334 
THE TERTIARY VOLCANOES 
BOOK VIII 
that after a long interval the basic masses were forced through them, partly 
consolidating underneath and partly appearing at the surface as the plateau- 
basalts. 1 That the order of appearance of the several rocks has been exactly 
the reverse of this supposed sequence was fully established by me in the 
year 1888, and has since been amply confirmed. 2 Professor Zirkel recog- 
nized that the gabbros are a dependence of the basalts, that they overlie 
them, and that on the naked flanks of the mountains they are regularly 
bedded with them. 3 
Up to the time of the publication of my memoir in 1888 no one had 
traced out in more detail the actual boundaries of the several rocks on the 
ground, so as to obtain evidence of their true relations to each other as 
regards structure and age. Some of the numerous impediments recorded by 
Maccullocli no doubt retarded the investigation. But, as Forbes so well 
pointed out, there is really no serious difficulty in determining the true 
structural connection of the amorphous rocks with each other and with the 
bedded basalts of the plateaux. I have ascertained them in each of the 
districts, 4 and have found that there cannot be the least doubt that the 
amorphous bosses, both basic and acid, are younger than the surrounding 
bedded basalts, and that the acid protrusions are on the whole younger than 
the basic. I shall now proceed to show how these conclusions are estab- 
lished by the evidence of each of the areas where the several kinds of rock 
occur. 
iii. DESCRIPTIONS OF THE SEVERAL GABBRO-DISTRICTS 
1. The Gabbro of Skye 
The largest, most picturesque, and to the geologist most important area of 
Tertiary gabbro in Britain, is that of Skye (Map. VI.). Though, like every 
other portion of the Tertiary volcanic districts, it has suffered enormous denuda- 
tion, and has thereby been trenched to the very core, it reveals, more conspicu- 
ously and clearly than can be seen anywhere else, the relation of the gabbro 
to the bedded basalts on the one hand, and to the acid protrusions on the other. 
Its chief portion is that which rises into the group of the Cuillin Hills, which 
for blackness of hue, ruggedness of surface, jaggedness of crest, and general 
grimness of aspect, have certainly no rivals within the limits of the British 
Isles (Fig. 381). It has long been known to extend eastwards into Blath 
Bheinn (Blaven) and its immediate northern neighbours. There is, indeed, 
no break whatever between the rock of the Cuillins and that of the hills on 
the east side of Strath ua Creitheach. In Strath More the gabbro is inter- 
rupted by the granitoid mass of the Bed Hills. Patches of it, however, 
occur further to the east, even as far as the Sound of Scalpa. If we throw 
1 Quart. Journ. Geot. Soe. xxx. (1874), p. 249. 
2 Trans. Hoy. Soc. Min. xxxv. (1888), pp. 122 et seq. ; Quart. Journ. Geol. Soc. vol. 1. (1894), 
pp. 216, 645 ; vol. Iii. (1896), p. 384, and Mr. Harker, ibid. p. 320. 
3 Zettschrift. Deutsch. Geol. Geselhch. xxiii. (1871), pp. 58, 92. 
4 In two of my excursions in Mull, and once in Skye, I was accompanied by my former 
colleague Mr. H. M. Cadell, wlio rendered me great assistance in mapping those regions. 
