352 
THE TERTIARY VOLCANOES 
BOOK VIII 
there its true brecciated nature is conspicuously revealed by prominent 
blocks of hardened sandstone. This band of breccia must in some places be 
150 or 200 feet broad. It has no distinct bedding, but seems to lie as a 
highly inclined bed dipping into the hill. It may possibly be a crush-breccia 
belonging to a period earlier than the volcanic eruptions. It is at once 
succeeded by a black flinty felsite like that of Mull. The groundmass of 
this rock, so thickly powdered with magnetite grains as to be almost opaque 
under the microscope, displays good flow-structure round the turbid crystals 
of orthoclase and the clear granules of quartz. Further up the hill, the 
rock becomes lighter in colour and less flinty in texture — a change which is 
Fig. 340. View of Allival, Rum, sketched from the base of the north-east side of the cone. 
found to arise from more complete devitrification, the groundmass having 
become a crystalline granular aggregate of quartz and felspar with scattered 
porphyritic crystals of these minerals '(microgranite). In some places, the 
felsite incloses fragments of other rocks. A specimen of this kind, taken 
from the head of Coire Dubh, shows under the microscope a brown micro- 
felsitic. groundmass, with crystals of felspar and augite, inclosing a piece 
of basalt, composed of fine laths of plagioclase, abundant magnetite and a 
smaller proportion of granules of augite. 
This band of felsite and microgranite may be traced continuously from 
Loch Gainmieh along the base of Barkeval and Allival, and similar rocks 
appear at intervals on the same line round the eastern base of the hills. 
