134 DR OEIKIE ON THE HISTORY OF VOLCANIC ACTION 



Along their southern border, from a point on the east coast near Bagh-na-h-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 Archaean series can hardly be doubted, and the vertical 

 separations and apparent transitions are probably repetitions of the faults and thrust- 

 planes of the north-west. 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 Lower 

 Silurian limestone of Skye. 



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 imbedded 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 but for the included stones it could 

 hardly be distinguished; so greatly has the mass been indurated that the stones seems to 

 shade off into the rest of the rock. But here and there its true brecciated nature is 

 conspicuously revealed by prominent blocks of hardened sandstone. This band of breccia 

 must in some places be 150 or 200 feet broad. It has no distinct bedding, but seems to 

 lie as a highly inclined bed dipping into the hill. It is at once succeeded by a black 

 flinty felsite like that of Mull. The ground-mass of this rock, so thickly powdered with 

 magnetite grains as to be almost opaque under the microscope, displays good flow- 

 structure round the turbid crystals of orthoclase and the clear granules of quartz. 

 Further up the hill, the rock becomes lighter in colour and less flinty in texture — a 

 change which is found to arise from more complete devitrification, the ground-mass 

 having become a crystalline granular aggregate of quartz and felspar with scattered 

 porphyritic crystals of these minerals (microgranite). In some places, the felsite incloses 

 fragments of other rocks. A specimen of this kind, taken from the head of Coire Dubh, 

 shows under the microscope a brown microfelsitic ground-mass, with crystals of felspar 

 and augite, inclosing a piece of basalt, composed of fine laths of plagioclase, abundant 

 magnetite, and a smaller proportion of granules of augite. 



