CHAP. XXXVIII 
RIVERS OF THE PERIOD 
front comes the pile of bedded basalts (c) with their slaggy lower and upper 
surfaces. But as we follow them round the east side, we find them to be 
abruptly cut off by a mass of conglomerate (</). That the vertical junction- 
line is not a fault is speedily ascertained. The lower platform of slaggy 
basalt runs on unbroken under both shales and conglomerate. Moreover, 
the line of meeting of this conglomerate 
with the basalts that overlie the shales is 
not a clean-cut straight wall, but displays 
projections and recesses of the igneous rocks, 
round and into which the materials of the 
conglomerate have been deposited. The 
pebbles may be seen filling up little crevices, 
passing under overhanging ledges of the 
basalts, and sharply truncating lines of 
scoriaceous structure in these rocks. The 
same relations may be observed on the west fki. 275. 
front of the stack. There the ashy shales 
and tuffs are sharply cut out by the con- 
glomerate, which wraps round and underlies 
a projecting cornice of the slaggy bottom of the basalt that res 
stratified band (Fig. 275). 
The conglomerate is rudely stratified horizontally, its bedding being 
best shown by occasional partings of greenish sandstone. It consists of 
well-rounded, polished, and waterworn stones, chiefly of members of the 
volcanic series — basalts, and dolerites, both compact and amygdaloidal or 
•Enlarged. Section on tlie western 
side of Dun Beag. 
a, amygdaloid ; h, tuft'; c, ashy shales ; U, layer 
of coaly shale ; c , amygdaloidal ijasalts ; V 
conglomerate. m 
slaggy — but with a conspicuous admixture of Torridon Sandstone, gneiss, 
grey granite, grit and different schists. The coarsest part of the deposit 
lies toward the bottom where the volcanic blocks, some of them being six 
and eight feet in diameter, may have originally fallen from the basalts 
against which the conglomerate now reposes. The far-transported stones 
are also of considerable size, pieces of granite and gneiss frequently 
exceeding a foot in length. The well-rounded pebbles of foreign materials 
have been washed into the interstices between the large volcanic blocks. 
It is, I think, tolerably clear that the wall of basalt against which this 
conglomerate has been laid down is one of erosion. The beds of basalt have 
here been trenched by some agent which has likewise scooped out the soft 
underlying shales, and even cut them away from under their protecting 
cover of basalt. There can be little hesitation in regarding this agent as a 
watercourse, which for some considerable interval of time continued to dig 
its channel through the hard basalts. There is not room enough between 
the basalt-wall of Dim Beag and the opposite cliffs of the shore (where no 
trace of this conglomerate is to be seen) for any large stream to have found 
its way. I do not therefore seek to identify this relic of an ancient water- 
way with the channel of the main river which deposited the conglomerate 
bands of Ganna and Sanday. More probably it was either a mere torrential 
chasm, or a tributary stream draining a certain part of the volcanic plateau 
