CHAPTER L 
EFFECTS OF DENUDATION 
Among the more impressive lessons which the basalt-plateaux of North-Western 
Europe teach the geologist, the enormous erosion of the surface of this part 
of the continental area since older Tertiary time takes a foremost place. He 
may be ready almost without question to accept the evidence adduced in 
favour of a vast amount of denudation among such soft and incoherent 
strata as those of the older Tertiary formations of the south-east of England 
or the north-west of France. But he is hardly prepared for the proofs 
which meet him among the north-western isles that such thick masses ot 
solid volcanic rocks have been removed during the same geological interval. 
To gain some idea of the amount of this waste we must, in the first 
place, picture to our minds the extent of ground over which the lavas were 
poured, and the depth to which they were piled upon it. Though we may 
never he able to ascertain whether the now isolated basalt- plateaux ot Britain 
were once united into a continuous plain of lava, we can be quite certain 
that every one of these plateaux was formerly more extensive than it is 
now, for each of them presents, as its terminal edge, a line of wall formed by 
the truncated ends of horizontal basalt -sheets. And there seems no 
improbability in the assumption that the whole of the great hollow from the 
centre of Antrim up to the Minch was flooded with lavas which flowed from 
many vents between the hills of ancient crystalline rocks forming the line 
of the Outer Hebrides on the west, and those of the mainland ot Scotland 
on the east. ... 
It is certain that the depth to which some parts of this long hollow 
were overflowed with lava exceeded 3000 feet, for more than that depth of 
rock can be shown to have been in some places removed. The original 
inequalities of surface were buried under the volcanic materials which were 
spread out in a vast plain or series of plains, like those that have been 
deluged by modern eruptions in Iceland. Owing, however, to a general but 
unequal movement of subsidence, the lava-fields sank down here and there 
to perhaps, an extent of several hundred feet, so that the old land-surface 
on which they began to be poured out now lies in those places below the 
level of the sea. _ 
I have shown that even during the volcanic period, while the lavas were 
