424 



RECENT WORK OF THE GEOLOGICAL SURVEY 



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of the hill, the " pipe-rock " appears 

 with an inverted dip, plunging under- 

 neath the basal quartzites, the beds being 

 inclined to the E.S.E. at tolerably high 

 angles. jN'ear the hill-top the uncon- 

 formable junction of the basal quartzites 

 on the Cambrian sandstones is exposed, 

 and when followed southwards the false- 

 hedded quartzites pass transgressively 

 across the Cambrian strata and rest 

 directly on the Archcean gneiss. Only a 

 small exposure of the gneiss is met 

 with, but it presents the normal charac- 

 teristics and contains one of the basic 

 dykes. 



This remarkable outlier is about a 

 mile in length, from north to south, the 

 outcrop of the thrust-plane forming a 

 striking feature round the hill. The 

 limestones are exposed in the various 

 swallow-holes adjoining the plane, and 

 the actual disruption-line is seen in 

 dip-section on the north-west shoulder 

 of the ridge. The AUt nan Uamh has 

 carved a deep channel through the 

 underlying limestones and has isolated 

 the outlier on Beinn Phuarain from that 

 on Beinn nan Cnaimhseag on the north 

 side of the stream. In the case of the 

 latter, the Archaean rocks are not ex- 

 posed, but the basal quartzites rest un- 

 conformably on the Cambrian sand- 

 stones. By means of these and other 

 outliers, originally continuous with the 

 strata lying above the main outcrop on 

 Sgonnan ITore, we can form a clear 

 conception of the enormous extent of 

 these terrestrial displacements and also 

 of the vast amount of denudation which 

 has since taken place. 



Only a brief reference can be made 

 to the large number of thrust-planes 

 repeating the Silurian strata, with occa- 

 sional wedges of Archaean rocks, to the 

 east of the Ben-^Iore Thrust-plane. 

 The alteration produced by these move- 

 ments in advance of the Moine Thrust- 

 plane is remarkable. So striking is the 

 change, as will be shown presently, that 

 Mr. Callaway grouped the Loch-Ailsh 



