i8o 
THE TERTIARY VOLCANOES 
BOOK VIII 
of feet of rock have been removed from parts of the surface of the land since 
the time of the uprise of the dykes. 1 The evidence of this denudation is 
singularly striking in such districts as that of Loch Lomond, where the 
difference of level between the outcrop of the dykes on the crest of the 
ridges and in the bottom of the valleys exceeds 3000 feet. It is quite 
obvious, for example, that had the deep hollow of Loch Lomond lain, as it 
now does, in the pathway of these dykes, the molten rock, instead of 
ascending to the summits of the hills, would have burst out on the floor of 
the valley. We are, therefore, forced to admit that a deep glen and lake- 
basin have been in great measure hollowed out since the time of the dykes. 
If a depth of many hundreds of feet of hard crystalline schists could have 
been removed in the interval, there need be no difficulty in understanding 
that by the same process of waste, many sheets of solid basalt may have been 
gradually stripped off the face of Central Scotland and Northern England. 
The association of fissures and dykes with the accumulation of thick 
and extensive volcanic plateaux, over so wide a region of North-western 
Europe as from Antrim to the North of Iceland, finds its parallel in different 
parts of the world. One of the closest analogies presents itself among the 
Ghauts of the Bombay Presidency, where vast basaltic sheets, probably of 
Cretaceous age, display topographical and structural features closely similar 
to those of the Tertiary volcanic plateaux of the British Isles. The dykes 
connected with these Indian basaltic outflows correspond almost exactly in 
their general character and stratigraphieal relations to those of this country. 
They occur in great numbers, rising through every rock in the district up 
to the crests of the Ghauts, 4000 feet above the sea. They vary from 1 or 
2 to 10, 20, 40, and even occasionally 100 or 150 feet in width, and are 
often many miles in length. They observe a general parallelism in one 
average direction, and show no perceptible difference in character even when 
traced up to elevations of 3000 and 4000 feet. 2 
Thousands of square miles in the Western States and Territories of the 
American Union have been similarly flooded with basic lavas. Denudation 
has not yet advanced far enough to lay bare much of the platform on which 
these lavas rest. But the dykes that traverse the rocks outside of the lava- 
deserts afford an example of the structure which will ultimately be revealed 
when the wide and continuous basalt-plains shall have been trenched by 
innumerable valleys and reduced to fragmentary plateaux with lofty 
escarpments (p. 267). 
It is to the modern eruptions of Iceland, however, that we turn for the 
completest illustration of the phenomena connected with dykes and fissures. 
An account of these eruptions will therefore be given in Chapter xl. as 
an explanation of the history of the Tertiary basalt-plateaux of Britain. 
1 Scenery of Scotland, 2nd edit. (1887), p. 149. But see the remarks already made (p. 150) on 
the curious coincidence sometimes observable between the upper limit of a dyke and the overlying 
inequalities of surface. 
Mr. O. T. Clark, Quavt. Journ. Geol. Soc. xxv. (1869) p. 163. For remarks on the con- 
nection of dykes with superficial lavas, see 'posted, p. 268. 
