288 
THE TERTIARY VOLCANOES 
BOOK VIII 
depressions of this lava comes a thin group of sedimentary strata from an 
inch or two to eighteen inches or more in thickness (b). These deposits 
consist of hardened shale charged with macerated fragments oi linear leaves 
and other plant-remains, including and passing into streaks of coal, which 
may he looked upon as probably occupying the 
same horizon with the coal of Portree. But here, 
instead of reposing on a mass of stratified tuff, 
the carbonaceous layers lie on one of the bedded 
lavas. The tuff has died out in the lintervening 
three miles', yet that some of the discharges of 
volcanic detritus reached even to this distance, and 
that they took place during the accumulation of 
these layers of mud and vegetation, is shown 
by the occurrence in the shales of pieces of 
finely amygdaloidal basalt, from less than an 
inch to six inches in length, likewise lapilli of 
a fine minutely cellular basic pumice, like 
some varieties of palagonite. The overlying 
dolerite (c) becomes finely prismatic at its junction with the sedimentary 
layers and has probably indurated them. 
This intercalation of a shaly and coaly band among the lavas can be 
followed northward along the coast. In some places it has been invaded by 
dykes, sills, and threads of basalt on the most remarkably minute scale, of 
which I shall give some account in Chapter xlii. (see Fig. 321). 
Fig. 305. — Section of the Vol- 
canic Series at Acli na Han- 
nait, south of Portree, Skye. 
North 
of Tianavaig Bay — that is, about three-quarters of a mile nearer to the 
Portree vent — a perceptible increase in the amount of volcanic material is 
observable among the shales and leaf-beds. Not only are lapilli of basic 
pumice abundant, but the volcanic detritus has accumulated here and there 
in sufficient amount to form a band of dull greenish-brown tuff. 
These coast-sections in the neighbourhood of Portree afford additional 
illustrations of the characteristic fact, on which I have already insisted, 
that the interstratifications of sedimentary material in the basalt- plateaux 
frequently terminate upward in leaf-beds, thin coals, or layers of shale, lull 
of indistinctly preserved remains of plants. As I have endeavoured to 
show, this vegetation, which was undoubtedly terrestrial, probably grew not 
far from the sites where its remains have been preserved. Leaves and seeds 
would naturally be blown or washed into pools on the lava-fields, and would 
gather there among the mud and sand carried by rain from the surrounding 
ground. Such a topography and such a sequence of events point to intervals 
of longer or shorter duration between the successive outpourings of basalt. 
It was probably during one of these intervals of quietude that the crater of 
the Portree volcano became a maar and was finally silted up. 
Reference has already been made to a conspicuous mass of agglomerate 
which occurs at the east end of the island ol Canna, and marks the site ol 
an important volcanic vent belonging to the Small Isles plateau. A portion 
of it projects from the grassy slopes, and rises vertically above the beach as 
