638 MESSES. CL0UGH, MATTEE, AND BAILEY ON [NOV. I9O9, 



unquestionable. Where it traverses reddened patches of this 

 character, the porphyrite, one need scarcely add, shows no indi- 

 cation of chilling at its margin. It is interesting to note that 

 in the permeation-phenomena outlined above, alkali-felspar plays 

 the predominant role, and it seems necessary to admit the operation 

 of diffusion, governed by some selective principle. 



In addition to the Cruachan mass, there is another important 

 granite, with which the fault-intrusion comes into contact. This 

 is the great Moor of Rannoch Granite, lying to the east. Its 

 interior portions, as seen in the neighbourhood of Kingshouse, 

 consist of grey hornblende-granitite ; but, for a considerable breadth 

 near its margin, it is free from hornblende, and contains large pink 

 orthoclase crystals. In many parts the mass shows a well-marked 

 parallel arrangement of its mineral constituents. Its junction with 

 the fault-intrusion is of a very intricate type, and is well exposed, 

 in both the Coupall l and the Etive rivers. The interpretation of 

 these difficult sections we owe entirely to Mr. Grabham. He has 

 shown that, although the margin is of the hybrid type, the fault- 

 intrusion is certainly the later rock. The most convincing section 

 is that in the Coupall River, where at one place scattered blocks of 

 the llannoch Granite lie in the grey fault-intrusion and each block 

 possesses a different orientation, as shown by its mineral banding. 



(6) The early fault-intrusions. — For a mile and a half to 

 the west of Stob Mhic Mhartuin a mass of rock is indicated on the 

 general map under the title of ' early fault-intrusion.' It is a grey 

 hornblendic porphyrite, frequently charged with xenoliths, but still 

 quite readily distinguishable from the previously described grey 

 fault-porphyrite of the fault- intrusion. For one thing, it is finer 

 in texture and less packed with phenocrysts. Its main charac- 

 teristics are summed up in its designation. It is a ' fault-intrusion ', 

 since, all along its course, its inner margin is chilled against an outer 

 branch of the boundary-fault, which is marked by a line of intense 

 shearing of the schists culminating in the production of flinty crush- 

 rock. It is ' early ' because, unlike its more important neighbour, 

 it has suffered from renewed shearing movement which came on 

 after its complete consolidation. Sometimes along the margin, 

 sometimes a little distance from it, and in many cases in the very 

 heart of the rock, shear-bands may be detected crossing this early 

 porphyrite. On a fractured face it maybe observed that the pheno- 

 crysts are broken and drawn out ; the porphyrite in this stage 

 assumes a dark colour, and is traversed by threads and bands of 

 flinty crush-rock formed by the destruction of its own material. 



This is not all. On Sron a' Choire Odhar-bhig the fault-intrusion, 

 sensu stricto, occupies the whole space between the two branches of 



1 The exposures of Rannoch Granite in the Coupall River are small and 

 outside the main mass. They are not indicated on the map (PI. XXXIV). 



