1889.] 071 the Highlands of Scotland and the West of Ireland. 545 



The last question on which I propose to touch is the geological 

 date of the extraordinary terrestrial disturbances to which the crystal- 

 line schists of the Highlands of Scotland and the north-west of 

 Ireland, owe their characteristic structures. The limit of its antiquity 

 is easily fixed. As these disturbances involve rocks containing fossils 

 of Lower Silurian age, they must obviously have taken place after 

 some part at least of the Lower Silurian period. In Scotland their 

 chronological limit in the other direction is determined by the fact 

 that the conglomerates of the Lower Old Red Sandstone, are largely 

 composed of the crystalline schists of the Highlands. They must 

 consequently have occurred before the deposition of some part at 

 least of the Lower Old Eed Sandstone. Here, then, is a long geo- 

 logical interval within which the gigantic upthrusts and meta- 

 morphism began and ended. 



But the evidence obtained in Ireland enables us to fill up this 

 interval with a little more definiteness. In southern Mayo and 

 northern Galway, as Professor Hull has pointed out, the Upper 

 Silurian rocks rest upon and contain abundant fragments of the 

 younger crystalline schists which stretch into these counties from 

 Donegal. And the inference has naturally been drawn that the great 

 terrestrial disturbances and metamorjohism occurred before the Upper 

 Silurian period. But a recent more critical examination of the 

 ground has satisfied me that this inference, though to a certain extent 

 correct, does not embrace the whole truth. 



Those who have visited Connemara may remember the singular 

 group of mountains, which hem in the Killary fjords, and rise in 

 Mweelrea and its neighbouring ridges to a height of more than 2600 

 feet above the sea that frets their base. These massive buttresses of 

 rock owe their distinctive forms to the thick beds of coarse grit and 

 conglomerate of which they are in great measure built up. An 

 abundant series of fossils proves that this mass of deposits is of Upper 

 Silurian age. It is the base of these exceedingly coarse sediments 

 which along their southern margin can be seen to rest upon the 

 upturned edges of the crystalline schists, and to be there largely 

 made up of fragments derived from that metamorphic platform. 

 The numerous bands of coarse conglomerate upon successive horizons 

 serve to indicate considerable terrestrial disturbance during their 

 deposition. That the commotion continued after that time is further 

 shown by the remarkable way in which the rocks have been dislocated. 

 These Upper Silurian sediments have been broken up into large 

 mountainous blocks which have been thrown on end or actually 

 pushed over each other. So violent has the movement been along 

 certain lines, that the bands of greywacke and shale have been 

 intensely crumpled and puckered, and have actually been converted 

 locally into fine micaceous schists. 



Hence it seems tolerably certain that though in the west of 

 Ireland the chief i^lications, fractures, and metamorphism were 

 completed before Upper Silurian time, and though a vast interval 



