14-J 



DR GEIKIE ON THE HISTORY OF VOLCANIC ACTION 



side being pushed up further or with a steeper slope than another side. But even in the 

 case of the Cuillin Hill area, it is conceivable that the total uplift produced at the surface 

 a gentle inclination of no more than 8° or 10°. 



It is along the periphery of a gabbro area that we may most hopefully search for 

 traces of this uplift. But unfortunately it is just there that the work of denudation has 

 been most destructive. There appears also to have been a general tendency to sagging 

 subsequent to the gabbro protrusions, and the inward dip thereby produced has probably 

 been instrumental in effacing at least the more gentle outward inclinations caused by the 

 uprise of the eruptive rock. In one striking locality, however, to which I have already 

 referred, the effects of both movements are, I think, preserved. The basalt-plateau 

 of Strathaird, which in its southern portion exhibits the ordinary nearly level bedding, 

 dips in its northern part at an unusually steep angle to the N.W., towards the gabbro 

 mass of Blath Bheinn. But before reachiDg that mountain the basalts, much interbanded 

 with sheets of dolerite and gabbro, suddenly bend up to form the prominent eminence of 



a c a 



Fig. 42. — Theoretical representation of the structure of one of the Gabbro Bosses of the Inner Hebrides, a, platform of 

 older rock on which the bedded basalts (b) have been poured out ; c, gabbro. 



An Stac, where they dip rapidly towards S.E. and S. (fig. 37). This steep dip away from 

 the central mass of gabbro, is repeated in the hills to the north, where the beds are 

 inclined to N.E., the angle gradually lessening northwards till they are truncated by 

 the granophyre of Strathmore. The theoretical structure of one of the gabbro bosses is 

 represented in fig. 42. 



7. The injection of so much igneous material among the bedded basalts has induced 

 in these rocks a certain amount of contact metamorphism. I have referred to it as 

 showing itself in the field as a marked induration, the rocks becoming closer grained, 

 dull, and splintery, weathering with a grey or white crust, while their amygdules lose 

 their definite outlines, and epidote and calcite run in strings, veins, and patches through 

 many parts of the rocks.* The microscopic characters of the altered basalts are described 

 at p. 167. 



* Many years ago I was much struck with the evidence of alteration in the igneous rocks of Mull, and referred to 

 it in several papers, Proc. Roy. Soc. Edin. (1866-67), vol. vi. p. 73; Quart. Jour. Geol. Soc, xxvii. (1871) p. 282, note. 



