Yol. 65.] THE CAULDRON-SUBSIDENCE OE GLEN COE. 661 



general inclination of the plane in this locality is slightly north of 

 west at about 47° ; and the rock above the plane, that is, on the 

 south-west side of the outcrop of the latter, consists of garnetiferous 

 mica-schist, while that below is quartzite. 



From the locality just mentioned the fault continues in a south- 

 easterly direction for about a quarter of a mile, and then turns and 

 runs southwards for nearly the same distance. Farther south still, 

 a broad mass of pink porphyrite has come up along the fault, and 

 continues southwards until the granitic rocks are reached. The 

 portion of the fault-line, so far described, which is free from fault- 

 porphyrite, is nearly half a mile in length ; but near the middle it 

 is occupied or closely accompanied by a thin porphyrite dyke which 

 locally gives up its usual north-north-easterly direction and proceeds 

 along the fault for about 170 yards. 



The same line of fault is seen again without any fault-porphyrite, 

 on the hillside nearly 1000 yards north-east of Stob Dubh. At 

 this spot it inclines northwards at about 28°, and separates the 

 underlying quartzite on the south side from the garnetiferous mica- 

 schist on the north. Thence it continues in a north-easterly 

 direction for about a third of a mile, forming a distinct boundary 

 all the way between the two Schists just alluded to. Its path 

 farther northwards is not so clear, chiefly owing to the abundance 

 of intrusive igneous rock, including both fault-porphyrite and also 

 a considerable amount of quartz-felsite, part of which is older than 

 the fault-porphyrite. In this direction, however, it is plain that 

 the fault under consideration must be approaching near to the 

 inner fault already described. 



IV. General Conclusions and Discussion. 



A brief summary has already been given (pp. 623, 624) of our 

 main conclusions based upon a study of the volcanic succession in 

 Glen Coe, its associated sediments, and its relations to the uneven, 

 eroded floor of Schists beneath. General and detailed descriptions 

 have followed, showing that :— 



(1) The Glen Coe volcanic rocks occupy the major portion of a 

 cauldron-subsidence, using this term in the sense in which it has 

 been employed by Suess. The boundary-fault delimiting the 

 cauldron .encloses an area roughly oval in shape, and measuring 

 9 miles in length by 5 in breadth. The area within the fault has 

 undergone extensive subsidence. Even near its edge the rocks 

 have sunk well over 1000 feet, while the total displacement has 

 been materially increased by sagging of the central parts. 



One-fifth of the circumference of the cauldron is now obscured, 

 it is true, by an offshoot of the Cruachan Granite, which has 

 invaded the foundered area from the south, and obliterated the 

 boundary-fault for a distance of about 4 miles. But it is obvious 

 that this fault once completed its circuit, for the truncated ends of 



