Yol. 50.] 



ACID ROCES OF THE INNER HEBRIDES. 



227 



to the crests of the Cuillin Hills, and on the western front which 

 plunges into the dark cauldron-like hollow of Cony na Creiche they 

 likewise occur. In short, as I have shown elsewhere, the gabbro 

 masses as well as the plateau-lavas are penetrated by a vast number 



Fig. 4. 



-Plan of quartz-felsiie dyke and veins traversing gabbro 

 (ivest of Druim an Eidhne, Skye). 



■ 



of felsite-veins and dykes. There must be an immense body of acid 

 material underlying the various rocks that surround the granophyre 

 bosses, these bosses being merely its upper visible projections. 



IV. Conclusion. 



The evidence which I have adduced in the present paper will, I 

 trust, establish beyond any further cavil or question the fact, about 

 which there really ought never to have been any dispute, that the 

 great granophyre bosses of the Inner Hebrides are of younger date 

 than the gabbros. Prof. Judd considers the determination of 

 the relative ages of these two groups of rock to be an " absolutely 

 crucial " point in the interpretation of the volcanic history of the 

 region. And in this judgment I entirely agree with him. But, 

 apart from its local importance, the question has much wider 

 bearings in regard to geological theory, and especially to an under- 

 standing of the sequence of volcanic rocks. Many examples are 

 now known of the ascent of basic material first and of acid material 

 last in a cycle of volcanic activity, and this sequence harmonizes 

 well with the known order of separation of the constituents in 

 deep-seated bosses of eruptive material. The magnificent cones as 

 well as dykes and veins of granophyre which have burst through 

 not only the basalt-plateaux but also the eruptive cores of gabbro, 

 all over the range of the Western Isles from Mull to St. Kilda, 

 present the noblest example of this sequence in our area. 



In fine, there is a feature of Prof. Judd : s paper which for 



