310 Proceedings of the Royal Irish Academy. 



the ice as it ploughed it^ way northward do^sii the valley. These three 

 deep basins, 10, 14l, and 9 fathoms deep respectively, are situated just where 

 the three quartzite ridges of Knockalla, Croaghuadownies and EosguiU are 

 severed by the fiord. There would appear to be a third such depression on 

 this line, namely, that underlying Lough Fern and now largely filled with 

 drift material, ililford stands on a rock bar. 



The Admiralty chart of Lough SwUly shows the existence of deeper 

 isolated basins separated from each other and from the deeper sea by shallow 

 bars. ITiese deeper pools are located not, as might perhaps be expected, in the 

 areas underlain by the softer schist, but just where the constrictions in the 

 fiord take place, i.e. where the hard quartzite ridges strike obliquely across 

 the lough. There are two such deep places marking the positions of greatest 

 erosion, the more southerly formiag the deep channel between the quartzites of 

 Eathmullan and of Inch Island, the more northerly lying wliere the quartzite 

 ridge of Knockalla is broken across by the SwiUy. In the former, a maximum 

 depth of 11 and several soundings of 10 fathoms are recorded. South of 

 the line of the Knoekalla ridge the depth decreases to 6 fathoms. Yet 

 along the Kuoekalla line a string of 10-fathom soundings is charted, to the 

 north of which the floor again rises to within 6 fathoms of the surface. 



XL — SuililAEY OF COXCLUSIOXS. 



In the north-west of Ireland there is unmistakable proof of a severe and 

 wide-spread glaciation ; its succe-ssive phases just passed in l•e^aew, and some 

 of the more general conclusions drawn from this study, may be briefly 

 summarized. 



The earliest phase of the glaciation is represented by the advance of ice 

 from the east-over the easterly part of the region, i.e. Inishowen, the western 

 portions of the counties of Londonderrj', Tyrone and Monaghan. This ice of 

 the earlier Scottish glaciation did not oveixide the whole of ]!J'orth Ireland. 

 Its western limit, assumed as coincident with that of the dispersal of erratics 

 from exclusively eastern sources, such as chalk, chalk- flint, Antrim basalt, and 

 Scottish rocks, e.g., the Ailsa Craig paisanite, is given hy a line running along 

 the lower reaches of Lough SwUly to Inch Island and Londonderry, across 

 tlie northern foot-hills of the Sperriu Mountains, east of Draperstown, and 

 south-westerly across County Tyrone to Slieve Beagh and County Monaghan 

 (PL IX). 'I'he re-entrant angle in this line, formed by the Sperrin Moimtains, 

 is probably to be ascribed to obstruction ofiered by a local ice-cap crowning 

 these hnis at this period. 



Pene-contempoi-aneous with or of slightly later date than this glaciation 

 was a radiation centred chiefly in the Bamesmore HUls and the mountains 



