CHAP. L 
EFFECTS OF DENUDA TION 
457 
on the one hand, vividly impress the imagination by the demonstration which 
they afford of the reality and magnitude of the denudation, and, on the other 
hand, furnish a measure by which the minimum amount ol tliis denudation 
may be actually computed. 
Availing ourselves of this kind of evidence it is easy to show that 
valleys many miles long, several miles broad, and from crest to bottom 
several thousand feet deep, have been excavated out of the basalt-plateaux 
since the close of the volcanic period. And if this conclusion can be 
demonstrated for these plateaux, it must obviously apply equally to the rest 
of the country. We thus obtain a most important contribution to the 
investigation of the origin and relative age of the present topographical 
features of the surface of the land. 
Let me give a few illustrations of the nature of the investigation and of 
the results to which it leads. Throughout the Western and haroe Islands 
the level bars of basalt present their truncated ends in the great escarpment- 
cliffs which wind mile after mile along their picturesque coasts. Where 
they front the open sea, it is obviously impossible to say how much fui flier 
seaward they once extended. But where they retire in fjords or sea-lochs, 
and sweep inland into glens, it is easy to measure the distance from the 
bottom of the eroded hollow to its bounding watersheds, and to estimate the 
amount of material that has been worn out of it. The only uncertainty in 
this computation arises from our inability to determine to what extent 
movements of subsidence may have come into play to aid in the disappear- 
ance of the basalts. Where the bottom of the lavas can be seen at the same 
level on either side of an inlet, with no evidence of faulting, or where a 
definite horizon in the volcanic series can be traced round the head of a glen 
or sea-locli, the influence of underground movements may be eliminated. 
The evidence of vast denudation is always visible, the proofs of subsidence 
are much less frequently observable. 
The island of Mull supplies many striking examples of the enormous 
waste of the basalt-plateau. The Sound ol Mull, for instance, has heen 
eroded out of the volcanic series for a distance of 20 miles, with a mean 
breadth of about two miles. From the deepest part of this fjord to the 
summit of the Mull plateau is a vertical height of 3600 feet. The whole 
of this vast excavation has taken place since older Tertiary time. On the 
opposite side of Mull the hollow of Loch Seridain has been eroded to a mean 
depth of at least 1200 feet below the average level of the surrounding 
plateau, with a breadth of rather more than a mile. 
The scattered islands which lie to the west of Mull tell the same tale. 
They are all outliers of the same basalt-plateau, and have not only been 
oreatly lowered by the removal of their upper lavas, but have been separated 
by the erosion of long and deep hollows between them. Thus from the 
summit of the Gribon cliffs in Mull to the -deepest part of the sea -floor 
between that precipice and the Treshnish Isles a vertical depth of at 
least 2000 feet of rock has been removed since the basalts ceased to be 
erupted. 
