CHAP. XXXVI 
NATURE OF THE FRAGMENTAL ROCKS 
•97 
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, Eoss-shire, and 
some curious plug-like masses of breccia, also made up of fragments of 
Torridouian strata, occur in the island of Eaasay. 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, 
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. 
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. 
In the group of strata between the two series of basalts in Antrim, some 
°t 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 ” 
Hu d “ lithomarge ” of Antrim, probably derived from pale rhyolitic tuffs 
UQ d conglomerates (p. 204). Associated with these deposits in the 
Sar >ie district, is a pisolitic haematite, which has been proved to occur 
" v er 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, pliuthite, etc.). The source of the iron-oxide 
ls doubtless to he traced to the decomposition of the basic lavas during the 
v °lcanie period. 
