230 
THE TER TIA R V VOLCANOES 
BOOK VII 
need not have been derived from the action of atmospheric waste on the 
lava-fields, hut might quite well have been mainly supplied by the demoli- 
tion of volcanic cones of fragmental materials. 
That such has really been the chief source of the blocks in the con- 
glomerates I cannot doubt. At the east end of Canna we actually detect a 
volcanic cone, partly washed down and overlain by a pile of river-shingle. 
There were probably many such mounds of slag and stones along lines of 
fissure all over the lava -fields. The river in its winding course might come 
upon one cone after another, and during times of flood, or when its waters 
Fig. 272. — View of the Dun Beag, Sanclay, seen from the south. 
(From a Photograph by Miss Thom.) 
burst through any temporary barrier created by volcanic operations it would 
attack the slopes of loose material and sweep their detritus onward. At the 
same time, the current would carry forward its own natural burden of far- 
transported sediment, and hence on its flood-plains, buried and preserved 
under sheets of basalt, we find abundant pebbles of the old Highland rocks 
which it had borne across the whole breadth of the basaltic lowland. 
But the destruction of volcanic cones was probably not the only source of 
the basaltic detritus in the conglomerates of Canna and Sanday. I have 
shown that these conglomerates pass laterally into tuffs, and are sometimes 
