174 ME. E. B. BAILEY AND MR. M. MACGREGOR ON" [June I912, 



he was the first to note its presence, it contains verj- little tremolite 

 indeed, and is most fitly described as a grey, highly calcareous 

 mica-schist, with occasional seams of cream-coloured limestone. 

 The next good exposures of the limestone are met with in Glen 

 Orchy, near a sheepfold close to the road and in several streams 

 draining the ridge on the north-west side of the valley ; in fact, 

 Mr. J. B. Hill shows it on the published 1-inch map, Sheet 45, 

 near the summit of this ridge, and has also noted its presence in 

 his field-maps — along a tributar}^ stream flowing in the opposite 

 direction into Glen Strae. In these exposures it is sometimes about 

 40 feet thick, and is a cream-coloured limestone, sandy and mica- 

 ceous, and, so far as has been observed, free from tremolite. On 

 the west of Glen Strae it has been traced in clear exposures for 

 half a mile from the margin of the Etive Granite towards Loch Awe ; 

 it is here 20 or 30 feet thick, and is in the condition of calc-silicate- 

 hornfels owing to the proximity of the Etive Granite. It is 

 similarly affected by the granite where it reappears in Allt Coire 

 an Easain and in the Windows of Etive. 



In these two last-mentioned exposures it has been proved that 

 the limestone belongs to the Ballachulish Limestone Group ; and 

 that it is underlain by a fold-fault, known as the Ballachulish Slide, 

 which cuts out the thick politic portion of the Leven Schists that 

 one might expect to find between this limestone and the Banded 

 Series below [1, pp. 607 & 609]. As the condition of affairs 

 characteristic of these north-eastern sections continues throughout 

 the region now under consideration, it seems necessary to suppose 

 that here also the Ballachulish Limestone rests upon the Balla- 

 chulish Slide ; and incidentally we are led to postulate a displace- 

 ment of 24 miles, or more, along this slide-plane. It is possible 

 that the marked transgression of the limestone, which brings it to 

 rest directly on Eilde Elags north of Beiun Doirean, is due to the 

 Ballachulish Slide ; but this is a point which has not been closely 

 investigated in the field. 



{d) The grey politic mica-schist which follows above the 

 limestone is a most useful group, from the map-maker's point of 

 view, on account of its being very thick and therefore conspicuous. 

 At the northern extremity of the Beinn Doirean range it contains 

 a considerable proportion of quartzose material ; but in Beinn 

 Achallader, Beinn Doirean, and Beinn a' Chaisteil it is very free 

 from such intercalations, and is a massive garnetiferous mica-schist 

 of the type that occurs in the heart of the Beinn Udlaidh Eold, 

 In some sections this massive mica-schist starts immediately on the 

 top of the limestone ; in others it contains a band or so of quartzite 

 interbedded near its base. South of Glen Lochy the mica-schist 

 is again garnetiferous, and contains quartzite bands — some of which 

 lie a little above the limestone. In the Glen Orchy exposures 

 and in the outcrop near Loch Awe, west of the Glen Strae Eault, 

 bands of quartzite are found in the mica-schist near the limestone ; 

 and above this comes massive grey politic schist, very seldom 



