GABBROS ETC. Ii«^ SCOTLAND AND IRELAND. 55 



Twenty miles east-south-east is a fourth centre of eruption lying in the 

 peninsula of Ardnamurcban. Still further in the same direction, at 

 the distance of another 20 miles, is the fifth centre, from which the 

 volcanic masses of Mull and Morvern with the adjoining islands 

 were erupted. 



Now in the midst of each of these five areas we find, as I have 

 already shown, representatives of the most highly crystalline basic 

 rocks (gabbros), which are seen, when traced towards the peripheral 

 portions of the area, graduating insensibly into dolerites, basalts, 

 and finally into basalt-glass or tachylyte. What is more, the masses 

 of gabbro and dolerite are found to be intrusive in other masses of 

 more acid types of rock — granite, quartz-diorite, and diorite — which 

 are also seen to graduate into the volcanic types known as rhyolites, 

 dacites, and andesites. The whole of these more acid rocks, though 

 older than the basic series, are still of post-Cretaceous age. 



Those who would insist that the highly crystalline rocks — granites, 

 diorites, and gabbros — are necessarily of pre-Tertiary age, and that 

 the appearances presented in the "Western Isles of Scotland may be 

 accounted for by suj^posing Tertiary lavas to have accumulated 

 above and around older highly crystalline rock-masses, are confronted 

 b)' the startling fact that at no less than jive distinct and ^9(?r/gci% 

 isolated centres, we have the same remarkable collocation of crystal- 

 line rocks with lavas of identical composition. But as a matter of 

 fact, as I have already shown, the transition of the most highly 

 crystalline types of the several rocks into less crystalline, thence into 

 true lavas, aud finally into vitreous types, is alike manifested whether 

 we study the rocks in the field or in the laboratory. 



There is reason for believing that in Arran, 70 miles east-south- 

 east of Mull, we have another centre of Tertiary eruption for rocks 

 mostly of acid types. Fifty miles south-west of Arran we find the 

 great basaltic plateaux of the north-east of Ireland. This is the 

 largest of these areas of Tertiary volcanic rocks, except that of Ice- 

 land ; but the structure of the igneous masses is not so admirabl)' 

 exposed by denudation as is the case with the Scottish examples. 

 The fine coast-sections, it is true, exhibit many intrusive masses, 

 like those of Poi-trush and Fair Head, which I have carefully studied ; 

 while other highly crystalline rocks are said to be exposed more or 

 less clearly in the interior of the district. 



Southward of the great basaltic plateaux of Antrim, however, we 

 find in the Carlingford district another great centre of eruption both 

 for the acid and basic rocks of this period. Here denudation has 

 exposed the deep-seated masses in the same manner as in the 

 Western Isles of Scotland, and the correspondence of the rocks of 

 the two areas in all their essential characters is most striking. 



Although no igneous masses which are certainly of Tertiary age 

 are found further to the south than the Carlingford district, yet a 

 great plexus of dykes, more or less distinctly connected with those 

 great centres of eruption, can be traced all over the West of Scotland 

 and the northern parts of England and Ireland. 



The great band of Tertiary eruptive rocks which we are con si- 



