CHAP. XXXVIII 
THE PLA TEA U OF SMALL ISLES 
219 
where the cliffs present the structure represented in Fig. 268. At the 
base, and passing under the level of the sea, lies the agglomerate (a) of 
a vent which will be described in Chapter xli., together with other eruptive 
orifices of the various plateaux (p. 288). This rock has a somewhat uneven 
upper surface which rises in places about 150 feet above high tide-mark. 
Here and there it shades off upward into the conglomerate that overlies it ; 
water-worn pebbles appear among its contents, and rude traces of bedding 
begin to show themselves, until, within the course of a few feet, we pass 
upward into an undoubted conglomerate. Elsewhere, however, and par- 
ticularly along the precipices west of Compass Hill, the two deposits are 
more distinctly marked off from each other. The agglomerate has there a 
hummocky, irregular upper surface, as if it had been thrown down in heaps. 
The hollows between these protuberances have been filled up with conglo- 
merate and sandstone, forming the base of the thick overlying deposit. 
It is thus clear that the loose materials of the vent were directly 
exposed at the surface when the conglomerate was accumulated, and, 
indeed, that these materials served to supply some of the detritus of which 
the conglomerate consists. The absence of any trace of a cone and crater 
at the vent may perhaps be explicable on the supposition that their inco- 
herent material was washed down by the currents that swept along and 
deposited the conglomerate. 
The mass of sedimentary material (6) which overlies the agglomerate 
of the vent forms a conspicuous feature along the lower half of the precipices 
at the eastern end of Canna. It rises to a height of 250 to 300 feet above 
sea-level, and must reach a maximum thickness of probably not less than 
100 to 150 feet. It gradually descends in a westward direction, both along 
the northern cliffs and in the lower ground round Canna Harbour, insomuch 
that in about a mile, owing to the gentle westerly dip of the whole volcanic 
series, combined with the effect of a number of small faults, it passes 
under the level of the sea. 
Great variation in the character of the detritus composing this thick 
group of strata may be observed as it is followed westward. On the cliffs 
below Compass Hill, as represented in Fig. 268, the coarse conglomerate 
with water -worn stones, hardly to be distinguished from the volcanic 
agglomerate of the vent, shows more or less distinct bedding, or at least 
a succession of coarser and finer bands. Towards its base it encloses 
numerous pieces of Torridon Sandstone, sometimes subangular, but often so 
well and smoothly rounded as to show that they must have been long sub- 
jected to the action of moving water. It is further observable that, while 
in the agglomerate the volcanic stones have rough surfaces, those in the 
conglomerate begin to show increasing evidence of attrition, until, as the 
deposit is traced upwards, they become almost as well rounded and water- 
worn as the non-volcanic stones. 
Yet amidst and overlying these proofs of transport from some little 
distance lie abundant huge slags and blocks of amygdaloidal lava, sometimes 
closely aggregated, sometimes scattered through a volcanic tuff or ashy sand- 
