Vol. 65.] THE CAULDRON-SUBSIDENCE OP GLEN COE. 625 



(1) Distribution of the Volcanic Series in relation 

 to the faulting. — The Volcanic Group of Glen Coe occupies a 

 compact area with a simple outline, as shown on the general map 

 (PI. XXXIV). In many cases the lavas abut against the steep 

 even plane of the bounding fault, and there stop abruptly. Such 

 a relation is especially well seen between An t-Sron and Dalness 

 (PI. XXXIII, Sections II & IV). 



There is usually also a marginal inward tilt of the volcanic rocks, 

 which is evidently the result of the subsidence, and more or less 

 clearly determined by proximity to the fault-line ; the strike of the 

 tilted rocks is everywhere in close agreement with that of the fault 

 in the immediate vicinity. 



This marginal tilting, to which reference has been made more 

 than once in the preceding descriptions, is most conspicuously 

 exhibited in the western portion of Bidean nam Bian. Here the 

 volcanic rocks, with one or two thin intercalations of shale, are 

 thrown into a vertical position and finally overturned, so that in 

 the peak west of Stob Coire nan Lochan the andesites of group 1 

 overlie the rhyolites of group 2 (PI. XXXIII, Section IV). The 

 diversity of the volcanic sequence brings out this structure very 

 clearly, and under favourable conditions of light it can be detected 

 even from the Glen Coe road (PI. XXXII). 



The disturbance just described continues for some distance to the 

 south, as may be seen from the manner in which the outcrops of the 

 various groups cross the contours. In Beinn Ceitlein too the rhyolitic 

 rocks appear to assume a vertical position near the fault, although 

 a few hundred yards away their inclination is quite low. 



North of Bidean nam Bian, one may notice the steep easterly 

 or south-easterly dip of the volcanic rocks, which is prominently 

 displayed where the base-line of the group leaving the fault, swings 

 across Glen Coe above Loch Achtriochtan. 



In Aonach Eagach the lavas again assume a vertical position 

 where they once more come into contact with the fault which here, 

 among other things, cuts out the basement conglomerate of the 

 series (fig. 11, p. 658). 



In Coire Cam, upon the other side of Aonach Eagach, the 

 Volcanic Series is suddenly overturned in the immediate proximity 

 of the fault. The andesites at one point plunge steeply beneath 

 the basal conglomerates, which, with their associated grits and 

 shales, dip at about; 70° under a small inverted patch of the schist 

 floor (figs. 11 & 12, pp. 658 & 659). By tracing the reddened tops 

 of successive lavas in the cliff-face of this corrie, it is possible 

 to show that the inversion just described is a sharp flexure confined 

 to the vicinity of the fault. Beyond this the beds dip steeply away 

 from the dislocation, the position of which is marked in Meall 

 Dearg, as at so many other places, by the fault-intrusion in the 

 manner to be described later (p. 656). 



The overturned base of the volcanic group is seen, but not very 

 clearly, in Coire nan Lab to the east (fig. 11, p. 658). 



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