546 Mr. Ardiihald GeiJcie on the Highlands of Scotland, &c. [June?, 



must have elapsed during which the progress of denudation laid bare 

 the younger schists and thereby provided materials for the Upper 

 Silurian conglomerates, the terrestrial disturbances nevertheless 

 continued during the deposition of these conglomerates, and were 

 renewed with increased vigour afterwards. 



If we compare the geological structure of the Silurian tracts of 

 England, Wales, the south of Scotland, and the east of Ireland, with 

 that of the areas of the younger crystalline schists, many points of 

 resemblance will be seen to occur between them. Towards the north 

 and north-west we find that the Archaean, Cambrian and oldest 

 Silurian rocks, now exposed there by the progress of denudation, 

 have been subjected to tlie intensest mechanical deformation, and 

 have assumed the most completely schistose structures. Coming 

 southward, we trace the younger crystalline schists of the central 

 Highlands and of Donegal thrown into innumerable north-east 

 and south-west folds, and becoming less and less metamorphosed as 

 they are followed towards the lower grounds. Still further south the 

 Lower and Uj^per Silurian rocks, plicated, crumpled and dislocated, 

 repeat the familiar structure of the southern Highlands, but with 

 only partial and feeble metamorphism. I am disposed to look upon 

 the whole of these structures as the result of one great succession 

 of terrestrial movements which began and reached their maximum of 

 intensity during some part of Lower Silurian time, but which con- 

 tinued to repeat themselves at intervals with greater or less vigour 

 through a long series of geological ages, down to the early part of 

 the Old Eed Sandstone period. 



As the consequence of this prolonged disturbance the Archaean 

 and older Palteozoic rocks have been thrown into those north-east 

 and south-west folds, which have in large part determined the trend 

 of the land in the north-west of Europe. The shaping of our 

 mountains into their present forms has been brought about by ages 

 of subsequent sculpture in which the agencies employed by nature 

 have operated mainly on the surface, but the carving of their 

 features has been guided by the internal structures developed by 

 those subterranean movements which we have been considering. 



[A. G.] 



