CHAP. XXXVIII 
RIVERS OF THE PERIOD 
241 
The river-channel of Eigg, since it was eroded long after the cessation 
of the outflows of basalt in the plateau of Small Isles, must he much 
later in origin than those of Canna and Sunday which, as we have seen, 
were contemporaneous with the basalt-eruptions. But the river that ex- 
cavated the channels and deposited the gravels may have been the same 
in both areas. 
In dealing with this subject, though the evidence is admittedly scanty, 
we are not left wholly to conjecture. A consideration of the general 
topographical features of the wide region of the Inner Hebrides, from the 
beginning of the volcanic period onward, will convince us that, in spite of 
the effects of prolonged basalt-eruptions, the persistent flow of the drainage 
of the Western Highlands must have taken a westerly direction. It was 
towards the west that the low grounds lay. Though the long and broad 
valley which stretched northwards from Antrim, between the line of the 
Outer Hebrides and the West of Scotland, was gradually buried under a 
depth of two or three thousand feet of lava, the volcanic plain that over- 
spread it probably remained even to the end lower than the mountainous 
Western Highlands. Hence the rivers, no matter how constantly they may 
have had their beds filled up and may have been driven into new channels, 
would nevertheless always seek their way westwards into the Atlantic. 
On Canna and Sunday the traces of a river are preserved which poured 
its flood-waters across the lava-fields in that part of the volcanic region, 
while streams of basalt were still from time to time issuing from vents and 
fissures. Not more than fourteen miles to south-east stands the Scuir of 
Eigg, with its buried river-channel and its striking evidence that there, 
also, a river flowed westwards, but at a far later time, when the basalt- 
eruptions had ceased and the volcanic plain had been already deeply 
trenched by erosion, yet before the subterranean fires were finally quenched, 
as the pitchstone of the Scuir abundantly proves. 
When one reflects upon the enormous denudation of this region, to 
which more special reference will be made in the sequel, one is not surprised 
that many connecting links should have been effaced. The astonishment 
rather arises that so continuous a story can still be deciphered. Even, how- 
ever, had the original record been left complete, it would have been exceed- 
ingly difficult to trace the successive mutations of a river-channel during 
long ages of volcanic eruptions. Such a channel would have been concealed 
from view by each lava-stream that poured into it, and would not have 
been again exposed save by the very process of erosion that destroys while 
it reveals. 
While, therefore, there is not and can never be any positive proof that 
in the fluviatile records of Canna, Sanday and Eigg successive phases are 
registered in the history of one single stream, I believe that this identity 
is highly probable. It was a river which seems to have risen among the 
mountains of Western Inverness-shire, and it had doubtless already taken 
its course to the sea before any volcanic eruptions began. It continued to 
flow westwards across the lava-floor that gradually spread over the plains. 
VOL. II R 
