DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 95 



central part of the plateau, forming the island of Rum, is, however, much less regular. 

 Four small outliers of the basalts lie at levels of 1200 feet and upwards, on the western 

 slope of that island. They are underlain by a thick mass of red (Cambrian or Torridon) 

 sandstones, which form the northern half of the island, and which southwards are 

 connected with a confused series of gneisses and schists. These rocks are doubtless a 

 continuation of the red sandstones and schists of Sleat, in Skye, and like them have been 

 subjected to those post-Silurian convolutions and metamorphism whereby Archaean 

 gneisses have been brought above younger rocks, and the whole have been crushed and 

 rolled out so as to assume a new schistose arrangement. Before the time when volcanic 

 action began, a mass of high ground, consisting of these ancient rocks, stood where the 

 island of Rum is now situated. The streams of basalt spread around it, not only covering 

 the surrounding low tracts of Jurassic rocks, but gradually accumulating against the hills, 

 and thus reducing them both in area and in height above the plain.* 



The plateau has been obliterated over the centre and south of Eum by the extrusion 

 of enormous masses of gabbro, and some later granitoid rocks. The most extensive of 

 its fragmentary portions is that of Eigg, where the sheets of basalt, resting on Jurassic 

 beds and dipping gently southwards, can be studied all round the island in a continuous 

 range of precipices (see fig. 62). 



The general aspect and succession of volcanic sheets in the area of Small Isles agree 

 with those of Antrim and of the older part of the plateau of Mull. The basalts in Eigg, 

 Canna, and Muck rise into ranges of fine sea-walls, sometimes five or six hundred feet 

 high. The thickest mass of them occurs in Eigg, where, lying unconformably upon 

 different platforms of the Jurassic rocks, they attain a thickness of about 1100 feet. 

 They consist of the usual types — black, fine-grained, columnar and amorphous basalts, 

 more coarsely crystalline dolerites, and dull earthy amygdaloids with red partings, and 

 occasional thin bands of basalt-conglomerate or tuff. The individual beds range in 

 thickness from 20 to 50 or 60 feet. Though they seem quite continuous when looked at 

 from the sea, yet, on closer examination, they are found not unfrequently to die out, 

 the place of one bed being taken by another, or even by more than one, in continuation of 

 the same horizon. The only marked petrographical variety which occurs among them is a 

 light-coloured band which stands out conspicuously among the darker ordinary sheets of 

 the escarpment on the east side of the island. The microscopic characters of this rock show 

 it to belong to the same series of highly felspathic lavas as the " pale group " of Ben 

 More, in Mull. It is strongly vesicular, and the cells are in some parts so flattened and 

 elongated as to impart a kind of fissile texture to the bed.t 



This plateau has suffered even more than that of Mull from the combined influence 



* That the lava-fields did not completely bury this nucleus of older rock has been supposed to be shown by the 

 fragments of red sandstone found in the ancient river-bed of Eigg, which, was scooped out of the basalt-plateau and 

 sealed up under pitchstone. But I am disposed to think that these fragments, together with those of Jurassic sandstone, 

 came, not from Rum, but from some district more to the north and east, as will be adverted to in a later part of this 

 paper. 



+ For further details regarding this plateau in Eigg, see my paper, Quart. Jour. Geol. Soc. xxvii. (1871) p. 290. 



