CHAP. XLI 
VENTS OF THE BASALT-PLATEAUX 
291 
a single recognizable fragment of any Jurassic sedimentary rock in the 
agglomerate of Canna. 
This Canna vent exhibits, better than is usually shown, the occurrence 
of dykes and irregular injections of lava through the agglomerate. A large 
mass of a finely columnar basalt runs up from the beach at Garbh Asgar- 
nisli. A similar rock forms several detached crags a little further south, 
particularly in the headland of Coroghon Mor and the island of Alman. 
Here the basalt is beautifully columnar, its slender prisms curving from a 
central line until their ends abut against the agglomerate. The truly 
intrusive character of this basalt is well shown on the southern front 
of Coroghon Mbr, and on the northern face of Alman, as represented in 
the accompanying diagrams (Figs. 307 and 308). 
Although there is 110 conclusive evidence that these intrusions belong to 
Fig, 307. — Columnar Basalt invading Agglo- 
merate of Volcanic Vent, Coroghon Mor, 
Isle of Canna. (Height a'bove 20 feet.) 
Fig. 308. — Columnar Basalt invading Volcanic 
Conglomerate, north side of Alman Islet, 
Canna. 
the time of the activity of the vent, yet they differ so much from the 
ordinary dykes (one of which also cuts the agglomerate and ascends through 
the conglomerates and basalts above), are confined so markedly to the vent 
and its immediate proximity, and resemble so closely the basalt-injections of 
other vents, such as those of the Carboniferous and Permian necks of Scot- 
land, that they may with probability be regarded as part of the mechanism 
of the Canna volcano. 
Though the form and size of the vent of this volcano cannot be pre- 
cisely defined, the upper part of its agglomerate, as we have seen {ante, p. 
219), is dovetailed in the most interesting way with the series of coarse con- 
glomerates, which indicate strong river-action in this part of the volcanic 
area during the time of the eruption of the plateau-basalts. 
The agglomerate vents described in the foregoing pages as occurring in 
Antrim and among the Inner Hebrides all appear either in the midst of the 
plateau-basalts or in close proximity to them. Before quitting the Scottish 
examples, I may refer to some that rise through much more ancient forma- 
tions at a distance from any portion of the volcanic plateaux, and yet may 
with probability be assigned to the Tertiary volcanic period. 
