636 MESSES. CL0TTGH, 3IAEFE, AND BAILEY ON [XoV. 1909, 



This grey tonalite stretches north-north-westwards, in a con- 

 tinuous helt without sensible variation, to the foot of Stob Beinn 

 a' Chrulaiste. At two points along this course, namely on Meall a' 

 Bhuiridh and in the Cam Ghleann, there are clear sections of the 

 boundary-fault, and the accompanying tonalite exhibits a typical 

 chilled inner margin. 



On Stob Beinn a' Chrulaiste the grey tonalite merges into coarse 

 grey porphyrite, packed with felspar phenocrysts. This rock is 

 chilled against the fault, and also along its outer irregular margin. 

 In Stob Mhic Mhartuin the grey porphyrite reappears with exactly 

 the same relations. 



From Coire Odhar-mhor westwards the fault-intrusion is repre- 

 sented in the main by pink porphyrite, associated with a considerable 

 mass of pink granitite. The latter does not come into actual contact 

 with the fault, and its junction with the intervening porphyrite can 

 readily be traced. This junction is exposed at the base of Sron 

 Garbh, and is not quite sharp, being marked by a foot or so of hybrid 

 rock. Probably, in this case, the granitite is of somewhat later 

 date than the porphyrite. 



The innumerable intrusions of pink porphyrite between Garbh 

 Bheinn and Glen Coe are especially interesting, for in this district 

 the porphyrite frequently fails to show any sign of chilling away 

 from the fault-line, and the schists which it traverses are altered 

 and permeated to a remarkable degree. 



In An t-Sron the pink granitite is found in intimate association 

 with grey tonalite. In the vicinity of tbe fault a chilled margin is 

 always developed, as elsewhere. 



South of the River Etive the fault-intrusion is represented by the 

 grey type of coarse porphyrite ; and, about a third of a mile south- 

 south-east of Dalness, a definite line can be drawn between this and 

 the adjoining pink granitite of the Cruachan mass. The exact 

 junction is not exposed, but the two rocks retain their distinctive 

 features within a foot of one another ; and specimens taken from the 

 mass of the porphyrite, even at a distance from the junction, show 

 marked contact-alteration. 



These facts seem to be most easily harmonized on the assumption : 

 (1) that the fault-intrusion came from the same deep reservoir as 

 the Cruachan Granite, and that differentiation in this reservoir had 

 already, before intrusion, separated out two distinct types, now 

 represented by grey and pink rocks respectively ; (2) that during 

 the long process of intrusion to higher levels, sufficient time occa- 

 sionally elapsed for the more or less complete consolidation of 

 outlying portions before later arrivals appeared ; and (3) that the 

 pink granitite, in some cases at least, was the last to appear. 



It has been stated above that the contact - metamorphism of 

 the schists clue to the fault-intrusion is practically limited to the 

 rocks outside of the Glen Coe cauldron. For instance, the phyl- 

 lites inside the fault on the slopes above Loch Achtriochtan have 



