37 2 
THE TERTIARY VOLCANOES 
BOOK VIII 
they graduate into each other, and that any attempt to discriminate between 
them as to relative age is superfluous. Yet he evidently felt that the 
contrasts of topography which he described could hardly fail to raise the 
question of whether rocks so distinct in outward form did not differ also in 
relative antiquity. But he dismissed the question without answering it, 
remarking that if there is any difference of age between the two kinds of rock, 
“ there appears no great prospect of discovering it.” 1 He records an instance 
of a vein of “ syenite ” traversing the “ hypersthene rock ” in the valley of 
Coruisk. “ If this vein,” he says, “ could be traced to the mass of syenite, 
it might be held a sufficient ground of judgment, but under the present circum- 
stances it is incapable of affording any assistance in solving the difficulty .” 2 
Instead, however, of being a solitary instance, it is only one of hundreds of 
similar intrusions which can be connected with the general body of granitic 
and granophyrie masses, and which put the relative ages of the several 
groups of rock beyond any further doubt. 
Boue, who knew the geology of some of the extinct volcanic regions of 
Europe, recognized the similarity of the Scottish masses to those of the 
Continent, and classed the acid rocks as “ trachytes.” He saw in each of 
the volcanic areas of the West of Scotland a trachytic centre, and supposed 
that the more granitoid parts might represent the centres in the European 
trachytic masses. He traced in imagination the flow of the lava-streams 
from these foci of volcanic activity, distinguishing them as products of 
different epochs of eruption, among the last of which he thought that the 
trachytic porphyries might have been discharged. He admitted, however, 
that his restoration could not be based on the few available data without 
recourse to theoretical notions drawn from the analogy of other regions . 3 ^ 
In the careful exploration of the central region of Skye made by Von 
Oeynhausen and Von Dechen, these able observers traced the boundary 
between the “ syenite ” and the “ hypersthene rock ” ; and as they found the 
former lying underneath the latter, they seem naturally to have considered 
it to be the older protrusion of the two . 4 Principal Eorbes came to a 
similar conclusion from the fact that he found the dark gabbro always over- 
lying the light-coloured felspathic masses . 5 Professor Zirkel also observed 
the same relative position, and adopted the same inference as to the relative 
a«-e of the rocks . 8 Professor Judd followed these writers in placing the acid 
rocks before the basic. He supposed the granitoid masses to form the 
cores of volcanic piles probably of Eocene age, through and over which 
the protrusions of gabbro and the eruptions of the plateau-basalts took 
place . 7 
1 Western Islands, i. p, 368 ; see also pp. 488, 575, 578. 2 Op. cit. p. 370. 
3 Essai Geologique sur I'Ecosse, pp. 291, 322, 327. 
4 Karsten's ArMv, i. p. 82. It will be shown in later pages that the apparent infraposition 
of the granopliyre is often deceptive, the real junction being vertical. 
5 Edin. New Phil. Jour. xl. (1846) p. 84. 
6 Zeitsch. Dcutsch. Geol. Gesellsch. xxiii. (1871) pp. 90, 95. He says that the gabbro seems 
to be the younger rock, so far as their relations to each other can be seen. 
7 Quart. Jour. Geol. Soc. xxx. (1874) p. 255. 
