part 1] AlfNIYERSART ADDEESS OF THE PRESIDENT. Ixxxi 



stratiform habit of the plntonic complex, and of its component 

 jDarts, and the regular sill-form of its satellites assure us that the 

 intrusions were effected during a time of relative rejDose ; but, as 

 regards their age, we knoAV only that they are younger than the 

 Durness limestones and older than the Caledonian overthrusting. 

 The lack of stricter chronological data and the isolated situation 

 of the district preclude any present discussion of the relations of 

 this interesting group. 



Towards the close of Silm-ian time the original geosyncline had 

 been filled in, and indeed was being replaced by a geanticline. 

 The thrust, which had been renewed, now attained a much greater 

 intensity, until it culminated in the main Caledonian crust- 

 movements. The rocks yielded hj folding, or cleavage, or over- 

 thrust, according to their lithological characters ; but, allowing 

 for this, we see that the effects become more marked, both in 

 separate areas such as Wales and the Lake District and in the 

 region as a whole, when we pass northwards or north-westwards. 

 The climax is reached Avhere the front of the great Highland 

 barrier itself has broken before the advancing earth-wave. Those 

 parts which had experienced to the full the effects of the Archaean 

 metamorphism and had cores of Archaean plutonic rocks, could 

 yield only en bloc. The movement, therefore, instead of being dis- 

 tributed, was localized in great dislocations along what Mr. Barrow 

 styles the ' outer margin of crystallization.' The displacement 

 at the Highland Border seems to be of the nature of an under- 

 thrust; a second master-fracture is represented by the line of 

 the Great Grlen ; and a third by the Moine overthrust, or by 

 the system of which it is the chief member. The south- 

 westward convergence of these three lines shows how the earth- 

 wave, travelling on the whole northwards, was swung round on 

 meeting an obstacle with a north-easterly and south-westerh^ 

 trend. The effect of these displacements was to sever the 

 Highland tract from the LoAvlands, and to divide it into three 

 broad belts, South-Eastern, Central, and North- Western, each 

 narrowing somewhat in the south-westerly direction. 



The Caledonian crust-movements were not immediately accom- 

 panied, like those of Archaean age, by igneous action. The S3^stem 

 of intrusions known to Highland geologists as the *Newer Granites,' 

 w^ith others of like age elsewhere in Britain, came somewhat 

 later, after crustal stresses had greatl}^ relaxed. Accordingly, 



TOL. LXXIII. f 



