Vol. 65.] THE CAULDRON-SUBSIDENCE OF GLEN COE. G33 



been separated by Mr. Kynaston into an earlier more basic rim, the 

 Cruachan Granite, and a later, coarser, porphyritic, and more acid 

 interior, called the Starav Granite. 1 



Here we are concerned merely with the Cruachan Granite, which, 

 as Mr. Kynaston has shown, extends from the south far into the 

 Gien Coe cauldron, and produces marked alteration in the volcanic 

 rocks. 2 This northern prolongation of the granite passes trans- 

 gressively beneath the lavas of the two Buachailles, which serve as 

 its roof, now, it is true, largely removed by erosion. It has not, 

 however, tilted these overlying lavas, so far as can be ascertained 

 in many clear sections, and nowhere do Ave find indications of its 

 base (PI. XXXIII). It would not be correct, therefore, to describe 

 it as a laceolite. 



The Cruachan Granite within the district embraced by PI. XXXIV 

 is mainly of one uniform type, which is best described as a pink 

 hornblende-granitite rich in plagioclase. Two conspicuous varieties 

 are also included within the mass, but it has not been possible to 

 map them out, owing to the absence of definite boundaries between 

 them and the normal rock. The one, a pink granitite of more acid 

 composition than the normal rock, and confined to certain parts of 

 the high ground, requires no further description here. The other 

 is a grey tonalitic variety occurring on the margin of the mass, in 

 the district south of Allt Coire an Easain. 



Under the title of ' fault-intrusion ' we include a series of more 

 or less isolated masses of granitite, tonalite, and porphyrite, which 

 form an interrupted girdle round the sunken area (PI. XXXIV). 

 Although the special name here introduced is justified, on account 

 of the very intimate relations which exist between the girdle of 

 intrusion and the boundary-fault, the evidence is strong that the 

 fault-intrusion is merely, as it were, an advance guard flung out in 

 front of the main mass of the Cruachan Granite. It presents four 

 principal types : namely, the grey tonalite and the normal pink 

 granitite already mentioned in connexion with the Cruachan Granite, 

 and also coarsely crystalline grey and pink porphyrites. 3 The 

 difference in composition between the types of porphyrite is probably 

 less than that between the tonalite and the granitite ; but, in most 

 localities where the fault-intrusion is represented by porphyrite, the 

 latter is of one prevalent variety, grey or pink as the case may be. 



A feature of the fault-intrusion, which in some localities is 

 exceedingly conspicuous, is the number of xenoliths enclosed by it. 



1 The name Starav has been taken from the beautiful mountain, Ben 

 Starav, entirely composed of the central granite. In previous descriptions 

 the central intrusion has been termed the Glen Etive or Blackmount G-ranite; 

 but both these names have had to be abandoned as geographically misleading. 



2 ' Summary of Progress of the G-eological Survey for 1900 ' (1901 ) pp. 82, 83. 



3 The types of ' fault-intrusion ' here termed porphyrite resemble closely 

 in the hand-specimen certain of the more coarsely crystalline dyke-rocks, but 

 it is probable that the former include rocks of 'acid' composition (that is, 

 containing more than 65 per cent, of silica). 



