Vol. 65.] THE CAULDKON-SUBSIDENCE OF GLEN COE. 629 



from the horizontal ; but along the opposite margin it inclines 

 towards the subsidence (PI. XXXIII). The fault-planes on either 

 side are thus roughly parallel, and one may imagine that the 

 cylindrical downthrown mass originally had perpendicular walls, 

 and then was tilted bodily into its present inclined position. In 

 support of this hypothesis we may quote the prevalent southerly dip 

 of the lavas in the heart of the cauldron. At the same time, 

 the outer branch of the fault on the slopes of Beiun Ceitlein 

 has a markedly reversed hade ; and, as it is quite impossible in 

 this case to explain the phenomenon on any hypothesis of later 

 tilting, it appears not improbable that the local reversal of hade 

 elsewhere may also be an original feature. The frequent sharp 

 flexuring, and even inversion, of the lavas in the vicinity of the 

 fault shows that the interior mass yielded as it sank, and it is 

 quite conceivable that this permitted an over-riding of the sub- 

 siding area, to use an expression introduced by Suess. 



(4) Mechanical evidence of faulting. — At a few favoured 

 points along the line of the fault there is clear evidence of intense 

 shearing, leading finally to the production of ' flinty crush-rock.' 



The macroscopic characteristics of this rock are flinty lustre, dark 

 colour, frequently conspicuous flow-banding, and an almost invariable 

 connexion in the field with obvious signs of shearing. tinder the 

 microscope the banded structure is often strongly pronounced, and 

 the rocks themselves are found to be richly charged with fragments 

 derived from the adjacent shear-zone, fragments which are as a 

 rule in a mylonized, crushed, or disintegrated condition. In the 

 Glen Coe district, in cases where flinty crush-rock has been produced 

 from quartzite or quartz-schist, a large proportion of the rock con- 

 sists of separate glistening grains of quartz, which can be detected 

 on a fractured surface even without the aid of a pocket-lens. 



Since flinty crush-rock plays an important part in the geology of 

 Glen Coe, we may indicate briefly the main points in regard to its 

 mode of occurrence in certain other localities. Flinty crush-rocks 

 have been described from the Cheviot granite of Lower Old Red 

 Sandstone age, and also from certain pre-Torridouian lines of move- 

 ment in the North-West Highlands. 1 " In the latter region, besides 

 the black flinty crush-rocks, which are in obvious genetic connexion 

 with planes of dislocation, there are found occasional 



' dark-brown, grey or black strings, rarely more than an inch thick, which do 

 not displace the' folia crossed by them. These strings sometimes bulge out 

 in rounded projections, or end bluntly and look like intrusive felsites. They 

 are, however, confined to zones which have been crushed, and it seems probable 

 that many of them are isolated on all sides by the adjoining rocks. It is 

 difficult to see how they can be intrusions of true igneous rock. Perhaps 

 by the intensity of the crushing near them sufficient heat may have been 

 generated to fuse small portions of the rock.' 



1 C. T. Clough, ' The Geology of the Cheviot Hills ' Mem. Geol. Surv. 

 1888, p. 22 ; 0. T. Clough & E. Greenly, in ' The Geological Structure of the 

 North- We^t Highlands of Scotland' Mem. Geol. Surv. 1907, pp. 249-50. 



