CHAP. XLII 
THE BASIC SILLS OF SKYE 
315 
inclined strata are cut through by many vertical basalt-dykes, some of which 
intersect each other, but among which by far the largest is the mass shown 
in the figure. This broad dyke consists of a dolerite or gabbro the largely 
crystalline texture of which marks it off at once from the others, which are 
of the usual dark, heavy, fine-grained type, with an occasional less basic and 
porphyritic variety. Traced up from the sea-margin, the dyke loses itself in 
a talus of blocks from the cliff above, so that its actual junction with the 
mural front of the sill cannot be seen. But that it joins that mass, with 
which it agrees in petrographical characters, hardly admits of question. 
The cliff consists of a thick sheet of coarsely crystalline dolerite or gabbro 
( d in Fig. 249), which in its general aspect at once recalls the rock of Fair 
Head. It varies considerably in texture, some parts of the mass are exceed- 
ingly coarse, like the Skye gabbros, and present a fibrous structure in their 
augite resembling that of the diallage in these rocks ; other portions assume 
the compactness of basalt. A specimen of medium grain under the micro- 
scope shows the typical ophitic structure so generally found among the 
dolerites both of the plateaux and of the intrusive sheets. This sill must 
be about 200 feet thick, and like the rock at Fair Head is traversed from 
top to bottom by joints that divide it into prisms. It appears to bifurcate 
eastward, one portion running with a tolerably uniform thickness of a few 
feet as a prominent band at the top of the shales and sandstones, the other 
slanting upwards and gradually thinning away in the granopliyre. 
Towards its base, near the contact with the underlying shales, the rock 
as usual becomes finer grained, and the thin band just referred to resembles 
in texture one of the wider basalt-dykes. Westwards the rock can be 
followed round the top of the grassy slopes formed by the decay of the shales. 
Though concealed by intervals of moorland and peat, it is visible in the 
stream sections, and I think must be continuous, as a band only a few 
yards thick, round the northern side of the hills as far as Beilin Bhuidhe, 
where a similar sill makes a prominent crag. Its total area measures a mile 
and a quarter in length by half a mile in breadth. The granopliyre which 
overlies it forms part of an interesting series of sheets which I have traced 
all the way from Suisnish to the braes above Skulamus. 
Whether or not the whole sheet of basic rock is continuous, and 
whether it all proceeded from the great Suisnish dyke, cannot be confidently 
decided until the ground is mapped in detail, though from the great thick- 
ness of the sill at the dyke, its attenuation outwards from that centre and 
its uniformity of petrographical character, I am disposed to answer affirma- 
tively. There is no other probable, vent to be seen in the neighbourhood, 
unless a massive dyke that runs from Loch Fada north-westwards into Glen 
Boreraig can be so regarded. 
Not far from the extreme southern point of Skye a singularly interesting 
example of a sill remains as a detached survival of the basaltic plateau and its 
accompaniments. In his map of Skye, Macculloch showed the position of 
this outlier, which he classed with the general “ trap ” formation of the 
island. The locality was visited by Professor Judd, who regarded the intrusive 
