CHAP. XXXVIII 
RIVERS OF THE PERIOD 
239 
end is truncated, so as to lay open a section of the gravelly deposit along 
which the pitchstone flowed. 
The accompanying diagram ( Fig. 279) represents the natural section 
there exposed. Eising over each other in successive beds, with a hardly 
perceptible southerly dip of 2°, the sheets of basalt form a mural clifl about 
700 feet high. The bedded character of these rocks and their alternations 
of compact, columnar, amorphous and amygdaloidal beds are here strikingly 
seen. They are traversed by veins and dykes of an exceedingly close- 
grained, sometimes almost flinty, basalt. But the conspicuous feature of 
the cliff is the hollow which has been worn out of these rocks, and which, 
after being partially filled with coarse conglomerate, has been buried under 
the huge pitchstone mass of the Scuir. The conglomerate consists of water - 
Fjg. 279. — Natural Section at the Cliff of Bhleann Boidheacli, north-west end of the Scuir of Eigg. 
aa, Beilded dolerites and basalts ; b, basalt dykes and veins ; r, ancient river-bed tilled with 
conglomerate ; p, pitchstone of the Scuir. 
worn fragments, chiefly of dolerite and basalt, but with some also ot the 
white Jurassic sandstones, imbedded in a compacted sand derived from the 
waste of the older volcanic rocks. The grey devitrified bands in the pitch- 
stone, so conspicuous at the east end of the Scuir, here disappear and leave 
the conglomerate covered by one huge overlying mass of glassy pitchstone. 
If any doubt could arise as to the origin of the mass of detritus exposed 
under the pitchstone at the east end of the Scuir it would be dispelled by 
the section at the west end, which shows with unmistakable clearness that 
the conglomerate is a fluviatile deposit and lies in the actual channel of the 
ancient river which was eroded out of the basalt plateau, and was subse- 
quently sealed up by streams of pitchstone-lava. 
An examination of the fragments of rock found in the conglomerate 
affords here, as in Canna and Sunday, some indication of the direction in 
which the river flowed. The occurrence of pieces of red sandstone, which 
