CHAP. XXXVI 
NATURE OF THE FRAGMENTAL ROCKS 
195 
agglomerates contain abundant detritus of a basic minutely cellular 
pumice. 
( b ) Volcanic Conglomerates and Breccias in beds intercalated between the 
floivs of Basalt. — These are of at least three kinds, (a) Basalt-conglomerates, 
composed mainly of rounded and subangular blocks of basalt (or allied basic 
lava), sometimes a yard or more in diameter, not unfrequently in the form 
of pieces of rough slag or even of true bombs, imbedded in a granular 
matrix of comminuted basalt-debris. In some cases, the stones form by far 
the most abundant constituents of the rock, which then resembles some of 
the coarse agglomerates just described. Perhaps the most remarkable accumu- 
lations of this kind are those intercalated among the basalts in the islands 
of Cantia and Sunday, of which a detailed account will be given in Chapter 
xxxviii. These conglomerates, besides their volcanic materials, contain rounded 
blocks of Torridon sandstone and other rocks, which must have been carried 
iroin the east by some tolerably powerful river that flowed across the basalt- 
plains during the volcanic period. Again, on the east side of Mull, the 
s l a ggy basalts of Beinn Chreagach Mhor are occasionally separated by vol- 
canic conglomerates. As a rule, however, such intercalations are seldom 
more than a few feet or yards in thickness. Their coarseness and repetition 
on successive horizons indicate that they probably accumulated in the near 
neighbourhood of one or more small vents, from which discharges of fragment- 
ary materials took place at the beginning or at the close of an outflow of lava, 
and that the stones were sometimes swept away from the cones and rolled 
about by streams before being buried under the succeeding lava -sheets. 
More commonly the dirty-green or dark-brown granular matrix exceeds in 
bulk the stones embedded in it. It has obviously been derived mainly from 
the trituration of already cooled basalt - masses, and probably also from 
explosions of the still molten rock in the vents. A striking illustration of 
this type of rock may be seen on the south side of Portree Harbour, where 
a mass of dark-green basalt-conglomerate, with a coaly layer above it, lies 
near the base of the bedded basalts, and attains at one part of its course a 
thickness of about 200 feet. This rock will be again referred to in con- 
nection with the vent from which its materials were probably derived. As 
m the case of the agglomerates of the vents, pieces of older acid lavas, and 
‘ s t ] ll more of the non-volcanic rocks that underlie the plateaux, are found in 
the bedded conglomerates and breccias. In Antrim and Mull, for instance, 
fragments of flint and chalk are of common occurrence. A characteristic 
e xaniple of this kind of rock forms the platform of the columnar bed 
"ut of which Fingal’s Cave, Staffa, has been excavated (Fig. 266a). 
(/3) Felsitic Breccia. — This variety, though of rare occurrence, is to be 
Seen in a number of localities in the island of Mull. It is composed in 
great measure of angular fragments of close-grained flinty felsitic or rhyolitic 
recks, sometimes showing beautiful flow-structure, together with pieces of 
fluartzite and amygdaloidal basalt, the dull dirty-green matrix appearing to 
he made up chiefly of basalt-dust. 
(y) Rhyolitic Conglomerate. — Between the upper and lower group of 
