634 MESSES. CL0TCH, MATTFE, AND BAILED ON []S"0V. I909, 



The great majority of these xenoliths are highly metamorphosed 

 fragments derived from the Highland Schists. 



The relation of the fault-intrusion to the fault will now be con- 

 sidered in greater detail. Enough has been said already to justify 

 a belief in the Glen Coe subsidence, and to indicate in many places 

 the precise position of the boundary-fault. The distribution of 

 the fault-intrusion is in obvious relation to the course of this 

 fault : in brief, the intrusion follows along its outer side. It often 

 occurs as a broad dyke-like mass, sharply bounded along its inner 

 margin by the fault, but with an outer margin of very irregular 

 form : often, too, it occurs in isolated masses, of many different 

 shapes, which flood the country-rock for a distance of about a mile 

 from the fault, but farther away are almost unrepresented. 



If we leave out of consideration the big but compact mass of the 

 Cruachan Granite, invading the south-eastern corner of the area, 

 only three or four small outcrops of the fault-intrusion are known 

 within the boundary-fault. These three or four little outcrops 

 comprise a small mass intruded into the andesites at the foot of 

 Meall Dearg in Coire Cam, apparently along a branch of the fault ; 

 a second small mass to the east of An t-Sron, also intrusive into 

 andesites and close to the fault ; and an ill-defined patch on the 

 south side of Glen Etive, a little above Dalness. 



The contact-metamorphism induced by the fault-intrusion is 

 similarly restricted, and the lavas and schists within the cauldron- 

 subsidence have escaped almost untouched, with the exception of 

 the rocks in the neighbourhood of the large offshoot of the Cruachan 

 Granite and those in immediate contact with the small masses 

 mentioned in the preceding paragraph. 



Along its external boundaries the fault-intrusion not infrequently 

 becomes finer in grain, or even shows actual chilling. In contact 

 with the boundary-fault the latter change is invariable, and the 

 rock passes into a fine-grained porphyritic marginal facies — a fact 

 which clearly proves that the intrusion is not of earlier 

 date than the faulting. 



The smooth chilled edge, which the intrusion presents against the 

 fault, is so characteristic a feature that it can be safely used, in 

 return, to fix the position of the fault in places where, owing to the 

 breadth of the intrusion, other evidence is obscure. Thus, between 

 Dalness and Bidean nam Bian, so long as the fault-intrusion is of 

 very moderate dimensions, it is obvious that its even north-eastern 

 margin is chilled against a powerful fault, throwing lavas on the 

 north-east down against schists on the south-west. On the other 

 hand, northwards, on An t-Sron, where the breadth of the fault- 

 intrusion is no less than a mile, the precise position of the fault can 

 only be inferred from the smooth course followed by the inner 

 chilled margin of the fault-intrusion, and from the later shattering 

 which has been guided by the old line of weakness. 



In this long section between Dalness and An t-Sron, and again 



