DURING THE TERTIARY PERIOD IN THE BRITISH ISLES. 155 



the south-eastern side, between the head of the Scarrisdale river and Loch Ba', the line 

 of junction between the two rocks is nearly vertical, but a body of black felsite 

 intervenes as a huge wall between the ordinary granophyre and the basalt. On Beinn 

 Fhada and Beinn a' Chraig the line of separation, as I have above remarked, is inclined 

 outwards, and plunges under the basalts at an angle of 30° to 40°. The terraced basalts 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. 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'Chioch and the upper part of Ben More. The 

 granophyre has usurped the place of the lower dolerites and basalts, but has left the 

 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 



Fig. 46. —Section on south side of Beinn Fhada, Mull, cm, bedded basalts and dolerites ; 

 b, "pale group " of Ben More ; c, granophyre. 



the edges of the lavas, and allows the junction of the basalts and the "pale group" to 

 be seen above it (fig. 46). 



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. In the field, 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 characterises the unaltered rocks. The microscopic changes will be described on 

 a later page, together with those of other districts. 



The position of this eruptive mass, quite a mile broad, breaking through, without 

 violently tilting them, 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 



