CHAP. XLIII 
THE GAB BROS OF SKYE 
33 7 
enclose it, the bedded basalts mount from the bottom of the valley, with 
their lines of parallel terrace dipping gently inward below the black rugged 
gabbro that crowns them and sweeps round to form the back or head of the 
corry. Down the whole length of Glen Brittle the same structure con- 
spicuously governs the topographical features. On the right hand, the 
ordinary terraced basalts form the slopes; and they rise for some 500 or 
600 feet up the eastern side, until they pass under the darker, more rugged, 
and less distinctly bedded rocks of the mountains (Fig. 332). The dip of 
the whole series is here at a gentle angle towards south-east, that is, into or 
under the main mass of the Cuillin group. 
When, however, we proceed to examine the junction between the two 
rocks we find it to be less simple than it appears. It is not an instance of 
mere superposition. The gabbro unquestionably overlies the basalts, and is 
therefore of younger date. But it overlies them, not as they rest on each 
other, in regular conformable sequence of eruption, but intrusively, as a sill 
does upon the rocks on which it appears to follow in the unbroken order of 
accumulation. This important structure may be ascertained in almost any 
of the many sections cut by the torrents which have so deeply trenched 
with gullies the flanks of the hills. Starting from the ordinary bedded 
basalts, we observe, in mounting the slopes and approaching the gabbro, that 
the rocks insensibly assume that indurated shattery character, which has 
been referred to as characteristic of them round the margins of vents, and 
which will be shown to be not less so in contact with large eruptive masses 
of basic or acid rock. 1 Beds of dolerite make their appearance among the 
basalts, so distinctly crystalline, and so similar in character to the rocks of 
the sills, that there can be little hesitation in regarding them as intrusive. 
These sills increase in size and number as we ascend, though hardened 
amygdaloidal basalts may still be observed. True gabbros then supervene 
in massive beds, and at last we find ourselves entirely within the gabbro 
area, where, however, thin bands of highly altered basalt may still for some 
distance appear. One further fact will generally be noticed, viz. that before 
reaching the main mass of gabbro, veins and sills of basalt, as well as of 
various felsitic and porphyritic members of the acid group, come in abun- 
dantly, crossing and recrossing each other in the most intricate network. 
The base of the thick gabbro-sheets is thus another horizon on which, as on 
that below the plateau-basalts, intrusive masses have been especially de- 
veloped. Through all these rocks numerous parallel basalt-dykes, running 
in a general persistent X.N.W. direction, with a later N.E. series, rise from 
below the sea-level up even to the very crests of the Cuillins (Fig. 333). 
The sections on the western side of the gabbro area of Skye thus prove 
that this rock inosculates with the bedded basalts by sending into them, 
between their bedding planes, sheets which vary in texture from fine dolerites 
1 This indurated, altered character of the bedded basalts near the intrusive bosses and sills 
"'ill be more particularly described in a later chapter in connection with the granophyre in- 
trusions (see p. 386). The metamorphism induced by the basic rocks has generally been less 
pronounced than that effected by the acid masses. 
VOL. II 
Z 
