[1] Geikie, _loc. cit._, p. 161, etc.

[2] _Physical Geology of Ireland_, 2nd edition, p. 174 (Fig. 21).
Professor Judd has also come to the conclusion that the granite of
Mourne is of Tertiary age, _Quart. Jour. Geol. Soc._, vol. xxx. p. 275.

[3] Judd, _loc. cit._, p. 254.

CHAPTER V.

THE SCUIR OF EIGG.

Amongst the more remarkable of the smaller islets are those of Eigg,
Rum, Canna, and Muck, lying between Mull on the south and Skye on the
north, and undoubtedly at one time physically connected together. The
Island of Eigg is especially remarkable for the fact, as stated by
Geikie, that here we have the one solitary case of "a true superficial
stream of acid lava--that of the Scuir of Eigg."[1] (Fig. 34.) This
forms a sinuous ridge, composed of pitchstone of several kinds, of over
two miles in length, rising from the midst of a tableland of bedded
basalt and tuff to a height of 1,289 feet above the ocean; the
plateau-basalt is traversed by basaltic dykes, ranging in a N.W.-S.E.
direction. But what is specially remarkable is the evidence afforded by
an examination of the course of the Scuir, that it follows the channel
of an ancient river-valley, which has been hollowed out in the surface
of the plateau. The course of this channel is indicated by the presence
of a deposit of river-gravel, which in some places forms a sort of
cushion between the base of the Scuir and the side of the channel. Over
this gravel-bed the viscous pitchstone-lava appears to have flowed,
taking possession of the river-channel, and also of the beds of several
small tributary streams which flowed into the channel of the Scuir. The
recent date of the pitchstone forming this remarkable mural ridge, once
occupying the bed of a river-channel, is shown by the fact that the
basaltic dykes which traverse the plateau-basalts are truncated by the
river-gravel, which is, therefore, more recent; and, as we have seen,
the pitchstone stream is more recent than the river-gravel. But at the
time when this last volcanic eruption took place, the physical geography
of the whole region must have been very different from that of the
present time. From the character and composition of the pebbles in the
old river-bed, amongst which are Cambrian sandstone, quartzite,
clay-slate, and white Jurassic limestone, Sir A. Geikie concludes that
when the river was flowing, the island must have been connected with the
mainland to the east where the parent masse