48 SECTIONAL ADDRESSES. 



responsible for the two suites of ring-dykes are separated from each other 

 by a distance of about two miles. The earlier centre lies to the south-east 

 and the later centre to the north-west, situated in the neighbourhood of 

 Loch Ba. 



It was not till the mapping of Mull had progressed towards completion 

 that it was recognised how characteristic and prevalent was the ring-dyke 

 type of intrusion, but Dr. Harker's description and mapping of central 

 Skye made it quite clear that ring-dykes were there represented, more 

 particularly in the region of the Red Hills. His description of the compo- 

 site nature of the main granophyre, its vertical western margin, its arcuate 

 inclusions of older plutonic rocks, and on the eastern side its relation to 

 the older sedimentary rocks, are in perfect agreement with those of the 

 ring-dykes of Mull. He emphasises the steeply inclined or vertical 

 boundaries and the almost flat or slightly domed roof so characteristic 

 of this type of intrusion. 



Since the detailed study of Skye and Rum three additional plutonic 

 centres have come under review, namely Ardnamurchan, the Mourne 

 Mountains and Arran. 



Mull must always be regarded as the type area of ring-dyke intrusion, 

 especially as it possesses in the Loch Ba felsite, one of the intrusions around 

 the later centre, the most perfect example yet met with. The neighbouring 

 centre of Ardnamurchan, however, offers most striking instances of ring- 

 dykes, and being less complex than Mull or Skye the relations of the rock- 

 bodies to each other and their forms are readily understood and appreciated. 

 The forthcoming memoir on this district should prove an interesting sequel 

 to those on Skye and Mull. 



Mr. Richey, who spent many years mapping the peninsula, has estab- 

 lished at least two suites of ring-dykes, one intersecting the other and with 

 centres of subsidence situated more than a mile apart. A third, but less 

 obvious centre, is the earliest, and, from the agglomerates which appear 

 to be related to it, marked the initiation of the Ardnamurchan centre as 

 a whole. It is the latest series of intrusions, ranging from eucrite to 

 monzonite, that most clearly exhibits the typical features of ring-dyke 

 injection. The steep junctions, the intercalation and metamorphism 

 of older rock between successive intrusions, the sharp transgression and 

 disregard of pre-existing structures, and the welding of intrusive junctions 

 with a general absence of structures due to chilling, are all features that 

 we have come to recognise as peculiar to ring-dykes. No roofs are now 

 preserved, denudation having been very severe, but it is practically certain 

 that several of the ring-dykes were connected across the area of subsidence 

 by transverse continuations. 



Carrying his work farther afield, Mr. Richey has lately demonstrated 

 that in the Tertiary granites of the Mourne Mountains we have a very 

 clear case of ring-dyke intrusion due to central subsidence. He has found 

 that the Mourne granite, instead of being one simple intrusion, com- 

 prises four successive intrusions, one within the other, the outermost 

 being the oldest and the innermost the most recent. Further, that these 

 intrusions were effected without any disturbance of the strike or dip of 

 the neighbouring Lower Palaeozoic rocks, a fact which shows that the 

 magma did not force apart the rocks into which it was intruded, but 



