360 
THE TERTIARY VOLCANOES 
BOOK VIII 
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 crystal- 
line 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. 1 
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 
tor the emission of the plateau- basalts and their pyroclastic accom- 
paniments. 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. Bound 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 
1 On this subject, see the papers by Professor Judd already cited. 
