Chapter xlvii

.). They consist entirely of non-volcanic stones and dust and are traceable for some miles along the line of a fissure. Where they have been discharged through granite they consist entirely of the detritus of that rock, but where they have been erupted in the Silurian area they consist of fragments of grits and shales. They seem to have been produced by æriform discharges, without the uprise of any volcanic magma, though eventually andesite and rhyolite ascended the fissure and became full of granitic and Silurian fragments.

Some remarkable necks filled almost entirely with fragments of Torridon Sandstone have been observed in the west of Applecross, Ross-shire, and some curious plug-like masses of breccia, also made up of fragments of Torridonian strata, occur in the island of Raasay. These examples will be more particularly described on later pages (pp. 292, 293).

(_c_) _Tuffs._--The tuffs intercalated in the basalt-plateaux generally consist essentially of basic materials, derived from the destruction of different varieties of basalts, though also containing occasional fragments of older felsitic rocks, as well as pieces of chalk, flint, quartz, and other non-volcanic materials. They are generally dull, dirty-green in colour, but become red, lilac, brown, and yellow, according to the amount and state of combination and oxidation of their ferruginous constituents. They usually contain abundant fragments of amygdaloidal and other basalts. As a rule, they are distinctly stratified, and occur in bands from a few inches to 50 feet or more in thickness. The matrix being soft and much decomposed, these bands crumble away under the action of the weather, and contribute to the abruptness of the basalt-escarpments that overlie them.

[Illustration: Fig. 262.--Breccia and Blocks of mica-schist, quartzite, etc., lying between bedded Basalts, Isle of Mull.

_a a_, Bedded basalts; _b_, Breccia; _d_, Basic dyke.]

In the group of strata between the two series of basalts in Antrim, some of the tuffs consist chiefly of rhyolitic detritus, both glassy and lithoid.

Where the tuffs become fine-grained and free from imbedded stones, they pass into variously-coloured clays. Among these are the "bauxite" and "lithomarge" of Antrim, probably derived from pale rhyolitic tuffs and conglomerates (p. 204). Associated with these deposits in the same district, is a pisolitic hæmatite, which has been proved to occur over a considerable area on the same horizon. Many of the clays are highly ferruginous. The red streaks that intervene between successive sheets of basalt are of this nature (bole, plinthite, etc.). The source of the iron-oxide is doubtless to be traced to the decomposition of the basic lavas during the volcanic period.

(_d_) There occur also grey and black clays and shales, of ordinary sedimentary materials, containing leaves of terrestrial plants (leaf-beds), with occasional wing-cases of beetles, sometimes associated with impure limestones, but more frequently with sandstones and indurated gravels or conglomerates containing pieces of fossil wood. These intercalated bands undoubtedly indicate the action of running water, sometimes even of river-floods, and the accumulation of sediment in hollows of the exposed flows of basalt at intervals during the piling up of the successive lava-sheets that form the plateaux. The alternation of fluviatile gravels with volcanic tuffs, fluviatile conglomerates, and lava-streams, is admirably displayed in the island of Canna, as will be narrated in detail in