656 MESSES. CL0TTGH, MAETE, AND BAILEY ON [Xov. I909 



along its outer margin by a dyke of typical fault-porphyrite. On 

 the lower slopes at this point it is flanked on the north by a 

 branch fault (F 2 ), accompanied in turn by a grey porphyrite. This 

 rock is very similar to, but scarcely identical with, tbe early 

 fault-intrusion already described farther to the north. Like the 

 latter it has suffered locally from shearing, and near the top of 

 the slope it is cut across obliquely by the fault-intrusion proper. 

 A sheared felsite (not shown in the section) occupies the angle 

 between the two intrusions ; probably it is one of the early felsite 

 dykes already referred to. Farther down the slope of the hill a 

 felsite or rhyolite dyke, which may, however, be of later date, 

 appears to accompany the inner fault-line. There is perhaps no more 

 complicated bit of ground in the district. At the same time, the 

 lesson to be learnt from it is the old one of repeated movement 

 and intrusion along the margin of the Glen Coe subsidence. 



Standing upon the ridge between Coire Odhar-mhor and Coire 

 Mhorair, the observer may at first be bewildered by the profusion 

 of later north-north-easterly porphyrite dykes. But the fault- 

 porphyrite itself is quite distinct, both in character and in texture,, 

 from these intrusions ; and its outcrop, running at right angles to 

 their direction, is a clear indication of the position of the fault, 

 which on the slopes beyond is again marked by later shattering. 

 The difference between the Schists on each side of the fault-line, 

 exposed on this ridge and in the corries to the north and south, is 

 very striking : inside the fault, pure white quartzites dip steeply 

 beneath thin phyllites and black slates, and these under calcareous 

 schists and limestones ; outside all is quartzite or quartzitic schist. 

 The strike is different in the two cases, but what first attracts 

 attention is the contrast in the condition of the rocks on each side 

 of the disturbance. The bedding of the Schists lying to the north 

 is often obscure, and local reddening is sometimes extremely well 

 marked. In addition to this, as already stated, the Schists are 

 crossed by abundant irregular intrusions of pink felsite, which are 

 quite unknown on the downthrow side of the fault, either in the 

 volcanic rocks or in the schists which underlie them. 



Meall Dearg and Coire Cam. — The fault can be traced, 

 coinciding with the inner margin of the fault-intrusion, from Coire 

 Mhorair along the watershed to the south of Sron Garbh. Its 

 steep descent into Coire nan Lab, farther west, is also clearly marked 

 by the smooth, chilled, and shattered edge of this intrusion. In 

 the floor of Coire nan Lab the fault is lost sight of for a short space, 

 but it soon reappears and ascends the Meall Dearg ridge beyond ; 

 here it once more overhangs markedly, being inclined at about 

 50° to the horizontal. Immediately south of the summit of Meall 

 Dearg, considerable masses of quartzite occur, almost surrounded 

 by the fault intrusion, but at the same time brought into contact 

 with the Old Bed Sandstone lavas along the line of dislocation 

 (fig. 11, p. 658). 



