660 U ESSES. CLOTJGH, MAEFE, AND BAILEr ON [Nov. 1909, 



in this locality steeply north-east, towards the subsidence, and 

 separates quartzite on the south-west side from altered breccias 

 and ashy sandstones on the north-east. These breccias and sand- 

 stones belong to a group which in this district comes between the 

 andesitic lavas of group 1 and the rhyolitic rocks of group 2. 

 They are separated from the quartzite on the south-west by a 

 highly sheared band, partly made up of igneous rocks ; and also by 

 a dyke of porphyrite, which leaves its usual north-north-easterly 

 direction and runs south-eastwards along the fault for more than 

 50 yards. It is quite free from crushing, and is not far distant 

 from other dykes which go without deflection through the fault. 



About 100 yards farther south-east the rhyolitic rocks of 

 group 2 are found in contact with the quartzite, without any 

 intervening intrusive rock. Close to the junction they are crossed 

 by various nearly vertical planes of shear, and by thin streaks of 

 black flinty crush-rock. The flinty rock in a specimen (11464) which 

 was obtained in this locality shows under the microscope incipient 

 crystallization. 



A little farther up the hill a fault-porphyrite, for the most part 

 of a grey colour, comes in between the quartzite and the volcanic 

 rhyolitic rocks of group 2, and it can be traced along or near the 

 fault for about a third of a mile. Its north-eastern margin is 

 distinctly chilled and nearly straight ; and, for a little distance off, 

 the bedding of the rhyolitic rocks appears to be nearly vertical, 

 though 300 or 400 yards away it is approximately horizontal. 

 The south-western side of the porphyrite is less regular than 

 the north-eastern, and sends out various processes in a south-south- 

 westerly direction, which are hard to distinguish from some of 

 the common dykes of the neighbourhood. It is crossed, however, 

 by a broad dyke of quartz-felsite, which is, in turn, cut by several 

 dykes of porphyrite. 



The ground farther south-east near the hilltop, just before the 

 Cruachan Granite is reached, is obscure. In this tract the fault 

 seems to be represented by at least three branches ; but the two 

 lying farther to the south-east are probably of no great importance. 

 The main mass of the fault-porphyrite goes off to the south-west 

 before the Cruachan Granite is reached. It is not clear that any 

 portion of this granite penetrates along the fault. 



On the west side of Beinn Ceitlein another important dislocation 

 has been detected, which resembles the inner fault in being 

 frequently accompanied by fault-porphyrite, and in being of earlier 

 date than the north-north-easterly dykes. It is, however, generally 

 less steeply inclined than the inner fault, and is more variable in 

 direction of inclination and of outcrop. 



On the hillside, about 680 yards north-east of the summit of 

 Stob Dubh, it is unaccompanied by any fault-porphyrite, and is 

 represented by a thin fracture-plane, no thicker than a knife- 

 blade, which might be taken for a mere joint-plane were it not for 

 the evident discordance between the Schists on the two sides. The 



