290 
THE TERTIARY VOLCANOES 
BOOK VIII 
oi these, which have evidently been broken off from already consolidated 
lavas, are angular or subangular in shape, and their steam-holes are cut 
across by the outer surfaces of the stones. Where they consist of calcite, 
zeolite, etc., the amygdales so exactly resemble those of the bedded basalts 
oi the plateaux that, as already remarked, we must believe them to have 
been already filled by infiltration before the disruption of the rocks by 
volcanic explosions. Other blocks are true bombs, with a fine-grained crust 
outside and a more cellular texture inside, the vesicles of the outer crust 
being sometimes dragged round the surface of the stone. The variety of 
materials included among the ejected blocks and the abundance of pieces of 
the red bole which so generally separates the plateau-basalts indicate that 
a considerable thickness of bedded lavas has probably been broken through 
by the vent. 
Beside the volcanic materials, occasional angular pieces of red (Torridon) 
sandstone may be observed in the agglomerate. The paste is a comminuted 
mass of the same material as the blocks, tolerably compact, and entirely 
without any trace of stratification. 
The actual margin of this vent has nowhere been detected by me. We 
never reach here the base of the volcanic series, for it is sunk under the- 
sea-level. On the other hand, the upper limits of the agglomerate have 
been partially effaced or obscured by the conglomerates which overlie it. 
From the breadth of ground across which the agglomerate can be followed 
along the shore, the vent might be regarded as having been perhaps not less 
than three-quarters of a mile in diameter. But there is the same difficulty 
here as at the Strath vent in Skye in determining the actual limits of the 
volcanic funnel. Possibly there may have been more than one vent in close 
proximity. Even if there was only one, the existing agglomerate may 
include not only what filled the chimney, but also a portion of what had 
accumulated round the orifice and formed the external cone. That the 
volcano continued for some time in vigorous eruption may be judged from 
the amount of material ejected from it, the large size of its blocks, and the 
distance to which they were sometimes thrown. 
The pieces of Torridon Sandstone were no doubt derived from the 
extension of that formation underneath Canna. On the opposite island of 
Bum, where these pre-Cambrian red sandstones are copiously developed, they 
form the platform through which the Tertiary volcanic series has been erupted. 
The several remaining outliers of the bedded basalts, referred to in a previous 
chapter (p. 215 and Eig. 267) as visible on the west side of this island, 
show that the basalt-plateau of Small Isles, which once covered that area, 
rested immediately on the inclined edges of the Torridon Sandstones. 
Probably the same structure stretches westward under Canna and Sunday. 
Bo traces of any Jurassic strata have been detected beneath the volcanic 
rocks of Bum, though they are so well developed a few miles to the east in 
the island of Eigg. Either they were not deposited over the pre-Cambrian 
rocks of Bum, or they had been removed from that ancient ridge before the 
beginning of the Tertiary volcanic period. Certainly I have not detected 
