1912-13.1 62 9 



conversion into gneiss by mechanical deformation ; (c) the 

 intrusion of four sets of igneous dykes ; (d) further earth move- 

 ments which converted the dykes into epi-diorites, hornblende- 

 schists, mica-schists, &c; (e) great denudation of the land surface, 

 which removed the cover under which the igneous rocks and 

 gneisses had been formed. The formation of the Torridon 

 Sandstone was followed by earth movements and extensive 

 denudation, and later by a downward movement of the land, 

 which sank beneath the level of the sea in which the Cambrian 

 rocks were subsequently deposited. At a later date after the 

 formation of the Ordovician and Silurian rocks a still more 

 stupendous series of earth movements set in, producing the 

 enormous thrust planes and reversed faults so characteristic of the 

 North-West Highlands. Attention was drawn to the natural 

 division of Scotland into four great structural areas — viz., (i) the 

 North-West Highlands, (2) the Eastern and Central Highlands, 

 (3) the Central Valley (Clyde and Forth Valleys), and (4) the 

 Southern Uplands, and the last three were shown to extend into 

 Ireland, and were traced across the island to the Atlantic coast. 

 Particular attention was drawn to the close similarity both in 

 nature and arrangement existing between the Ordovician and 

 Silurian rocks of the Southern Uplands of Scotland and those of 

 County Down and the neighbouring districts, while many of the 

 quartzites and limestones of the Central Highlands between the 

 Caledonian Canal and the Grampian Fault were shown to have 

 their counterparts in the counties of Londonderry, Donegal, and 

 Tyrone. 



At the conclusion of the lecture Mr. R. J. Welch raised 

 some very interesting questions on the subject matter of the paper, 

 and thanked the lecturer for the lucid explanation he had given 

 of such difficult problems. 



Mr. W. J. C. Tomlinson also commented on the interest of 

 the paper, and pointed out how necessary it was that more work 

 should be done on the same lines in Ireland, as has been done by 

 the Geological Survey in Scotland. 

 E 



