Vol. 65.] THE CAULDRON-SUBSIDENCE OF GLEN COE. 669 



due to the explosive tendency of the steam occluded in the 

 rising magma, and to treat the Rudloff Crater as a rim-volcano 

 essentially contemporaneous with the Knebel cauldron. 



It is particularly interesting to note that, whereas the rim- 

 volcanoes of Askja poured out basalt, the Eudloff Crater brought 

 up obsidian and pumice from the depths. We have, in this, yet 

 another parallel with Glen Coe, where augite- and hornblende- 

 andesite lavas are found together in the closest association with 

 flows of rhyolite. 



(c) The Etive Granite. — The Cruachan Granite, as exposed 

 at the surface, is divisible into two portions, which are confluent, 

 but still more or less distinct (see fig. 1, p. 614). The one 

 extends northwards into the heart of the Glen Coe cauldron, while 

 the other (including within it the great core of the more acid 

 granite of Starav) forms the main mass of the Etive plutonic 

 complex. The outcrop of aplitic granite forming a strip between 

 Stob Dubh and Meall Odhar emphasizes the individuality of the 

 two lobes of the Cruachan Granite (see PI. XXXIV). Eor half its 

 course, to the north-west, it is a marginal intrusion insinuated 

 between the Cruachan Granite on the south-west, and the Schists 

 on the north-east. For the other half of its course, it continues its 

 path south-eastwards without any indication of change, although 

 here it separates merely the two lobes of the Cruachan Granite. 



The northern mass, although it must have entered the precincts 

 of the Glen Coe cauldron after subsidence en bloc had ceased, 

 merges with the fault-intrusion in the district of Allt Coire an 

 Easain (p. 635). The interval of respite in the plutonic history of 

 Glen Coe appears, therefore, to have been but brief; at the same 

 time it was sufficiently long to permit of consolidation of the fault- 

 intrusion in certain localities, as at Dalness. 



The form of this northern invading mass has only been partly 

 revealed by denudation. All that is exposed to view is the domed 

 roof of a great intrusion which cuts across, but does not tilt, the 

 superincumbent lavas. It is difficult to avoid the conclusion that 

 the invasion of the Cruachan Granite in this instance 

 has not interrupted, so much as modified, the history 

 of subsidence of the Glen Coe cauldron. It appears 

 probable that, since its roof shows no signs of tilting up, its floor 

 must have sunk in order to make room for the invading magma. 



But if it be admitted as likely that this semi-detached northern 

 mass of the Cruachan Granite, lying in the Glen Coe area of 

 subsidence, is merely the plutonic infilling of a subterranean 

 cauldron, then it is extremely reasonable to interpret the southern 

 mass in the same manner also. Such a conception is, in fact, in 

 close agreement with Mr. Kynaston's observations. 1 The eastern 



1 ' The Geology of the Country neai- Oban & Dalmally ' Mem. Greol. Surr. 

 1908, chap. viii. 



