140 DR GEIKIE ON THE HISTORY OE VOLCANIC ACTION 



between Loch Spelve and Fislinish Bay. Though obviously intrusive, they lie on the whole 

 parallel to the bedding of the basalts. The latter rocks exhibit the usual dull indurated 

 shattery character which they assume where the gabbro has invaded them, and which 

 gradually disappears as we follow them down hill away from the intrusive sheets to the 

 shores of the Sound. They dip towards the centre of the hill group, that is to S. W. in the 

 ridge of Mainnir nam Fiadh, Dun da Ghaoithe, and Beinn Meadhon, the angle increasing 

 southwards to 15°-20°, and at the south end reaching as much as 35°-40°. Some fine 

 crags of gabbro and dolerite form a prominent spur on the east side of the ridge of Ben 

 Talaidh, in the upper part of Glen Forsa. These consist of successive sheets bedded with 

 the basalts, and dipping S.W. A large sheet stands out conspicuously on the north front 

 of Ben More, lying at the base of the " pale lavas," and immediately above the ordinary 

 basalts, and circles round the fine corry between Ben More and A'Chioch, some of its domes 

 being there beautifully ice-worn. This is the highest platform to which I was able satis- 

 factorily to trace any of the intrusive sheets of Mull. Another dyke-like mass emerges 

 from beneath the talus slopes of A'Chioch, on the southern side, and runs eastward across 

 the col between the Clachaig Glen and Loch Scridain. 



§ 3. Structure of the Gabbro Areas. 



We are now in a position to draw, from the observations which have here been given 

 regarding the different areas of gabbro in the Tertiary volcanic region of Britain, some 

 general conclusions with respect to the type of geological structure which they illustrate. 



1. No evidence exists to show that the extruded masses of gabbro ever communicated 

 directly with the surface. They never exhibit the cellular, slaggy and other structures so 

 characteristic of surface-flows. They are, on the whole, free from included masses of 

 breccia and agglomerate, though portions of such rocks have been detected among the 

 boulders derived from one part of the Cuillin Hills. If the gabbro-bosses ever were con- 

 tinuous with sheets of rock emitted above ground, all such upward continuations have 

 been entirely removed. In any case, we may be quite certain that in an outburst at 

 the surface the rock would not have appeared in the form of a coarsely crystalline or 

 granitoid gabbro. 



2. The crystalline structures of the gabbros point unmistakably to slow cooling and 

 consolidation at some considerable depth beneath the surface. The most coarsely- 

 crystalline varieties, and those with the best developed segregation-veins, occur in the 

 largest bodies of rock, where the cooling and consolidation would be most prolonged.* 



3. From the occurrence of bands and more irregular portions of considerably different 

 texture and even mineralogical comj)osition, it may be confidently inferred that even 

 what appears now as one continuous mass was produced by more than one eruption. 



4. In every case there would necessarily be one or more pipes up which the igneous 

 materia] rose. These channels might be supplied by wider parts of fissures, such as those 

 filled by the dykes. But more probably they were determined by older vents, which had 



* On this subject, see the papers by Professor Judd already cited. 



