CHAP. XL1V 
HISTORY OF THE GABBRO INTRUSIONS 
359 
a close parallel to the gabbros and associated rocks of Skye. But in one 
important respect they stand alone. No certain trace remains of any 
basalt-plateau at St. Kilda sucli as those through which the gabbros of Skye, 
Mull and Ardnamurchan have been injected. In regard to their mode of pro- 
duction they have doubtless been intruded at some considerable depth beneath 
the surface. But no relic appears to have survived of the overlying cover of 
rock under which they consolidated, and into which they were injected. 
In the remarkable volcanic district of the north-east of Ireland a series 
of basic rocks appears, which in its mode of occurrence and its relation to 
the other members of the series presents many points of resemblance to the 
gabbros of the Inner Hebrides. Tire Irish gabbros are well developed in the 
Carliugford district, where they form intrusive bosses and sheets which have 
been erupted through the Palaeozoic rocks (Map YII.). They are themselves 
pierced by later masses of granophyrc and other acid rocks. Further refer- 
ence will be made to these gabbros in later pages, where an account will be 
given of the granite masses of Mourne, Barnavave and Slieve Gullion. 
It is interesting to observe that, while in St. Kilda no relic of any 
basaltic plateau has been preserved, in the Faroe Islands, on the other hand, 
no sign has been revealed by denudation that the volcanic plateau of that 
region is pierced by any eruptive core of gabbro or of granophyre. During 
my cruises round these islands and through their channels, I was ever on 
the outlook for any difference in topography that might indicate the presence 
of some eruptive boss like the gabbro and granophyre masses of the Inner 
Hebrides. But nothing of that nature could be discerned. Everywhere the 
long level lines of the bedded basalts were seen mounting up to the crests of 
the ridges and the tops of the highest peaks. Though I cannot assert that 
no intrusions of gabbro or of granophyre exist among the laroe Islands, I 
feel confident that any such masses which . may appear at the surface must 
be of quite insignificant dimensions, and do not make the important feature 
in geology and topography which they do among the Inner Hebrides. It 
is, of course, possible that, vast as the denudation of these islands has 
undoubtedly been, it has not yet trenched the plateau deeply enough to 
expose any great intrusive bosses and sills which may underlie and invade 
the basalts. 
iv. HISTORY OF THE GABBRO INTRUSIONS 
We are now in a position to draw, from the observations which have 
been given in this and the preceding chapter regarding the different areas 
of gabbro in the Tertiary volcanic region of Britain, some general conclusions 
with respect to the type of geological structure and the phases of volcanic 
energy which they illustrate. 
1. No evidence exists to show that the masses of gabbro ever communi- 
cated 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 
