632 MESSBS. CL0TTGH, JIATJFE, AND BAILEY ON [jSov. 1909, 



to be described (p. 635). It is clear, in fact, that tbe Glen Coe 

 fault has proved a ready guide to all later earth-tremors which 

 have affected the district, and that the rocks in and near it have 

 suffered accordingly. Among these latter may be mentioned a 

 basalt dyke of presumably Tertiary age, which has been intruded 

 along the fault-plane some distance north-west of Dalness, and has 

 later been brecciated in the self-same manner as the porphyrite 

 dykes of Old lied Sandstone age already mentioned. 



The shattering which has been induced along the line of the 

 boundary-fault is, therefore, of small significance as an independent 

 phenomenon ; but it has helped lis greatly in tracing the course of 

 the fault. In fact, where the original fault-line has not been used 

 again by later disturbances, it is sometimes very difficult to detect : 

 for the rocks are by no means invariably sheared in its vicinity to 

 the same extent as at Stob" Mhic Mhartuin. Sections will be 

 described later, in the neighbourhood of Dalness, where there is 

 such an absence of disturbance along the outcrop of one important 

 branch of the fault, that its existence can only be proved by the 

 discordant relation of the Highland Schists upon its two sides. 



At one point along its course — namely, on the eastern slope of 

 Me all a' Bhuiridh — the position of the fault is marked by an 

 abundance of quartz veins. This, however, is most unusual, and, 

 like the smashing, must be due to subsequent changes affecting 

 the original fault-rock, for one of the later north-north-easterly 

 porphyrite dykes is pierced by the veins. 



"Where consolidated igneous masses have been exposed to the 

 stresses which accompanied the Glen Coe subsidence, they have, as 

 a rule, been sheared and eventually broken down into flinty crush- 

 rock, in just the same manner as the schists into which they are 

 intruded. This behaviour is clearly seen in an important intrusion 

 which extends westwards from Stob Mhic Mhartuin, and lies to the 

 north-east of the outer of the two branches into which the Glen Coe 

 fault is divided in this locality. 



In one instance, however, strongly marked shearing of a different 

 type has been noticed in an igneous rock. In this case the most 

 obvious result of the stresses has been the production of long parallel 

 ribbons, derived from the destruction of the ferromagnesian pheno- 

 crysts. The shear-zone in which these ribbons occur is only a few 

 inches wide, and is well seen in the steep slopes on both sides of 

 Allt Coire an Easain, where it furnishes the main evidence for the 

 inner of the two branches of the fault shown upon the map in this 

 locality. The zone is vertical and strikes in a general north-north- 

 east and south-south-west direction. 



(5) The fault-intrusion, and its relation to the other 

 plutonic rocks of the district. — The Glen Coe volcanic rocks 

 lie on the northern margin of one of the largest granitic intrusions of 

 Scotland (see fig. 1, p. 614). This great mass, the Etive Granite, has 



