222 
THE TERTIARY VOLCANOES 
BOOK VIII 
cinders, or from the scoriaceous surface of a lava - stream. Where the 
vesicles were still empty, the large boulders could be more easily swept 
along by moving water. But a powerful current must have been needed 
to transport and wear down into more or less rounded forms blocks of basic 
lava, many of which must weigh several tons. The large block on Eilean 
a’ Bhaird probably exceeds 12 tons in weight. 
Besides the obviously volcanic contents of the conglomerate there occur 
here also, as in the Compass Hill cliffs, abundant pieces of Torridon Sand- 
stone. These stones are notably smaller in size and more perfectly water- 
worn and even polished than the blocks of lava. Obviously they have 
travelled further and have undergone more prolonged attrition. 
The matrix of the rock consists essentially of the fine detritus of basic 
lavas, probably mingled with true volcanic dust. The coarser parts dis- 
play only the feeblest indication of stratification ; indeed, in a limited 
exposure the rock might be regarded as a tumultuous agglomerate. But the 
manner in which the deposit is intercalated with, and sometimes overlies, 
green tuffs and shales, together with the water-worn condition of its stones, 
shows that it has not been accumulated in a volcanic chimney, but has been 
thrown down by some powerful body of water, with probably the co-opera- 
tion of volcanic discharges. 
While the composition of the conglomerate suffices to indicate that this 
deposit was formed at a time when some volcano was active in the imme- 
diate neighbourhood, singularly convincing proofs of the work of this vent 
are to be seen in the form of intercalated sheets of lava. Thus on Eilean 
a’ Bhaird the boulders of the conglomerate are overlain and wrapped round 
by a sheet of rudely prismatic basalt, with lines of vesicles arranged in the 
direction of the bedding. A similar relation can be traced along the beach 
between Canna House and the wooden pier, where successive sheets of basalt 
have flowed over the conglomerate (Fig. 269). 
But, besides coarse volcanic detritus, the sedimentary platform repre- 
sented by the lower conglomerate of Compass Hill includes other deposits 
of which good sections may be examined all round Canna Harbour. Beds 
of fine well-stratified dull-green tuff pass by an admixture of pebbles into 
fine ashy conglomerate or pebbly sandstone, and by an increase in the pro- 
portion of their fine detritus into volcanic mudstone and fine shales. The 
shales vary from a pale grey or white tone into blackish grey, brown, and 
black. They are well stratified and are frequently interleaved with layers 
of fine tuff. The darker bands are carbonaceous, and are not infrequently 
full of ill-preserved vegetation. Indeed, leaves and stems in a rather 
macerated condition are of common occurrence in all the shaly layers. 
Here and there, especially in some ashy shales in front of Canna House, 
I observed a recognisable Sequoia. The mudstones are dull green, close- 
grained shattery rocks composed of fine volcanic detritus, and pass both 
laterally and vertically into shales, tuffs, and conglomerates. They suggest 
showers of fine dust or streams of volcanic mud. They, too, contain 
fragmentary plants. 
