CHAP. XXXVIII 
THE PLATEAU OF SMALL ISLES 
22 I 
Hill. It is well seen in the ravine above the Coroghon, but cannot be 
followed further westward among the basalt -terraces. Yet, though this 
stratified intercalation is not traceable far as a band of conglomerate, the 
same stratigraphical horizon is probably indicated elsewhere by other kinds of 
sedimentary deposits, to which further reference will be made in the sequel. 
The section now described establishes the existence of at least two 
successive platforms of conglomerate in the volcanic series. Following 
these platforms along their outcrop, we obtain additional light on their 
origin, and on the topographical conditions under which they were de- 
posited, and we learn further that other prolonged intervals, which were 
likewise marked by intercalations of sedimentary material, occurred in the 
outpouring of the basalts. 
Taking first the lower conglomerate of Compass Hill and tracing it 
westward, we find it to form the depression in which the sheltered inlet of 
Canna Harbour lies. It is exposed along the shores and also in the islands 
enclosed within the same bay. But it is not traceable further west, possibly 
because it seems to sink beneath the level of the sea. To the south-east, 
though it is there likewise for the most part concealed under the waves, it 
rises above them in one or two parts of the coast-line of Sunday, particularly 
at the TJamh Buadh or Red Cave, and likewise on a surf-beaten skerry off 
Ceann an Eilein, the highest part of the Sunday cliffs — a distance of about 
a mile and a half from Compass Hill. Throughout this space it retains its 
remarkably coarse character and is mainly made up of volcanic material. 
The numerous sections exposed in Canna Harbour enable us to study 
the composition and local variations of this curious deposit. On the north 
side of the basin, while the lower part of the sedimentary series continues 
to be an exceedingly coarse volcanic conglomerate, it passes upward into 
finer conglomerates, tuffs, and shales. In front of Canna House the 
imbedded blocks are of large size, occasionally as much as three or lour 
feet in diameter. They are still more gigantic on the island of Eilean a’ 
Bhaird, where I found one to contain 150 cubic feet in the exposed part, the 
rest being still concealed in the matrix. As they are generally somewhat 
rounded, here and there markedly so, most of these stones have probably 
undergone a certain amount of attrition in water. The great majority 
of them, and certainly all those of larger size, are pieces of basalt, 
dolerite, andesite, etc. Among them huge blocks of amygdaloid and 
coarsely vesicular lava are specially abundant. Some of these look like 
pieces of slag torn from the upper surface of lava-streams. Others, display- 
ing a highly vesicular centre and a close-grained outer crust, are suggestive 
of bombs. It is interesting to note here again that the amygdaloidal blocks 
present their zeolitic infiltrations so precisely like those of the amygdaloids 
of the plateau that it seems reasonable to suppose the carbonate of lime, 
zeolites, etc. to have been introduced before the blocks were imbedded in the 
conglomerate. 
The whole aspect of this deposit is eminently volcanic. It looks like a 
vast sheet of lava-fragments swept away from one or more cones of slags and 
