Vol. 65.] THE CAULDRON-SUBSIDENCE OF GLEN COE. 615 



II. The Glen Coe Volcanic Series. 



(a) Type Section. 



The Coire nam Beith section, south of Loch Achtriochtan 

 (PI. XXXII & PL XXXIII, Sections II & IV) furnishes the best 

 introduction to the study of the volcanic series of Glen Coe, in- 

 cluding, as it does, an excellent exposure of the base of the group 

 resting upon the Highland Schists. After gaining the head of the 

 valley, Bidean nam Bian should be ascended and the traverse con- 

 tinued to the southern summit of Beinn Phada, in order to reach 

 the youngest volcanic rocks preserved anywhere in the district. 

 The full sequence thus encountered may be stated as follows : — 



Thickness in feet. 



7. Andesites and rhyolites about 300 



6. Shales and grits 50 



5. Rhyolite 250 



4. Hornblende-andesites 900 



3. Agglomerates and shales 250 



2. Rhyolites (and andesites elsewhere) 450 



1. Augite-andesites 1500 



Total 3700 



(1) Augite-andesites. — The junction of the augite-andesites 

 of group (1) with the underlying schists is well exposed in the 

 course of the stream. Some 2 feet of purple sandy shale, with slivers 

 of phyllite, occur beneath the bottom lava. There are in all about 

 seventeen flows of augite-andesite in this section, and their bedded 

 nature is well shown on the face of Aonach Dubh. 



Like the majority of the lavas of Glen Coe, they are vesicular only 

 in a minor degree. Their contemporaneous nature can, in fact, best 

 be inferred from the occasional intercalation between them of beds 

 of shale. In some cases, too, they have become brecciated, through 

 and through, during the process of cooling, and the interstices sub- 

 sequently rilled in with thin seams of horizontally bedded shale. 

 Breccias of this type are especially well seen near the summit of 

 the group in Coire nam Beith. They can be distinguished from 

 agglomerates, since each fragment possesses identical lithological 

 character, and its boundaries frequently conform accurately to the 

 irregularities in neighbouring blocks. The breccias are also quite 

 different from the brecciated lavas so common among the rhyolites 

 to be described later, where brecciation and inclusion of xenoliths 

 occurred long before movement in mass had ceased. 



The upper surfaces of some of the andesitic lavas are reddened 

 in a manner which at once suggests contemporaneous weathering. 

 Petrographically, too, these rocks are typical lavas, with a fine- 

 grained ground-mass exhibiting flow-structure, and including small 

 phenocrysts of augite, often accompanied by pseudomorphs of 

 iddingsite after olivine. 



