1. No evidence exists to show that the 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 pyroclastic material, though masses of agglomerate are enclosed in, and have probably been invaded by, the gabbro of the Cuillin Hills. If the gabbro-bosses ever were continuous 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 depth beneath the surface. The most coarsely-crystalline varieties, and those with the best developed banded structure, occur in the largest bodies of rock, where the cooling and consolidation would be most prolonged.[356]
[356] On this subject, see the papers by Professor Judd already cited.
3. The remarkable differences in composition between the dark and pale layers in the banded gabbros cannot be accounted for by segregation or successive intrusion, but seem to point to the existence of a heterogeneous magma from which these distinct varieties of material were simultaneously intruded.
4. From the prevalence of a bedded structure and the occurrence of bands and more irregular portions of considerably different texture and even mineralogical composition which intersect each other, it may be confidently inferred that even what appears now as one continuous mass was produced by more than one intrusion.
5. In every case there would necessarily be one or more pipes up which the igneous material rose. These channels might sometimes be wider parts of fissures, such as those filled by the dykes. In other places, they may have been determined by older vents, which had 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, as will be pointed out in the following chapters. In the case of the gabbros, however, the position of the vents seems to have been generally concealed by the tendency of these rocks to spread out laterally. Denudation has cut deeply into the gabbro-masses, but apparently not deep enough to isolate any of the pipes from the larger bodies of material which issued from them, and thus 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 lies above the true pipe or pipes, and has spread out on all sides from the centre of eruption. The prevalence of rude bedding and a banded structure indicate that most of the visible rock occurs in the form of sills, successively injected not only into the plateau-basalts, but between and across each other. Round the margin of the gabbro we undoubtedly reach horizons below that rock, and see that it lies as a cake or series of cakes upon the plateau-basalts. The actual pipe or fissure of supply must in each case lie further inward, away from the margin, and may be of comparatively small diameter.
6. From the central pipe or group of pipes or fissures 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 near the base of the basalts at Loch Scavaig; that is to say, their lowest members lie at least 1600 feet below those on the opposite margin.
7. 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 side being pushed up further or with a steeper slope than another side. But even in the case of the Cuillin Hill area, it is conceivable that the total uplift produced at the surface a gentle inclination of no more than 8° or 10°.
It is along the periphery of a gabbro area that we may most hopefully search for traces of this uplift. But unfortunately it is just there that the work of denudation has been most destructive. There appears also to have been a general tendency to sagging subsequent to the gabbro protrusions, and the inward dip thereby produced has probably been instrumental in effacing at least the more gentle outward inclinations caused by the uprise of the eruptive rock. In one striking locality, however, to which I have already referred, the effects of both movements are, I think, preserved. The basalt-plateau of Strathaird, which in its southern portion exhibits the ordinary nearly level bedding, dips in its northern part at an unusually steep angle to the north-west, towards the gabbro mass of Blath Bheinn. But before reaching that mountain the basalts, much interbanded with sheets of dolerite and gabbro, suddenly bend up to form the prominent eminence of An Stac, where they dip rapidly towards south-east and south ([Fig. 334]). This steep dip away from the central mass of gabbro, is repeated in the hills to the north, where the beds are inclined to north-east, the angle gradually lessening northwards till they are truncated by the granophyre of Strathmore. The mass of Blath Bheinn thus occupies the centre of the dome or anticline. The theoretical structure of one of the gabbro bosses is represented in [Fig. 343]. It will be understood, however, that what for the sake of clearness is here represented in one uniform tint of black in reality consists of an exceedingly complex network of sheets and dykes differing from each other in texture and structure, as well as in the relative dates of their intrusion.
8. The injection of so much igneous material among the bedded basalts has induced in these rocks a certain amount of contact metamorphism. I have referred to it as showing itself in the field as a marked induration, the rocks becoming closer grained, dull, splintery, and weathering, with a grey or white crust, while their amygdales lose their definite outlines, and epidote and calcite run in strings, veins and patches through many parts of the rocks. As already remarked, it is difficult to determine how much of this change should be referred to the influence of the gabbro, and how much to that of the numerous intrusions of granophyre which may be apophyses of much larger bodies of that rock lying not far underneath. On account of this difficulty, the more detailed description of the metamorphism of the plateau-basalts is reserved for Chapter xlvi., where it will find a place in connection with the effects produced by the intruded granophyres, which have undoubtedly been more extensive than those effected by the gabbros.