220 
THE TERTIARY VOLCANOES 
BOOK VIII 
stone. The composition and structure of these stones, and the manner of 
their dispersion through the deposit, leave little doubt that they were 
ejected from the vent. We are thus confronted with the interesting fact 
that, while the materials of the volcanic cone were being washed down by 
running water, eruptions were still taking place. But by degrees these 
indications of contemporaneous volcanic activity diminish. The detrital 
materials become coarser and more distinctly water-rolled until they pass 
into greenish sandstones and tine conglomerates. Yet the matrix even of 
these higher sediments is largely composed of fine volcanic detritus, and 
probably points to occasional discharges of dust and ashes. 
Various sills or intrusive sheets have been injected into this sedimentary 
group along the precipices at the east end of Canna, and form there len- 
ticular bands. One of these (c) is shown in Big. 2G8. 
Immediately above the massive greenish pebbly sandstone (d) which 
caps the stratified series lies a group of basalts (e), composed of several 
distinct beds, having a united thickness of from 80 to 100 feet. The lowest 
of these has a regular columnar structure, while those overlying it exhibit 
the confused starch-like grouping of curved and rather indistinctly-formed 
prisms. 
The next band in upward succession is one of conglomerate (/), which 
runs as a continuous and conspicuous feature along the upper part of the 
cliff. This rock presents in many respects a strong contrast to the con- 
glomerates underneath. It is dull-green to yellow in colour, and is well 
stratified, being marked by the interstratification of finer layers, and passing 
down into a band of pebbly sandstone, which rests immediately on the 
basalt (e). Its component stones are thoroughly water-worn, ranging up to 
six inches or even more in length. But its most distinctive character lies in 
the nature of its pebbles. Instead of consisting mainly of volcanic materials, 
these stones have almost all been transported for some distance. They 
include abundant fragments of Torridon Sandstone, gneiss, schists, grits, and 
other rocks like those in Bum and Western Inverness-shire. No such rocks 
exist in situ in Canna. The nearest tract of Torridon Sandstone is in Bum, 
about four miles to the eastward. But the pieces of schist and epidotic 
grit like the rocks of the Western Highlands, have probably travelled at least 
30 miles. 
It is important to observe that all these transported stones indicate a 
derivation from some source lying to the eastward of Canna. The evidence 
in this respect agrees with that furnished by the ancient river-gravel under 
the pitchstone of the Scuir of Eigg. It is clear that the waters which 
found their way across the lava-fields of this part of the Inner Hebrides 
took their rise somewhere to the eastward, probably among the mountains 
of Inverness-shire. 
The conglomerate now described is from 40 to 50 feet thick. It can be 
followed along the face of the cliffs for more than a mile on the north side 
of Canna. Less persistent on the south side, its outcrop strikes from the 
edge of the precipice inland, keeping to the south of the top of Compass 
