CHAP. XI.VII 
THE ACID BOSSES OF MULL 
399 
outwards, and plunges under the basalts at an angle of 30° to 40°. The 
terraced basaits and dolerites are not sensibly disturbed, but end off abruptly 
against the steep face of intrusive rock. We might suppose that in this 
case the younger rock had merely carried upward the continuation of the 
beds that are truncated by it, as if an orifice had been punched out for its 
ascent. But on the top of the ridge of Beinn a’ Chraig we find that the 
outliers which there remain are not portions of the lower basalts, but of the 
upper “ pale group ” of Ben More. The same rocks are prolonged on the 
other side of the Scarrisdale Glen, sweep over the summit of Beinn Fhada, 
and run on continuously into the crest of A’Cliioeli and the upper part of 
Ben More. The granophyre has usurped the place of the lower dolerites 
and basalts, but has left the more felspathic lavas of the “ pale group ” in 
their proper position. And to make this remarkable structure still more 
clear, sections may be seen on the southern flanks of Beinn Fhada, where the 
upper surface of the granophyre comes down obliquely across the edges of the 
lavas, and allows the junction of the basalts and the “ pale group ” to be seen 
a, bedded basalts and dolerites ; 6, “ pale group” of Ben More ; c, granophyre. 
above it (Fig. 355). As in the case of Beinn an Dubhaich, it is as if the grano- 
phyre had eaten its way upward and dissolved the rocks which it has replaced. 
The usual kind of contact-metamorphism has been produced around this 
intrusive boss. It is most marked in the outliers that cap Beinn a’ Chraig 
and on the two ridges to the south-west, where it is seen to consist in a 
high degree of induration, the production of a shattery, irregularly-jointed 
structure, and the effacement of the obvious bedding which characterizes the 
unaltered rocks. 
The position of this eruptive mass, quite a mile broad, breaking through, 
without violently tilting, more than 1800 feet of the bedded basalts, 
and then stopping short about the base of the “ pale group,” presents a 
curious problem to the student of geological physics. It at once reminds 
him of many sections among Palaeozoic granites where an eruptive boss has 
ascended and taken the place of an equivalent volume of the surrounding 
rocks, which, though more or less metamorphosed, are not made to dip away 
from it as from a solid wedge driven upwards through them. In this Mull 
case, however, there are some peculiar features that deserve consideration, for 
they seem to show that here, as elsewhere, passages for the uprise of the 
intrusive rock were already provided by the presence of volcanic pipes, which, 
