Vol. 65.] THE CAULDRON-SUBSIDENCE OF GLEN COE. 637 



undergone only an insignificant induration, while the rocks of the 

 same group outside the fault show traces of alteration for about half 

 a mile from the An t-Sron granitite, and at the actual junction with 

 the granitite have been converted into andalusite-hornfels. Without 

 entering into a detailed description of this contact-alteration, it may 

 be said that the mineral changes are quite similar in kind to those, 

 already described by Dr. Teall, 1 in the Highland Schists at the 

 southern margin of the Oruachan Granite, iu the neighbourhood of 

 Loch Awe. 



An interesting feature observed between An t-Sron and Stob 

 Mhic Mhartuin is the bright pink or red colour frequently assumed 

 by the schists included within the peripheral zone of alteration. 

 In the case of the andalusite-hornfels this coloration is relatively 

 slight, and is due to the development of abundant fresh orthoclase, 

 evidently at the expense of the original constituents of the rock. 

 There has been, however, in many places an injection of granitic 

 magma into the phyllites, in the form of highly felspathic pink 

 veins. The veins follow the foliation-planes of the phyllites, or 

 transgress them but slightly ; consequently they illustrate on a small 

 scale the lit par lit type of injection. But the introduction of 

 material from the invading magma takes place on a greater scale in 

 the case of the quartzite, and is particularly well exemplified 

 in the country between Garbh Bheinn and the watershed north of 

 Glen Coe. 



This is a quartzite country, riddled with small irregular masses 

 of pink porphyrite, for the most part packed with xenoliths, as is so 

 frequently the case with outcrops of the fault-intrusion. At many 

 points the quartzite becomes bright pink or red, owing to the intro- 

 duction of felspar, and this has locally proceeded to such an extent as 

 to yield a pseudo-granitoid rock. The clearest sections exhibiting the 

 connexion of this permeation with the injection of the pink porphyrite 

 phase of the fault-intrusion occur in the stream-courses south-west 

 of Garbh Bheinn. Remarkable intrusion-breccias are here exposed, 

 containing large blocks of schist enclosed in a porphyrite matrix. 

 In some places this matrix has derived so many quartz grains 

 from the quartzite that it might easily be taken for red felspar- 

 bearing quartzite itself, and yet it was evidently at one time fluid 

 enough to flow around and enclose xenoliths. A close inspection 

 of many of the quartzite enclosures in these breccias shows that 

 the constituent grains increase in size as they approach the margin, 

 and then are separated by the intervention of felspathic material 

 coming from the porphyrite. Here the porphyrite receives quartz 

 grains from its xenoliths, and an imbedded rock results. In other 

 cases, however, felspar has been introduced into the quartzites 

 without disturbing their bedding, while the rocks are so veined 

 with recognizable porphyritic substance that the connexion between 

 the felspathization and the intrusion of the porphyrite seems 



1 'The G-eologv of the Country near Oban & D aim ally ' Mem. Geol. Surv. 

 1908, p. 140. 



