618 MESSES. CLOUGH, MATJFE, AND BAILEY ON [Nov. 1909, 



that in the type section. This agglomerate forms an isolated out- 

 crop on the Buachaille Etive Mor, where it is once more found to he 

 inconstant in a southerly direction. 



Farther south-eastwards, across Glen Etive, the deposit is very 

 strongly developed, and extends from Sron na Creise, round the head 

 of the Cam Ghleann into Meall a' Bhuiridh. In the latter hill it 

 comes into direct unfaulted contact with the Highland Schists, thus 

 indicating a very remarkable overlap. The greater part of its mass 

 is here a mixed breccia of schist and rhyolite fragments, but the 

 upper portion near the overlying andesites also contains numerous 

 andesitic fragments. 



Returning to the type section on Bidean nam Bian, it is found 

 that the agglomerate cannot be traced far to the south-east, and 

 the position of the group has not been detected in the neighbour- 

 hood of Dalness. 



The abundant well-bedded rocks in group (3) give much assistance 

 in working out the structure of the district. It is found that the 

 sediments and lavas of the central portion of the Glen Coe cauldron 

 dip in a southerly direction, sometimes at low and sometimes at 

 rather high angles. This structure can only be detected away from 

 the margins of the subsidence: for, in the proximity of the boundary- 

 fault, the dominant characteristic is a steep upturning of the various 

 subdivisions, occasionally leading to actual inversion (PI. XXXIII, 

 Section IV). 



The local variations of group (2), represented by rhyolites 

 alone in the type section, are extremely interesting. Traced into 

 the western peak of Bidean nam Bian, where the rocks stand on 

 end in proximity to the boundary-fault (PI. XXXII), the rhyolites 

 die out entirely. Followed to the east, on the other hand, through 

 the ' Three Sisters of Glen Coe ' (Aonach Dubh, Gearr Aonach, and 

 Beinn Fhada), the group expands, and in the last of the three, 

 Beinn Fhada, and also in the opposite slopes of the Buachaille Etive 

 Beag, it includes a very large proportion of andesitic flows. Among 

 these latter both augite- and hornblende-andesites occur, and their 

 intercalation with the rhyolites, which they more or less replace in 

 the succession, can clearly be traced in Beinn Fhada. The thickness 

 of the group in this locality must be about 2000 feet (PI. XXXIII, 

 Sections I, II, & III). 



Farther east again, the andesite lavas of group (2) disappear 

 even more suddenly than they came. Not one of them extends 

 round the northern face of the Buachaille Etive Beag, for this 

 consists entirely of rhyolite. This rhyolitic development, locally 

 intei-rupted by the important agglomerate of Lochan na Fola, con- 

 tinues south-eastwards into the Buachaille Etive Mor (PL XXXIII, 

 Section II), and across the Biver Etive into the Cam Ghleann ; on 

 the northern face of Glen Coe, however, andesites greatly predo- 

 minate, and group (2) is represented in much the same manner as 

 in some parts of Beinn Fhada to the south. 



