CHAP. XXXVIII 
RIVERS OF THE PERIOD 
247 
these two rocks. Mr. Harker has examined a thin slice prepared from the 
Hysgeir pitchstone, and remarks regarding it that “ the large felspars are 
not the only porphyritic element. The microscope shows the presence also 
of smaller imperfect crystals of angite, very faint green in the slice, and 
small grains of magnetite. The felspars have been deeply corroded by the 
enveloping magma, and irregular included patches of the groundmass occupy 
nearly half the bulk of some of the crystals. This latter feature is seen 
especially in some of the larger crystals, which seem to be sanidine. They 
are, for the most part, apparently simple crystals, but in places there is a 
scarcely defined lamellar twinning, or, again, small patches not extinguishing 
with the rest ; so that we are probably dealing with some perthitic inter- 
growth on a minute scale. 1 
“ Rather smaller felspar-crystals are rounded by corrosion, but lack the 
inclusions of groundmass ; these have albite- and sometimes pericline- 
lamellation, and may be referred to oligoclase-andesine. The groundmass of 
the rock is a brown glass with perlitic cracks, enclosing very numerous 
microlites of felspar about '001 inch in length [6619]. The rock is pro- 
bably to be regarded as a dacite rather than a rhyolite, and thus agrees with 
Mr. Barker North’s analysis of the Eigg pitchstone.” 2 
There is no trace of any conglomerate in situ like that under the Scuir 
of Eigg, nor of any other rock, aqueous or igneous. As the pitchstone 
everywhere slips under the sea, its geological relations are entirely con- 
cealed. 
The great variety of materials met with in the form of boulders on the 
island is a testimony to the transport of erratics from the neighbouring 
islands and the mainland during the Glacial Period. The most abundant 
rock in these boulders is Torridon Sandstone, derived no doubt from the hills 
of Rum, but there occur also various kinds of schist, gneisses, quartzites, 
granites, porphyries, probably from the west of Inverness-shire, as well as 
pieces of white sandstone, probably Jurassic, which may have come from 
Eigg. 
That the pitchstone of Hysgeir is a continuation of that of the Scuir 
may be regarded as highly probable. If not a continuation, it must be 
another stream of the same kind, and doubtless of the same date. If it be 
regarded as probably a westward prolongation of the Eigg rock, and if it be 
about as thick as that mass at the west end of the Scuir, then its bottom 
lies 200 or 300 feet under the waves. The river-channel occupied by the 
Eigg pitchstone undoubtedly sloped from east to west. The position of 
Hysgeir, 18 miles further west, may indicate a further fall in the same direction 
at the rate of perhaps as much as 35 feet in the mile. 3 Unfortunately, 
however, as no trace of the river-bed can now be seen on this island, any 
statement in regard to it must rest on mere conjecture. 
Although the question of the denudation of the basalt-plateaux since 
1 Comp. Prof. Judd's remarks 011 the Scuir of Eigg rock, Quart. Journ. Geol. Soc. vol. xlvi. 
(1890), p. 380. 
2 Op. tit. p. 379. 
3 Rep. Brit. Assoc. 1894, p. 653. 
