark grey, forming a marked 

 contrast to the underlying red, granitoid gneiss. As the incli- 

 nation of the thrust-plane is variable, numerous outliers of the 

 reconstructed gneiss and schist have been formed. In the heart of 

 the main mass, however, there is a patch of Silurian limestone be- 

 longing to the Eilean Dubh and Ghrudaidh groups. The occurrence 

 of this large mass of Silurian limestone is of special importance, as 

 it shows how calcareous bands are formed among the eastern schists, 

 their geological relations being determined by mechanical movement. 

 Advancing still further eastwards, this belt of crushed and recon- 

 structed gneiss and schist is buried underneath the dark micaceous 

 flagstones lying above the Moine Thrust-plane. 



Along this line of section, the geological relations of the strata 

 are somewhat different from the foregoing. Towards the southern 

 margin of Loch an Leathaid Bhuain, the basal quartzites rest uncon- 

 formably on the Archaean gneiss, followed by the " pipe-rock," " Fu- 

 coid-beds," and Serpulite-grit. These are traversed by a major thrust- 

 plane, along which the " Fucoid-beds," Serpulite-grit, and basal lime- 

 stones have been piled up by minor thrusts. To these succeed the 

 Glencoul Thrust-plane, above which there is a great slice of Archaean 

 rocks with no basic dykes. This mass has been laid bare for a distance 

 of nearly two miles, from Ben a Ghrianain to Ben a Bhutha ; but to 

 the east of the latter hill there is a splendid development of Silurian 

 rocks resting naturally on the slice of the old Archaean platform. 

 All the Silurian zones, from the basal quartzites up to the horizon 

 of the Eilean Dubh limestones, are met with, thrown into a series of 

 inverted synclinal folds. On the hill to the west of Lochan nan Eala- 

 chan, a small patch of the old Archaean gneiss has been exposed on 

 the crest of the arch by the denudation of the basal quartzites. Ad- 

 vancing eastwards, the Silurian zones are met with up to the horizon 

 of the limestone, the latter being truncated by the maximum thrust 

 bringing in the sheared gneiss and green schist ; and within a short 

 distance the micaceous flagstones above the Moine Thrust-plane 

 appear on the slopes of Ben Lice. 



