xciv PROCEEDi:N'as of the geological society, [vol. Ixxiii, 



to special tectonic features such as ring-like faults and folds. At 

 centres where the disturbance was of a less violent kind there is a 

 tendency to a laccolitic or roughly stratiform habit. Of the minor 

 intrusions the most remarkable are the groups of inclined sheets, 

 dipping inward at angles up to 45°. Such sheets are very numerous 

 in the Cuillin Hills of Skye, and are represented in less force about 

 the centres of Rum and Ardnamurchan. In Mull the}^ occur in 

 extraordinary numbers, and are disposed about two neighbom-ing 

 points. About some of the plutonic centres there are local groups 

 of dykes with a radiate arrangement ; or, again, the d^d^ies of 

 the regional series become unusually'' numerous, and are drawn 

 into a sheaf -like disposition. In all these phenomena we see the 

 evidence of a special distribution of stress related to the prescribed 

 centres. 



Petrographically the local groups of rocks show more variety 

 than the regional series. Apart from some complications in the 

 Mull district, the sequence, when complete, ranges from ultrabasic 

 to acid in the plutonic phase, and the reverse in the phase of minor 

 intrusions. The parent stock of all must have been the same basic 

 magma as that which provided the plateau-basalts of the region and 

 other rocks with evident alkaline affinities. None the less we find 

 that, about the centres of acute disturbance in the interior of the 

 ^ti*act, the local groups of rocks belong to distinctively calcic t^^pes : 

 peridotite and allivalite, eucrite, gabbro and norite, augite- and 

 hornblende-gi-anite, and biotite-granite.i Towards the limits of 

 the tract, where the locally developed stresses were much less 

 intense, the alkaline facies reasserts itself in the plutonic rocks. 

 This is shown in the north by the massive sheets of riebeckite- 

 granite on Raasay, while in the south it is suggested by the Arran 

 gi-anites, rich in micropei-thite, and still more clearly b}-- the 

 paisanite of Ailsa Craig. 



The same law is illustrated by the rocks of regional distribution 

 themselves, where they come within the influence of the special 

 centres of stress. The basaltic lavas of Centml Skye seem to be 

 decidedly of less alkaline composition than those in the northern 

 part of the island. A like conclusion emerges when we compare 

 the dolerite sills in the interior of the specified tract with those of 



^ An anoiualy, from this point of view, is the occurrence of an alkali- syenite 

 forming a reef opposite Carsaig, on the south side of Mull. 



