File:The Quarterly journal of the Geological Society of London (12734285014).jpg

282 J. W. JTJDD ON THE SECONDARY ROCKS OF SCOTLAND.
will serve to illustrate the peculiar relations of the several rocks in
these cases.
The rocks on which the volcanic series of Lorn usually rests are
black clay-slates, abounding in fine cubical crystals of pyrites and
often traversed by numerous veins of quartz. This formation is
perhaps tbe highest member preserved to us of that great series of
Lower Silurian rocks which, bent into endless folds aud greatly me-
tamorphosed, occupies so large a portion of the Scottish Highlands.
At Seil and Easdale it is extensively worked for roofing-slates

as
yet, unfortunately, it has not yielded any fossils

but it exhibits in
the island of Seil interbedded igneous rocks, apparently of contem-
poraneous character.
The only point at which the series of rocks so well displayed in
Lorn is seen in juxtaposition with the Mesozoic sedimentary rocks
and the Tertiary lavas is in the south-eastern part of Mull. Here,
unfortunately, among the wonderfully disturbed and greatly meta-
morphosed rocks, which are exposed only in precipitous and altogether
inaccessible cliffs, I have sought in vain for any simple section illus-
trating the relations of these three series of deposits. Nevertheless,
after carefully tracing the positions of such masses as can be reached
and studied, there appears to be no room for doubt that the various
members of the Mesozoic series rest indifferently upon the denuded
lavas of Lorn and the older rocks, and that they are themselves
covered unconformably by the Tertiary volcanic rocks.
4. Succession of Rocks in Lorn. — The series of rocks which we have
been describing has usually been represented as consisting of beds of
conglomerate and sandstone at the base, overlain by a great mass of
" trap " rocks. A careful examination of the district, however,
proves that its structure is by no means so simple as this statement
would imply. The outpouring of the great lava streams was, in part
at least, contemporaneous with, as well as subsequent to, the depo-
sition of the conglomerates and sandstones. This is proved by the
alternation of the " trap " rocks with the conglomerates and sand-
stones, and by the fact that the materials of the latter are to a
great extent derived from the former. Good examples of the alter-
nations of the traps with the sandstones and conglomerates are to be
seen near Dunolly.
At the base of the whole series of the Lorn rocks, and in imme-
diate contact with the subjacent slates, is often found a breccia of
very interesting character. It is wholly composed of perfectly an-
gular fragments, sometimes of considerable size, of the slate and
quartz rocks on which it rests

this breccia exhibits no trace of
stratification or of its materials having been sorted or acted upon
by water. It sometimes forms masses of considerable thickness,
which appear to have been accumulated upon old terrestrial surfaces
of the slate rocks by purely subaerial agencies.
The conglomerates of the Lorn series present very remarkable
characters, which are familiar to all who have examined the pictur-
esque cliffs on either side of the beautiful Bay of Oban. They are
made up of blocks of very various sizes, occasionally angular