418 MAJOR A. E. DWERKTHOTTSE OX THE [vol. lxxix, 



the glaciation of the North-East of Ireland was the Firth-of- Clyde 

 ■Glacier. This great mer de glace had its origin in the High- 

 lands, but reached its maximum development only when the 

 eastern exit from the Central Valley of Scotland, the Firth of 

 Forth, was closed by the advance of the North Sea Glacier from 

 Scandinavia. At this stage the ice from the Grampians, and from 

 the Southern Uplands as well, must have escaped westwards, 

 largely by way of the Firth-of- Clyde Glacier. When this glacier 

 reached the Irish coast, which a glance at a map of the British 

 Isles will show to lie full in its track, it was cloven, part passing 

 westwards to the Atlantic and part southwards through the North 

 Channel to the Irish Sea. 



As the ice increased in thickness, it gradually overtopped and 

 submerged the cliffs of the Antrim coast and the Silurian uplands 

 of County Down. The track of this ice is marked by the occur- 

 rence of Scottish erratics, including several easily identifiable rocks 

 from Arran, and, most important of all. the riebeckite-eurite of 

 Ailsa Craig. It has been shown that the ice of this glacier covered 

 the whole of the counties of Antrim and Down and extended at 

 least as far south-westwards as the town of Monaghan, whence 

 the Ailsa Craig rock has been recorded. 



Prof. J. K. Charlesworth informs me that he has found pebbles 

 of flint in the drift on the flanks of Slieve Beagh, an observation 

 which confirms the westward movement of the ice in this region, 

 since the Cretaceous rocks occur only north-east of that locahty. 



Though much of the country formerly covered by the Scottish 

 ice was subsecmently glaciated from the west, sufficient evidence 

 remains in the distribution of remanie pebbles of Ailsa-Craig 

 eurite and in the occasional occurrence of relics of an older boulder- 

 <clay (the contents of which indicate a movement of ice from the 

 north-west) to support the conclusion that nearly the whole of the 

 area described in this paper was invaded by the Scottish ice. The 

 general trend of the ice-movement at this early stage is shown on 

 the map (fig. 12, p. 419). 



During this period there was doubtless much ice among the 

 hills of Donegal, and an extension thereof accompanied by a 

 shrinkage of the Scottish ice was responsible for the second phase 

 of the glaciation of the north-eastern counties. 



Two views are possible as regards the transition from Scottish 

 to Irish glaciation : either the Scottish ice retreated, and left the 

 ground vacant for the subsequent advance of the western glacier, 

 or the two ice-sheets were in contact throughout the period of the 

 Scottish retreat. I am of opinion that the latter of these hypo- 

 theses is the true one, and I base my view on the following facts 

 The track of the western ice throughout this district is marked by 

 the presence of erratics from the Tyrone Axis ; and the absence of 

 these rocks from a large part of the area east of Lough Neagh 

 and the line of the railway near Antrim Town and Cookstown 

 Junction has already been discussed. The glacier Avhich carried 

 the Tyrone rocks to Randalstown and to Moira was sufficiently 



