IN- THE NORTH-WEST HIGHLANDS OF SCOTLAND. 



427 



basal qiiartzites (fig. 21). 

 There can be little doubt, 

 from the evidence obtained 

 at Am Pollan (Loch Uri- 

 gill), in the valley of the 

 Achall, and at Ullapool, 

 that this mass of Archaean 

 rocks and basal quartzite 

 has been borne along by 

 the Ben More Thrust, and 

 that the materials rest on 

 the piled-up Silurian strata 

 underneath. 



Passing southwards to 

 the Achall valley, there 

 is a regular ascending 

 series from the basal quart- 

 zites, resting unconform- 

 ably on the Cambrian 

 sandstones, up to the Ser- 

 pulite-grit and a portion 

 of the basal limestone. 

 The latter zones are 

 abruptly truncated by a 

 powerful major thrust 

 driving forward nearly all 

 the zones of the Ghrudaidh 

 and Eilean Dubh lime- 

 stone-groups, repeated by 

 numerous reversed faults. 

 The limestones are over- 

 lapped in turn by the 

 materials lying above the 

 Ben-More Thrust-plane, 

 consisting of a great deve- 

 lopment of Archaean gneiss 

 with the basic dykes 

 covered unconforraably by 

 the Cambrian conglome- 

 rate and sandstones, with 

 the basal quartzites resting 

 unconformably on the 

 latter. The serpentine re- 

 ferred to by Prof. Mcol 

 as occurring in the Achall 

 valley is one of the ultra- 

 basic dykes in the thrust 

 Archaean gneiss. The Ben- 

 More Thrust-plane in the 

 Achall valley is inclined to 



