part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixxxiii 



diorites, quartz-diorites, and hornblende- and biotite-granites. In 

 this area, presumably near the principal focus of intrusion, the 

 more basic types are often well represented ; but farther awa}?-, in 

 Galloway, the Lake District, and Leinster, they are either lacking 

 or of small importance. On the other hand, muscovite-bearing 

 granites, Avhich are little in evidence in the Highlands, become 

 the dominant type in the most outlying belt. In Leinster we 

 find granites rich in microcline and muscovite. Here, too, south- 

 east of the main chain, and, as Prof. W. J. Sollas has shown, 

 of somewhat later intrusion, is a group of soda- granites, including 

 some rich in albite. Some of the minor intrusions presmnably 

 related to the granites are of thoroughly alkaline composition, and 

 it is clearly suggested that we touch here the limit of the 

 distinctively calcic province which goes with the Caledonian dis- 

 turbance. 



The minor intrusions of the region in general, usually taking 

 the form of dykes, belong to simple petrographical types. There 

 are first porphyrites, perhaps not differing materially in composi- 

 tion from the average magma of the region, and then lamprophyres 

 with their complementary acid rocks. The distribution of stress 

 during a dyke-phase is necessarily more complex than during the 

 preceding plutonic phase. It is compounded of the regional 

 S3'stem together with local stresses centreing in the plutonic intru- 

 sions, at least when these have the boss-habit. In the areas of 

 maximum disturbance the regional element is usualty dominant, 

 but in outlying districts the local stresses pi-evail. Thus the 

 Caledonian dykes in the South-Eastern Highlands tend to have 

 the strike-direction, and those in Galloway the direction of dip ; 

 but round the Shap granite the dykes have a markedly radiate 

 arrangement. 



A great system of crust-movement is not completed at one 

 defined epoch. It appears indeed that some part of the Cale- 

 donian overthrusting in the Highlands was effected during Old 

 Red Sandstone times : but, what is most in evidence during this 

 waning phase is not the lateral displacement, so much as the 

 differential vertical movement which went with it. Broad folds 

 were gradually formed, in the troughs of which the Old Ked 

 Sandstone was deposited. The conditions during this period, and 

 especially the manner in which the vulcanicity was related to 

 the broad geographical features, have been so well described by 

 Sir Archibald Geikie that a brief summary will suffice here. The 



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