Vol. 65.] THE CATJXDRON-ST/BSIDESTCE OF GLEN COE. 663 



Starav Granite, forms a huge boss, 15 miles long and 10 miles broad. 

 Collectively, the granitic complex of Cruachan and Starav is known 

 as the Etive Granite. 



(7) Between the periods of intrusion of the Cruacban and Starav 

 granites intervened an important epoch marked by the injection 

 of a great host of parallel north-north-easterly dykes (fig. 1, 

 p. 614). For the most part they are porphyrites, and obviously 

 closely related in composition to the granites. 



The small aplitic granite strip traced north-westwards from Meall 

 Odbar is certainly an intrusion dating from the general period of 

 the dyke-phase. It is earlier than some of the dykes, later than 

 others. 



The dykes are ' regional ' in their constancy of direction, for 

 they exhibit not the slightest tendency to radial arrangement ; but 

 they are 'local' in their marked concentration into a definite belt 

 with the Etive mass at its centre. 



The dykes of this suite definitely add their own thickness to the 

 width of the country which they traverse. Their injection has 

 been accompanied by an opening of fissures in the country-rock 

 and an outward displacement of the walls of these fissures ; at the 

 same time, it has not been accompanied by faulting. 



(8) In addition to the dominant north-north-easterly porphyrite 

 dykes, which are distinguished by their constant direction and 

 wide distribution, there is in Glen Coe an earlier local set of dykes 

 of irregular trend (fig. 5, p. 646). All these are cut by the north- 

 north-easterly porphyrites and some of them can be shown to be of 

 even earlier date than the fault-intrusion. They consist, for the 

 most part, of volcanic types — andesites, rhyolites, and also felsites 

 and quartz-porphyries. 



A theoretical discussion of the results enumerated above may 

 now be undertaken. Eour main points will be dealt with: — 



(a) The contemporaneity of the subsidence of the Glen Coe 



cauldron and the uprise of the marginal fault-intrusion. 

 This important matter will be examined first, in the light of the 

 asymmetrical distribution of the fault-intrusion with respect to the 

 boundary-fault, throughout the district as a whole ; and secondly, in 

 regard to its intimate relations with the flinty crush-rock, which is so 

 well exposed in the typical section of Stob Mhic Mhartuin. 



(b) A comparison between the Grlen Coe cauldron of Old Red 



Sandstone age, and the modern Askja caldera, in Iceland. 



(c) A suggestion as to the form and history of the Etive 



granite complex. 



(d) The place of the dyke-phase of activity in the igneous 



phenomena of Etive and Glen Coe. 



(a) The contemporaneity of faulting and intrusion. — 

 The distribution of the fault-intrusion on each side of the boundary- 

 fault is so strikingly asymmetrical, that we are bound to seek some 

 element of asymmetry in the conditions which governed the 

 intrusion. 



