Yol. 65.] THE CAULDRON-SUBSIDENCE OF GLEN COE. 639 



the fault, each marked by a line of flinty crush-rock ; and a 

 little way down the western face of the ridge a junction with the 

 early fault-porphyrite is exposed (fig. 9, p. 654). Here the contrast 

 between the relations of the two porphyrites is clear, since the 

 ■earlier intrusion on the north is greatly sheared along the contact- 

 plane, while its fellow on the south is quite unmoved. 



The same early fault-intrusion comes into conjunction also with 

 the pink granitite mass which lies at the foot of Sron Grarbh. The 

 junction is known only in one stream running down the scree-slope 

 from the east into Coire Mhorair. Microscopic examination of a 

 specimen from this stream shows that the granitite has induced 

 marked contact-metamorphism in the porphyrite, including the 

 replacement of the hornblende phenocrysts by aggregates of biotite 

 (12358). 



A distinct but much smaller mass of sheared early fault- intrusion 

 lies immediately outside the southern branch of the fault in 

 Coire Odhar-mh6r (see figs. 9 & 10, pp. 654 & 655). Here the 

 ' early ' age of the intrusion is again clear : for the main fault- 

 porphyrite cuts across it obliquely, without itself showing any 

 sign of disturbance. 



An outcrop of dark diorite exposed in the Cam Ghleann, a little 

 distance within the main boundary-fault, probably also belongs to 

 the category of early fault-intrusions. It lies with an even base 

 upon a lowly inclined plane, marked by intense shearing of the 

 underlying schists. This plane dips down stream, away from 

 the centre of subsidence, and is evidently connected with the 

 system of faults which surround the sunken area. The diorite 

 here shows a marginal felsitic facies in contact with the plane. 

 ]S"o clear signs of movement have been detected in this marginal 

 rock, and the main outcrop of diorite is certainly quite unsheared. 

 The rock is, however, so distinct in appearance from the big outcrop 

 of grey tonalite, representing the fault-intrusion proper in this 

 neighbourhood, that it almost certainly belongs to a distinct epoch 

 in the history of the subsidence. In the next paragraph, reason 

 will be given for believing that this epoch was of early date. 



A dark diorite similar to that of the Cam Grhleann, just de- 

 scribed, crops out on both sides of Ailt Coire an Easain. It has 

 already been mentioned, on account of the peculiar indications 

 of shearing which its inner margin exhibits. In its main relations 

 it is a typical fault-intrusion, for it has a smooth almost vertical 

 inner margin, excellently exhibited on the bare hill-slopes, and in 

 the immediate proximity of this margin it passes into a fine-grained 

 porphyrite. There can be little doubt that this smooth face is 

 determined by a fault-plane. The chilled margin, as already 

 noticed, is intensely sheared. The curious fact about it is that 

 the shearing has not given rise to the type of brecciation which 



Q. J. C. S. Xo. 260. 2 x 



