THE TERTIAR Y . VOLCANOES 
BOOK VIII 
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hills above Loch Don, probably Mainnir nam Fiadh (2483 feet), which he 
found to consist of “ strata of basalt and greenstone,'’ with some basalt-breccia 
or tuff and a capping of basalt. He speaks of the “ singular scorified-like 
aspect ” of the weathered greenstone — a description which applies to some 
of the coarser gabbro bands of that locality. But he appears to have 
recognized the general bedded arrangement of the rocks up even to the 
summit of the hill. 1 
It was not, however, until the visit of Professor Zirkel in 1868, that the 
true petrographical characters of the gabbro of Mull were recognized. This 
observer remarked that the rock is regularly interstratified with the 
basalt. 2 Professor Judd, as already stated, has supposed the gabbros to 
be the deep-seated portion of the masses which when poured out at the surface 
became the plateau-basalts, and he represents them in his map and sections 
of Mull as ramifying through the granitic rocks. 3 
In Mull the disposition of the gabbro in beds, sheets or sills is well 
displayed, for there is here no great central complicated mass of interlacing 
banded and amorphous sheets. We have seen that a higher group of 
plateau-basalts has survived in this island better than in the other plateaux, 
and it would seem that denudation has not yet succeeded here in cutting 
down so deeply into the gabbro core as in Skye, Bum and Ardnamurchan. 
Only the upper or outer fringe of intrusive sheets among the bedded basalts 
has been laid bare. The district within which this fringe may be observed 
is tolerably well-defined by the difference of contour between the long terraced 
uplands of the ordinary basalts and the more conical forms of the southern 
group of gabbro hills between Loch na Keal and Loch Spelve. The number 
and thickness of the gabbro-sheets increase as we proceed inwards from the 
basalt-plateau. These sheets are specially prominent along the higher parts 
of the ridge that runs northwards from the northern end of Loch Spelve, and 
along the west side of Glen Forsa. But they swell out into the thickest 
mass in the south-western part of the hilly ground, where, from above Craig, 
in Glenmore, they cross that valley, and form the rugged ridge that rises 
into Ben Buy (2354 feet), and stretches eastward to near Ardara (Map VI.). 
It is in this southern mass that the Mull gabbro approaches nearest in general 
characters to that of Skye. But even here its true intercalation above a 
great mass of bedded basalt may readily be ascertained in any of the numerous 
ravines and rocky declivities. 
One of the best lines of section for exhibiting the relations of the rocks is 
the declivity to the west of Ben Buy and Loch Fhuaran. Ascending from the 
west side, we walk over successive low escarpments and terraces of the plateau- 
basalts with a gentle inclination towards north-east or east. These rocks 
weather in the usual way, some into a brown loam, others into spheroidal ex- 
foliating masses. But as we advance uphill they gradually assume the peculiar 
indurated shattery character already referred to. The soft earthy amygdaloids 
become dull splintery rocks, in which the amygdales are no longer sharply 
1 Mineralogy of the Scottish Isles i. p. 205. 
2 Zeitsch. Deutsch. Geol. Gesellsch. xxiii. (1871) p. 58. 3 Quart. Jour. Geol. Soc. xxx. (1874). 
