DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 



131 



structure that at once recall him to the internal arrangements of the thicker intrusive 

 sheets, but they are displayed here on a still greater scale, as from the bulk of the huge 

 boss of the Cuillin gabbro, might be expected. Portions of the rock show a remarkable 

 segregation or flow-structure, the several minerals being arranged in parallel layers, 

 which sometimes simulate the puckering of true schists. This structure is shown in 

 fig. 38, the light bands consisting mainly of felspar, the darker of the ordinary gabbro, 

 but here and there with the magnetite separated out into distinct lenticular folia. It is 

 singular to find, in the midst of such coarsely crystalline material, exceedingly fine-grained 

 masses of basalt, some of which are amygdaloidal. At the time when I made my last 

 examination of this region, the six-inch Ordnance maps were not issued, and I found it 

 impossible to trace out such details of geological structure on the uncontoured one-inch 

 map, which was the only Ordnance sheet then available. I have every hope, however, 

 that when the mapping of the Cuillin Hills is undertaken on the large scale maps, it 

 will be possible to work out an exceedingly complex structure even in what might be 

 thought to be thoroughly amorphous masses. 



Fig. 38. — Segregation-structure in the Gabbro, from the ridge between Meall Dearg and Loch Coruisk. 



There is one important feature which only a minute and patient survey can elucidate, 

 Though I found among the Cuillins no distinct proof that the mass of gabbro ever gave rise 

 to discharges of material, either lava-form or fragmentary, which reached the surface, I 

 obtained unquestionable evidence of explosions and the production of pyroclastic masses. 

 Among the moraine-mounds of Harta Corry, blocks of basalt-agglomerate are strewn 

 about, full of angular fragments of altered basalt, sometimes highly amygdaloidal, and also 

 boulders in which lumps of coarse gabbro are enveloped in a matrix of finer material. But 

 I did not find the parent rocks from which these glacier-borne masses had been derived. 

 That this huge boss of gabbro in Skye, besides invading and altering the bedded basalts, 

 may have communicated eventually with the surface, and have given rise to superficial 

 discharges, is not at all improbable, but of any such outflows not a vestige appears now 

 to remain. We must remember, however, that the gabbro no doubt in many places 

 found its readiest upward ascent in vents belonging to the plateau-period, and that 

 portions of the agglomerates of these earlier vents may be expected to be found involved 

 in it, like that of the great vent of the Red Hills. 



Before quitting this area, I will refer to the detached portions of gabbro inclosed in 

 and lying to the east of the mass of the Red Hills. One of the best-marked of these forms 



