156 Dll'GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



they seem to show that here as elsewhere passages for the uprise of the intrusive 

 rock were already provided by the presence of volcanic pipes, which, even if filled up 

 with fragmentary materials, would no doubt continue to be points of weakness. 

 Round the flanks of the Loch Ba' boss, and here and there on its surface, patches 

 of intensely indurated volcanic agglomerate may be detected. A little to the south 

 of the tarn called Loch na Dairidh, the granophyre is succeeded by the black flinty 

 felsite already referred to. This rock in some places exhibits a beautiful flow- 

 structure, with large porphyritic felspars, and incloses a great many fragments of 

 dolerite and gabbro, varying from the size of a pea up to blocks several inches in 

 diameter. Lying on its surface are detached knolls of much altered dolerite, basalt, 

 and coarse breccia or agglomerate. On its southern margin one of these patches of 

 agglomerate contains abundant fragments of various felsitic rocks, among which are 

 pieces of a compact rock with flow-structure like that found in place immediately to 

 the north ; also rounded pieces of quartzite, and of compact and amygdaloidal basalt 

 wrapped up in a very hard matrix which seems to consist largely of basalt-dust. No 

 bedding can be made out in this rock, and the mass looks like part of a true neck. 

 Further down the slope the bedded basalts appear. The actual junctions of the different 



e 



Fig. 47. — Section to south of Loch na Dairidh, Mull, a, basalts ; b, dolerites ; c, volcanic agglomerate ; 



d, black felsite ; c, granophyre. 



rocks cannot be satisfactorily traced, but the structure of the ground appears to me to 

 be as shown in fig. 47. A patch of similar agglomerate appears a little to the south- 

 west of the last section in front of a cliff of the felsite, and seems to be enclosed in 

 the latter rock, and other exposures of agglomerate, underlain and intensely indurated 

 by the felsite, may be noticed on the ground that slopes towards Loch Ba'. 



That these agglomerates do not belong to the period of the eruption of the grano- 

 phyre and felsite, but to that of the bedded basalts, may be inferred from their intense 

 induration next the acid rocks, and also from the fact that similar breccias are actually 

 found here interposed between the bedded basalts. This is well shown on the hill above 

 the Coille na Srbine, where the accompanying section can be seen (fig. 48). The broad 

 dyke-like mass of black flinty felsite already referred to runs as a prominent rib over the 

 southern end of Beinn a' Chraig into the head of the Scarrisdale glen (see fig. 43). It 

 cuts across the bedded basalts, and immediately to the south of where these appear, a thin 

 intercalated bed of breccia crops out, of the usual dull-green colour, with abundant frag- 

 ments of basalt and many of yellow and grey felsite. 



From these various facts we may, I think, conclude that along the strip of ground now 

 occupied by the Loch Ba' boss of granophyre and felsite, there once stood a line or 



