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DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



at the immediate contact with the surrounding rock. One of the most striking proofs 

 of the posteriority of these veins is furnished by the perfect flow-structure they not 

 infrequently exhibit along their margins, their long felspar crystals being arranged 

 parallel to the walls in lines that follow the sinuosities of the boundary between the two 

 rocks (fig. 49). Patches of gabbro and of the indurated basalts may be seen lying on 



Fig. 49. — Mass of dark Gabbro about two feet in diameter traversed by pale veins of Granophyre, 

 lying on north slope of Creag na h-Iolaire, Mull, 



the granophyre, from which veins and strings ramify through them (fig. 50). Similar 

 veins can be traced upward into the main body of coarse gabbro, forming the ridge of 

 Ben Buy. Some of them are of the usual granular granophyric texture, others are dull 

 fine-grained porphyries (claystones of the older authors). 



Hence it is evident that the granophyres in Mull have been protruded not only after 



Fig. 50. — Section at Creag na h-Iolaire, Glen More, Mull, showing Basalts and Gabbros resting on and pierced by Granophyre. 

 a, much indurated and altered basalts and dolerites ; bb, gabbro ; c, granophyre ; dd, basalt dykes. 



the accumulation of the plateau-basalts, but after these were traversed by the sheets and 

 veins of gabbro. The amount of acid rock injected into these older rocks over the 

 mountainous part of the island is enormous ; but I reserve further reference to it for the 

 section on dykes and veins, for these are the forms in which it chiefly occurs. It should 

 be added, that in the localities here referred to basalt-veins and dykes are generally 



