DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



135 



This band of felsite and microgranite may be traced continuously from Loch 

 Gainmich along the base of Barkeval and Allival, and similar rocks appear at intervals 

 along the same line round the eastern base of the hills. Immediately above this belt of 

 felsitic protrusions comes the great body of gabbro. It will be observed that here, as in 

 Skye, the base of the gabbro-mass presents a horizon on which injections of acid rocks 

 have been particularly abundant. If the breccias are not the result of rock-crushings 

 during Palaeozoic time, but are really due to volcanic explosions during the Tertiary period, 

 they are evidently older than the eruption of the gabbros. They might be compared 

 with the agglomerate-necks through which the youngest eruptive bosses of Skye have 

 made their way ; but in their case the component materials have been derived from the 

 surrounding platform of ancient rocks, and not from subterranean lavas. 



v 1 1 







Fig. 40. — View of Allival, Rum, sketched from the base of the north-east side of the cone. 



For my present purpose, however, the chief point of importance is the structure of 

 the gabbro mass that springs from that platform into the great conical hills of Rum. 

 The accompanying sketch (fig. 40) will convey a better idea of this structure than a 

 mere description. At the base, immediately above the felsite just referred to, bedded 

 dolerites mark their appearance, much intersected with veins from the siliceous rock. 

 Veins and dykes of basalt also cut all the rocks here, the newest being those which run 

 in a N.W. direction. The lowest beds of dolerite are succeeded by overlying sheets of 

 coarser dolerites, gabbros, troctolites, Sec, which are as regular in their thickness and 

 continuity as the ordinary basalts of the plateaux. The band of light-coloured troctolite, 

 in particular, about twenty to thirty feet thick, which has been already referred to, 



