Vol. 65.] THE CAULDRON-SUBSIDENCE OF GLEN COE. 657 



The fault is splendidly exposed in the precipitous western slopes 

 of Meall Dearg. It overhangs just as conspicuously here as it does 

 on the other side of the ridge, and the lavas in its vicinity are 

 steeply reversed ; and in one locality, as already described, the basal 

 conglomerates and the old schist-floor are also brought to light in a 

 similar inverted position. 



In Coire Cam the fault performs the most striking of its many 

 changes of direction ; whereas it descends into the corrie from 

 Meall Dearg with an east-and-wesfc trend and a northerly inclination 

 of about 50°, it ascends Aonach Eagach with a south-west strike, 

 and so steeply inclined as to be practically vertical. The precise 

 position of this sudden bend is unfortunately not exposed. 



North of Aonach Eagach the fault which now runs south-west- 

 wards is marked by shattering, and also by the omission of the 

 basement conglomerate of the Volcanic Series ; the lavas are 

 here thrown directly against the quartzite outside, and along the 

 summit of Aonach Eagach they assume a vertical position near 

 the disturbance. South of this ridge the fault is equally clear, for 

 it throws phyllites, to the east, against quartzites, to the west. 

 At intervals along the line from Coire Cam to the bottom of 

 Glen Coe, the fault has been followed by a dyke of xenolithic 

 rhyolite, sometimes as much as 50 yards in width. This greatly 

 resembles in appearance many of the lavas of the sunken area, 

 and is earlier than the porphyrite dykes with which it comes into 

 contact; its age, in relation to the fault-intrusion, however, has 

 not been determined. 



The manner in which the zone infested by the fault-intrusion 

 follows the same abrupt bend as the fault itself, is best appreciated 

 by a glance at the general map. The alteration due to this suite 

 of intrusions is almost completely limited to the outside of the 

 fault-line. Both these points have been dealt with already, and 

 so we are now free to pass on to the consideration of another 

 aspect of the mode of occurrence of the fault-intrusion, which has 

 not yet received attention. Many of the minor masses of this 

 intrusion are obviously laccolitic in nature. In fact, a good 

 example of a small laccolite with neatly arched roof is a conspicuous 

 feature on the northern slope of Glen Coe, above Clachaig. Erom 

 this it is probable that some of the larger masses may also be of 

 laccolitic form. It can at any rate be shown, conclusively enough, 

 that the large body of porphyrite forming the Meall Dearg ridge 

 has a comparatively flat and even base. Away from the fault the 

 margin of this intrusive mass follows an approximately level course 

 on both sides of the ridge. The main outcrop of the porphyrite 

 stretches in this manner for a mile north-east from the fault; but, 

 about half way along its course, it is completely severed by an 

 important felsite dyke, although, strange to say, the latter is the 

 older rock : in fact, the fault-porphyrite is clearly chilled, at one 

 point, against the felsite dyke which interrupts its continuity 

 (fig. 11, p. 658). 



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