344 
THE TERTIARY VOLCANOES 
BOOK VIII 
actual plication of a group of successive bands, as shown in Fig. 337, 
wherein such a group about ten feet thick is shown to have been 
doubly folded between parallel bands above and below. This structure 
is not due to any deformation of the gabbro long subsequent to the 
consolidation of the mass. It belongs to the phenomena ol protrusion 
and solidification. An examination of thin slices of these rocks under the 
microscope reveals no evidence of crushing. On the contrary, the minerals 
of one band interlock with those of the band adjoining, in such a manner as 
to prove that the differences of composition cannot be due to crushing and 
shearing or to successive intrusion, but must have been present before the 
final consolidation of the whole rock. 1 
The conclusion which seems most consonant with the facts is that the 
magma which supplied the visible masses of gabbro in Skye existed below 
in a heterogeneous condition, that portions of it, differing considerably from 
each other in composition, were simultaneously intruded, and that by the 
deformation of these portions during their intrusion their present plicated 
structures were produced. A careful study of these banded gabbros otters 
many suggestive points of comparison with the gneisses and anorthosite 
(Norian) rocks of pre-Cambrian age. It seems in the highest degree prob- 
able that the banded structures and peculiar mineral aggregation in these 
ancient rocks arose under conditions closely analogous to, it not identical 
with, those in which the Tertiary gabbros of Skye originated. 2 
Similar structures are found to be widely developed through the 
gabbros of the Cuillin Hills. Not only are these rocks disposed in distinct 
beds, but many of the beds display the most perfect banding. Thus the 
mountains that surround the head of Loch Scavaig and sweep round Loch 
Coruisk up to the great splintered crests of Sgurr na Banachdich display 
on their bare black crags a distinct bedded structure. On the east side of 
Loch Scavaig the rock presents a rudely -banded character, the bands or 
beds being piled over each other from the sea-level up to the summits of the 
rugged precipices, and dipping into the hill at angles of 25° to 35 . Abun- 
dant dykes and veins of various basic, intermediate and acid rocks cut this 
structure. The individual layers here show sometimes the wavy and 
puckered condition already referred to. 
Even from a distance the alternating lighter and darker bands can 
readily be seen, so that this structure, with the variations in its inclination, can 
be followed from hill to hill (Fig. 338). The regularity of the arrangement, 
however, is often less pronounced on closer inspection. While the gabbro is 
rudely disposed in thick beds, indicative ol different intrusive sheets or sills, 
with which the banding is generally parallel, considerable irregularities may 
be observed in the arrangement of the structure ol individual sheets. These 
sheets may be parallel to each other, and yet, while in some the banding is 
tolerably regular in the direction of the planes of the sheets, in others it is 
much twisted or inclined at various angles. 
1 Mr. J. J. H. Teall and A. G., Quart. Journ. Geol. Soc. vol. 1. (1894), p. 652. 
2 Consult the Memoirs cited in the footnote on p. 342. 
