166 DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



two groups of rock cannot be well examined there. But the basalts, which present their 

 ordinary characters to the north of the Inn, are observed to become more and more 

 indurated, close-grained, dull, and splintery as they draw nearer to the granophyre of 

 Marsco. This part of the district furnishes the clearest evidence of the posteriority of 

 the great cones of Glamaig and its neighbours to the plateau-basalts which come up to 

 the very base of these hills. 



Round the eastern group of cones some interesting fragments of the once continuous 

 sheet of basic rocks remain, to show the same relation of the acid protrusions on that side. 

 One of these lies on the granophyre of the flanks of Beinn na Caillich, a little to the west 

 of the loch at the northern base of that hill. Another of larger size forms a prominent 

 knob about three-quarters of a mile further west, and is prolonged into the huge dark 

 excrescence of Creagan Dubha, which rises in such striking contrast to the smooth red 

 declivities of the granophyre cones around it. This prominence at its eastern and 

 northern parts consists of highly indurated splintery basalt in distinct beds, some of 

 which are strongly amygdaloidal. The bedding is nearly vertical, but with an inclination 

 inwards to the hill. Towards the south-west end a thin band of basalt-breccia makes its 

 appearance between two beds of basalt. Its thickness rapidly increases southward until 

 it is the only rock adhering to the granophyre. Beyond the foot of the hill, Lower 

 Silurian limestone and quartzite occupy for some distance the bottom of Strath Beg, 

 much invaded by masses of quartz-porphyry. At the summit of Creagan Dubha 

 abundant veins run into the basic rocks from the granophyre, which, as usual, is finer 

 grained towards the margin ; and there are likewise veins of quartz-porphyry which, 

 though their actual connection with the main mass of granophyre cannot be seen, are no 

 doubt apophyses from it. 



This outlier of altered basalt and breccia appears to me to be a fragment of the 

 plateau-basalts which once overlay the Silurian rocks of Strath Beg, and were dis- 

 rupted by the uprise of the granophyre. It continues to adhere to the wall of the 

 eruptive mass that broke up and baked its rocks. Its breccia, passing southward into a 

 coarse agglomerate, is doubtless a product of the same vent that discharged the great 

 agglomerate mass above Kilbride and Kilchrist. I have already (p. 109) referred to 

 what appears to be another outlier of the basalts on the south side of Beinn Dearg. 



On the northern and southern flanks of Beinn na Cro, similar evidence may be 

 observed of the posteriority of the granophyre to the basic rocks. Round the northern 

 base of the hill a continuous tract of basalts, dolerites, and gabbros forms the ridge 

 between Strathmore and Strathbeg. There is an admirable section of the relation of the 

 two groups of rock on the eastern side of the western glen. Along the lower part of 

 the declivity, coarsely-crystalline gabbros, like some of those in the Cuillin Hills, are- 

 succeeded by sheets of dolerite and basalt, the whole forming an ascending succession of 

 beds to the summit of the ridge. The edges of these beds are obliquely truncated by the 

 body of granophyre, which slants up the hill across them and sends veins into them. They 

 are further traversed by basalt dykes, which here as almost everywhere abound (fig. 55). 



