264 Proceedings of the Royal Physical Society. 
assume that under the conditions most likely to have 
obtained in Pliocene and Pleistocene times, that denudation 
went on more rapidly than is usually the case. Bearing 
this in mind, we will assume that an estimated 1 foot in 2000 
years for the erosion accomplished would be one certainly 
on the safe side. From the summit level of the Cuchullin 
Hills to the sea-level is now over 3100 feet. These hills 
consist of a kind of gabbro, a plutonic rock that, whatever its 
nature and origin, could not possibly have been formed 
within a short distance of the surface. One would be 
almost justified in regarding it as a rock of deep-seated 
origin, for even if it be, as I have suggested in the Geological 
Magazine, that this gabbro represents basaltic lavas re- 
crystallised by later intrusions, the metamorphism would 
still require that a considerable thickness of over-lying rock 
should have once covered it, and have since been removed. 
There are no exact means of arriving at an estimate of what 
that thickness was. But assuming that the Skye Volcano 
was of the same nature as, and had slopes as low as occur in, 
the volcanic mass of Hawaii, that alone would give us an 
elevation of its central portions of, at least, 8000 feet. In 
the period since the old volcano died out it has been 
trenched by rain, rivers, and ice to its very core. If this 
denudation has gone on at the moderate rate of only 1 foot 
in 2000 years, this rate gives us 16,000,000 as the time 
required for the formation of the valleys. The quantity of 
rock that has been removed since the close of the volcanic 
period can be shown by other examples to be enormous. Sir 
Archibald Geikie, in his well-known “Scenery of Scotland,” 
refers especially to the excavation since that period of the 
great valley in which hes Loch Scridain in Mull; also to 
the extensive removal of volcanic material implied by the 
shaping of the Sound of Mull; and, further, to the evidence 
afforded by the dykes. For instance, referring to the dyke 
that crosses Loch Lomond and rises to near the summit of 
Ben Voirlich—and assuming that the particular dyke in 
question is of the same age as the volcanic rocks of Skye, 
Rum, Mull, etc.—he points out that, if the great valley in 
which Loch Lomond is situated was actually in existence at 
