328 Reports and Proceedings — Geological Societ;/ of London. 



The volcanic pile, with patches of conglomerate and breccia at the 

 base, rests upon an uneven floor, evidently a land-surface, of the 

 Highland Schists ; and further, the eruptions appear to have been 

 subaerial. 



The cauldron subsidence, which let down the volcanic rocks and 

 the underlying schists some thousands of feet, affected an area roughly 

 oval in shape and measuring 8 miles by 5. It is delimited by a fault, 

 the hade of which is sometimes normal, sometimes reversed. The 

 lavas abut against the fault-plane, and are frequently tilted up into 

 a vertical or even overturned position near it. Further evidence for 

 the boundary fault is afforded by the displacement of the outcrops of 

 several members of the Highland Schists. 



A. zone about a mile wide lying immediately outside the boundary 

 fault has been invaded by a number of masses of granite and porphyrite, 

 spoken of collectively as the ' fault- intrusion '. Where the fault- 

 intrusion comes against the boundary fault it is chilled, and its margin 

 is a smooth even plane, while its junction with the schists is highly 

 irregular. The fault-intrusion causes contact-alteration in the schists 

 outside the fault, but the volcanic rocks and schists inside the fault 

 are scarcely affected by it. 



The movement along the fault-plane has caused intense shearing 

 and crushing, leading finally to the production of ' flinty crush-rock '. 

 The latter owes its characters to extreme trituration, probably accom- 

 panied by incipient fusion due to frictionally generated heat. 



In certain places flanking faults older than the main boundary 

 fault, and accompanied in each case by a mass of igneous rock on their 

 outer walls, are found near the main boundary fault and parallel to it. 

 The subsidence therefore took place in at least two stnges. 



After subsidence in the cauldron had ceased, a multitude of dykes, 

 mainly porphyrites, but including quartz-porphyries and lamprophyres 

 also, were intruded along lines trending north-north-eastwards and 

 south-south-westwards. It is shown that the dykes add their width 

 to that of the country traversed, and that they have their focus within 

 the Etive granite mass. They have a parallel, not a radial arrange- 

 ment; and a vast majoritj' are concentrated into two swarms, which 

 extend north-north-eastwards and south-south-westwards from the 

 granite. 



The authors discuss several questions arising out of their con- 

 clusions. With reference to the relative age of the faulting and the 

 fault-intrusion, it is concluded that they are contemporaneous, and 

 that the uprise of the magma may be considered as complementary 

 to the subsidence. The Glen Coe subsidence is compared with the 

 subsidence which took place in the Askja caldera in Iceland in 1875. 



It is considered probable that the lobe of the Cruachan granite, 

 which invaded the sunken area of Glen Coe, was admitted by a further 

 subsidence of part of the rock-mass within the cauldron, and that the 

 granite occupied the cavity thus formed. 



A theory is advanced that the Cruachan granite mass also originated 

 in a subsidence of the schists in place of which the granite is now 

 found, the magma welling up the sides of the sinking mass and filling 

 in the subterranean cauldron. Evidence for this is adduced in the 



