CHAP. XLVIII 
THE ACID SILLS 
431 
the great gabbro mass of Ben Buy similar protrusions occur. But as we 
retire from the mountainous tract into tlie undisturbed basalts of the plateau, 
these acid intercalations gradually disappear. 
In the islands of Eigg and Bum, excellent examples occur of the 
tendency which the sheets of porphyry or granophyre manifest to appear at 
or about the base of the bedded basalts. I have already alluded to 
the boss or sheet at the north end of the former island. A still 
more striking illustration occurs in Bum. All along the base of the great 
mass of gabbro, protrusions of various kinds of acid rock have taken place. 
The great mass of Orval, already described, is one of these. Below Barkeval 
and round the foot of the hills to the south-east of that eminence an inter- 
rupted band of quartz-porphyry may be traced, from which veins proceed 
into the gabbros and dolerites. 
But it is in Skye and Baasay that the intrusive sheets of the acid group 
of rocks reach their chief development. They have been most abundantly 
injected underneath the bedded basalts, particularly among the Jurassic 
strata. A band or belt of them, though not continuous, can be traced round 
the east side of the main body of granophyre, at a distance of from a mile 
and a half to about three miles. Beginning near the point of Suisnish, 
this belt curves through the hilly ground for some five miles, until it dies 
out on the slopes above Skulamus. It may be found again on the west 
side of the ridge of Beinu Suardal, and on the moors above Corry, till it 
reaches the shore at the Budh’ an Eireannich (Irishman’s Point). It skirts 
the west side of Scalpa Island, and runs for some miles through Baasay. 
Another series of sills occurs below the basalts and gabbros in the Blaven 
group of hills. 
Over a large part of their course, the rocks of the eastern belt rest in great 
overlying sheets upon the Jurassic strata, which may almost everywhere be 
seen dipping under them. From the analogy of other districts, we may, I 
think, infer that the position of these sills here points to their having been in- 
truded at the base of the plateau-basalts which have since been removed 
from almost the whole tract. Fortunately, a portion of the basalts remains in 
Baasay, and enables us to connect that island with the great plateau of Skye 
of which it once formed a part. There can be no doubt that the basalts 
of the Dun Caan ridge once extended westwards across the tract of granophyre 
which now forms most of the surface between that ridge and the Sound of 
Baasay. A thin sheet of quartz-porphyry, interposed among the Oolitic 
strata, may be seen a little inland from the top of the great eastern cliff and 
below the position of the bedded basalts. 
The great sheet, or rather series of sheets, which stretches north- 
eastwards from Suisnish at the mouth of Loch Eishort in Skye, consists of a 
rock which for the most part may readily be distinguished in the field from 
the granitoid material of the bosses. It appears to the naked eye to be a 
rather close-grained or finely crystalline-granular quartz-porphyry, with 
scattered blebs or bi-pyramidal crystals of quartz and crystals of orthoclase. 
At the contact with adjacent rocks, the texture becomes more felsitic, some- 
