The Cauldron Subsidence of Glen Coe. 335 



hornblende-andesite, and rhyolite. Agglomerates, tuffs, and sedi- 

 ments form but a small portion of the sequence. The Lower Old 

 Red Sandstone age of the rocks is proved by the occurrence of 

 plant-remains in shales at the base. The sequence is divisible 

 into groups, which are not, however, persistent over the whole 

 area. Each group may contain different types of lava, which 

 interdigitate one with the other. It is probable that the district 

 was supplied from more than one centre, the foci being independent 

 as regards type of material erupted, although their periods of activity 

 overlapped. 



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 Anally to the production of 'flinty crush-rock." 

 The latter owes its characters to extreme trituration, probablv 

 accompanied 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 

 stages. 



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, arrangement ; and a vast majority are concen- 

 trated 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 



