CHAP. XXXVIII 
RIVERS OF THE PERIOD 
229 
calated among the basalts of Canna and Sanday as the successive flood-plains 
of a river which, like the rivers that traverse the lava-deserts of Iceland, 
flowed perhaps in many separate channels across the basalt-fields of the 
Inner Hebrides, and was liable to have its course shifted from time to time 
by fresh volcanic eruptions. That this river came from the east or north- 
east and had its source among the Western Highlands of Inverness-shire, may 
be inferred from the nature of the stones which it has carried for 30 miles 
or more along its bed. And that it crossed in its course the tract of 
Torridon Sandstone, of which a portion still remains in Rum, is manifest 
from the abundance of the fragments of that formation in the con- 
glomerates. 
With the remarkable exception of the section on Dim Beag, to be 
immediately referred to, no trace of any eroded channel of this river through 
the lavas of the great volcanic plain has been preserved. Possibly frequent 
invasions of its bed by streams of basalt from different vents hindered it 
from remaining long enough in one course to erode anything like a gorge or 
canon. But, in any case, the main channel of the river probably lay rather 
to the east of the present islands of Canna and Sanday, on ground 
which is now covered by the sea. The banks or sheets of boulder-con- 
glomerate undoubtedly show where its current sw'ept with great force over the 
lava-plain, but the manner in which these coarser materials are so often 
covered with fine silt suggests that the sedimentary materials now visible 
were rather deposited on the low grounds over which the steam rushed in 
times of flood. Pools of water would often be left after such inundations, 
and in these depressions silt would gradually accumulate, partly carried in 
suspension by the river, partly washed in by rain, while drift-wood that 
found its way into these eddies, and leaves blown into them from the trees 
and shrubs of the surrounding country, would remain for some time afloat 
and would be the last of the detritus to sink to the bottom. Hence, no 
doubt, the carbonaceous character of the hardened silt in the upper part of 
each intercalation of sediment. 
If we were to look upon the volcanic materials in the conglomerates as 
derived from the subserial disintegration of the fields of basalt, we should be 
compelled to admit a very large amount of erosion of the surface of the volcanic 
plain during the period when the river flowed over that tract. It would be 
necessary to suppose not only that there was a considerable rainfall, but 
that the differences of temperature, either from day to night, or from 
summer to winter, were so great as to split up the lavas at the surface, in 
order to provide the river with the blocks which it has rolled into rounded 
boulders. I do not think, however, that such a deduction would be sound. 
If we compare the materials that have filled up the large eruptive vent at 
the east end of Canna (to be afterwards described) with the great majority 
of the blocks in the coarse conglomerates, we cannot fail to note their strong 
resemblance. The abundance of lumps of slaggy lava in the river-shingle 
corresponds with their predominance in the agglomerate of the vent. The 
boulders of basalt, dolerite, and andesite which crowd the conglomerates 
