CHAP. XLIV 
THE GABBRO OF RUM 
353 
Immediately above this belt of felsitic protrusions comes the great body of 
gabbro. It will be observed that here, as in Skye, the base of the gabbro 
mass presents a horizon on which injections of add rocks have been particu- 
larly abundant. Whether the breccias be regarded as the result of earlier 
rock-crushings, or as due to volcanic explosions during the Tertiary period, 
they are evidently older than the eruption of the gabbros. In that respect 
they may be compared with the agglomerates through which the youngest 
eruptive bosses of Skye have made their way ; but their component materials 
have been derived from the surrounding platform of ancient rocks, and not 
from subterranean lavas. 
For my present purpose, however, the chief point of importance is the 
structure of the gabbro mass that springs from that platform into the great 
conical hills of Eum. The accompanying sketch (Fig. 340) will convey a 
better idea of this structure than a mere description. At the base, 
immediately above the felsite just referred to, bedded dolerites make their 
appearance, much intersected with veins from the siliceous rock. Veins and 
Fig. 341. — Section of foliated gabbros in the Tertiary volcanic series of Allival, Rum. 
a, massive gabbro with rude lamination parallel to bedding, only seen in some weathered surfaces ; b, laminated 
troctolite ; e, massive coarsely crystalline gabbro rudely laminated. 
dykes of basalt also cut all the rocks here, the newest being those which run 
in a north-west direction. The lowest sheets of dolerite are succeeded by 
overlying sills of coarser dolerites, gabbros, troctolites, etc., which are as regular 
in their thickness and continuity as the ordinary basalts of the plateaux. The 
band of light-coloured troctolite, in particular (Fig. 341), about 20 to 30 feet 
thick, which has been already referred to for its remarkable laminar struc- 
ture, can be followed for some distance along the base of the hill as a 
marked projecting escarpment. This rock at once arrests attention by its 
platy or fissile structure, parallel to the bedding -surfaces of the sheet. 
Indeed hand-specimens of it, as I have said, might readily pass for pieces of 
schistose limestone, especially if taken from the upper part. It consists of 
successive layers, which on the weathered surface divide it into beds almost 
as regular as those of a flagstone, each bed being further separated into 
laminae marked off by the darker and lighter tints of their mineral constitu- 
ents. The darker layers consist of olivine, and the lighter of plagioclase. 
This segregation here and there takes the form of rounded masses, where the 
minerals are more indefinitely gathered together. The affinity of the 
rock with intrusive sheets is further displayed by the occurrence of abundant 
VOL. ii 2 a 
