﻿Vol. 66.'] SCHISTS OF THE SCOTTISH HIGHLANDS. 601 



by no less a thickness of flags which have been described already 

 as belonging to the Eilde Group (9) : see PI. XLI1I, Sections 

 A&D. 



In the Onich district, where the core is deep enough to include 

 all groups from 2 to 6, it might be argued that the flags on the 

 north-west are merely the disguised representatives of the Leven 

 Schists, emerging from beneath the syncline with a new facies ; but 

 on the slopes above Glen Nevis no such explanation can be main- 

 tained, since the core here contains only the Ballachulish Limestone, 

 and yet the contrast between the Leven Schists on the one side and 

 the flaggy series on the other is as marked as ever. 



We are thus driven to postulate a fold-fault or slide as the north- 

 western boundary of the Appin Core between Port William and 

 Onich — a slide which cuts out the Leven Schists (7) and Glen Coe 

 Quartzite (8), and brings the Eilde Plags (9) into conjunction with the 

 Ballachulish Limestone (6). This inference is greatly strengthened 

 by a detailed examination of the junction-line. It is found that 



(1) the Leven Schists and Glen Coe Quartzite are not always entirely- 

 missing, for they are locally represented in a very attenuated, or, aa 

 Swiss geologists say, a ' laminated ' form ; and 



(2) the Ballachulish Limestone itself, about 3 miles north of Onieh, suffers 



a similar attenuation (PI. XLIII, Section D), and for a space is 

 entirely absent. 



The type section of the Fort W'illiam Slide is afforded by the 

 river that drains westwards from the Lairigmor Valley. The slide 

 is twice exposed in the bed of this river, owing to normal faulting; 

 but in the down-stream exposure the limestone is only just seen 

 before it is cut out by the fault which is responsible for the 

 repetition of the section. 



Both Leven Schists and Glen Coe Quartzite, with a thickness of 

 about 10 or 20 feet each, are here represented in their natural 

 position between the limestone and the flags. The quartzite has a 

 more flaggy structure than usual, and minute examination reveals 

 three or four instances of very gradual transgression of one division- 

 plane across another. But, although the whole of the attenuated 

 quartzite, from its banded edge against the Leven Schists to its 

 contact with the Eilde Plags, is laid bare in the river, there is no 

 plane marked out by special signs of shearing, much less by any 

 overt evidence of disruption. 



It is, in fact, a feature, not only of the Port William Slide, but 

 also of by far the greater number of the slides of this district as a 

 whole, that no one could possibly suspect their importance from 

 a mere examination of the slide-plane itself. It seems probable, 

 too, that in almost every case sliding has not been confined to a 

 single isolated plane, but rather has been distributed over a host of 

 close-set parallel planes ; doubtless, too, sliding has been associated, 

 to an important extent, with phenomena of plastic deformation. 

 In this way we can account for the simultaneous thinning of 

 neighbouring groups of rocks in connexion with these slides — a 

 phenomenon often observed. 



