DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 141 



served for the emission of the plateau-basalts and their pyroclastic accompaniments. 

 There can be no doubt that some of these vents afforded egress for the subsequent eruption 

 of granitoid rocks. In the case of the gabbros, however, they seem to have been generally 

 concealed by the tendency of these rocks to spread out laterally. Denudation has cut 

 deeply into the gabbro-masses, but not deep enough to isolate any of the pipes from the 

 material which issued from them, so as to leave solitary necks like those in and around 

 the basalt-plateaux. In Skye, where the central core of gabbro is largest and most 

 completely encircled, we cannot tell how much of it which is amorphous and resembles 

 what might be supposed to be the material filling the actual vent, really lies above the 

 true pipe or pipes, and has spread out on all sides from the centre of eruption. All that 

 we know is that round the margin of the gabbro we can reach horizons below that rock, 

 and see that it lies as a cake or series of cakes upon the plateau-basalts. The actual pipe 

 of supply must lie further inward, away from the margin, and may be of comparatively 

 small diameter. 



5. From the central pipe or group of pipes which rose from the platform of older 

 rocks into the thick mass of the basalt-plateaux, successive sheets of dolerite and gabbro 

 were forced outward between the layers of basalt. This took place all round the orifices 

 of supply, on many different horizons, and doubtless at many different times. In some 

 cases, the intrusive sheets were injected into the very bottom of the basalts, and even 

 between these rocks and the older surface on which they rested. This is particularly the 

 case in Rum, where the gabbro-cones spring almost directly from the ancient grits, schists, 

 and sandstones on which they rest. The intrusive sheets have likewise found egress at 

 every higher platform in the basalt-series, up at least to the base of the pale group in 

 Mull — that is, through a continuous pile of more than 2000 feet of bedded basalt. But 

 the intrusion did not proceed equally all round an orifice. At all events, the progress of 

 denudation has revealed that on one side of a gabbro area the injected portions may occur 

 on a lower stratigraphical level than they do on the opposite side. At the Cuillin Hills, 

 for example, the visible sheets of dolerite and gabbro to the north of Coire na Creiche 

 begin about 1600 feet above the sea, which must be much more than that distance above 

 the bottom of the basalts. On the south-east side, however, they come down to the 

 Torridon sandstone at Loch Scavaig ; that is to say, their lowest members lie about 

 the base of the bedded basalts, or more than 1600 feet below those on the opposite 

 margin. 



6. The uprise of so much igneous material in one or more funnels, and its injection 

 between the beds of plateau-basalt, would necessarily elevate the surface of the ground 

 immediately above, even if we believe that surface to have been eventually disrupted and 

 superficial discharges to have been established. If no disruption took place, then the ground 

 would probably be upraised into a smooth dome, the older lavas being bent up over the 

 cone of injected gabbro until the portion of the plateau so pushed upward had risen some 

 hundreds of feet above the surrounding country. The amount of elevation, which would 

 of course be greatest at the centre of the dome, might be far from equable all round, one 



VOL. XXXV. PAET 2. T 



