342 
VOLCANOES OF THE LOWER OLD RED SANDSTONE book v 
the intervals of quieter sedinieiitatioii indicated by the fine sandstones and 
sliales, volcanic explosions continued, as may he seen hy the occurrence of 
occasional large bombs which have fallen upon and pressed down the fine 
ashy silt that was gatliering on the bottom. 
It would seem from the cliaraeters of some of the strata in this sedimen- 
tary series that over the area of deposit portions of the shallower waters 
were occasionally laid bare to the sun and air. Among the conglomerates 
there lie certain bands of reddish sandy, ripple-marked, sun-cracked and 
rain-pitted shales and fine sandstones. Such accumulations, indicative of 
the ultimate exposure of fine sediment that silted up hollows in the great 
banks of coarse shingle, may be noticed at the south end of the Island of 
Kerrera, on at least two horizons which are separated from each other by 
thick masses of conglomerate and fine felspathic grit. We may infer, 
therefore, that the fine littoral mud, which gathered during pauses in the 
heaping up of the coarse gravel and shingle, was occasionally laid dry. 
Similar strata may be observed behind Oban, where the alternation of 
exceedingly fine sediment and granular ashy bands is w'ell exhibited. 
But the explosions that gave rise to the volcanic materials so largely 
represented in tliese lower conglomerates, were merely preliminary to those 
which led to the outflow of 
the great slieets of lava that 
now constitute so large a part 
of the hills of Lome. In the 
few traverses which I have 
made across different parts of 
this district I have noted tlie 
general resemblance of the 
lavas to those of the Lower 
Old Eed Sandstone of the 
Midland Valley of Scotland, 
their bedded character, and 
the occurrence of occasional 
layers of stratified material 
between them. The prominent features of these rocks, and their relation to 
the volcanic conglomerates below them, and to the imderlying slates and 
schists are well displayed on Beinu Lora at the mouth of Loch Etive (Fig. 
100). There the black slates of the district are unconformably covered by 
the coarse volcanic conglomerate, formed chiefly of blocks of andesite, 
cemented in a hard matrix of similar composition. About 150 or 200 feet 
of this material underlie the great escarpment of the lavas, which here rise 
in successive beds to the top of the hill, 1000 feet alcove its base. 
On the south side of Loch Etive the base of the volcanic scries, with its 
underlying conglomerate, may be followed westward to Oban and thence 
southward to Loch Feochau. Tlie lavas cover most of the ground from the 
western shore eastwards to near Loch Awe. But this area is still very 
imperfectly known. The Geological Survey, however, has now advanced 
Fig. 100. ShcIIoji ol’ lava-cscarinueut at Beinn Lora, north 
sidb of luoutli of Locli Etivb, Argyllsliire. 
1. riiyllites 
; 2. Thick conglomerate ; 
andesite. 
3. Successive slieets of 
