CHAP. XI.IV 
THE GABBRO OF RUM 
35i 
island a thickness of some 10,000 feet. Tlieir red or rather pinkish tint 
seems mainly to arise from the pink felspar so abundant in them, for in 
many places they really consist of a kind of arkose. Pebbly bands with 
rounded pieces of quartz are of common occurrence throughout the whole 
formation. Dykes and veins of basalt are profusely abundant. Sometimes 
these run with the bedding, and might at a distance be taken for dark layers 
among the pink sandstones. They often also strike obliquely up the face of 
the cliffs like ribbons. 
But, notwithstanding their apparent continuity, there can he no doubt 
that these sandstones have suffered from those powerful terrestrial disturb- 
ances which have affected all the older rocks of the North-West Highlands. 
On the west side, where they plunge steeply into the sea, they have under- 
gone a change into line laminated rocks, which might at first be mistaken 
for shales, but which owe their fissility to shearing movements. Along their 
southern border, from a point on the east coast near Bagh-na-li-Uamha, south 
of Loch Scresort, to the head of Kilmory Glen, they are abruptly truncated 
against a group of dark, flaggy and fissile schists and fine quartzites or grits, 
which in some places are black and massive like basalt, and in others are 
associated with coarse grey gneiss. That some of these rocks are portions of 
the Lewisian series can hardly be doubted, and their structure and relations 
are probably repetitions of those between the Lewisian gneiss and Torridon 
sandstone of Sleat in Skye. I found also on the northern slopes of 
Glen Dibidil a patch of much altered grey and white limestone or marble, 
which reminded me of the Cambrian limestone of Skye. The red sandstones 
in a more or less altered condition are prolonged to the south-east promontory 
of the island. 
In passing over the zone of these more ancient rocks, we find them to 
present increasing signs of alteration as they are traced up the slopes towards 
the great central mass of erupted material. The pink sandstones gradually 
lose their characteristic tint, and grow much harder and more compact, while 
the veins and dykes of basalt and sheets of dolerite intersecting them increase 
in number. The zone of black compact quartzite, which lies to the south of 
the sandstones, and which at one point reminds us of basalt, at another of 
the flinty slate of the schistose series, likewise displays increasing induration. 
Its bedding, not always to be detected, is often vertical and crumpled. But 
the most remarkable point in its structure is the intercalation in it of bands 
of breccia. These vary from less than an inch to Several yards in diameter ; 
they run mostly with the bedding, but occasionally across it. The stones 
in them are fragments of the surrounding rock embedded in a matrix of the 
same material, but also with pieces of a somewhat coarser grit or quartzite. 
A band of coarse breccia forms the southern limit of this zone along the 
northern base of Barkeval and Allival. In general character it resembles 
the thinner seams of the same material just referred to. The matrix so 
closely agrees with the black flinty quartzite, that hut for the included stones 
it could hardly be distinguished ; so greatly has the mass been indurated 
that the stones seem to shade off into the rest of the rock. But here and 
