354 MAJOK A. It. DWEERi HOUSE ON THE [vol. lxxix, 



at Toome Bridge, flowing through Lough Beg and thence north- 

 wards to the sea near Port Stewart ; while the Biver Main rises in 

 the northern part of the central valley, flows southwards, and, 

 after receiving several tributaries from the eastern hills, enters the 

 northern end of Lough Neagh at Bandalstown. 



There is thus presented the unusual phenomenon of two parallel 

 streams flowing through the same depression in opposite directions. 

 In the latitude of Portglenone the streams approach within 5 miles 

 of each other, and in no case is the watershed between them more 

 than 300 feet above the bed of the Main, or more than 600 feet 

 above that of the Bann. 



(2) The Belfast Valley. 



This valley with its seaward extension, Belfast Lough, lies 

 between the basalt escarpment on the one hand and the Silurian 

 uplands on the other. It is floored by deposits of Triassic age 

 which are much softer than the surrounding rocks, and it is doubt- 

 less owing to this difference that the valley owes its existence. 



It is the valley of the lower part of the Biver Lagan and of a 

 smaller parallel stream, the Blackstaff, both of which flow over 

 Glacial deposits, but cut in places into the underlying Trias. 



The Upper Bann passes across the head of this valley on its way 

 to Lough Neagh, and is separated from the Lagan by an alluvial 

 plain, the deposits of which rest upon Glacial drift of considerable 

 thickness. Pending further investigation of the form of the rock- 

 head below the drift, the pre-Gdacial drainage of this area is a 

 matter of conjecture. 



(3) The Palaeozoic Country of County Down. 



This country is undulating and in many places hummocky. It 

 rises to over 700 feet in its northern part, which is separated from 

 the southern part by a narrow Trias-filled valley (under 200 feet) 

 that runs eastwards from Belfast by way of Comber to the head 

 of Strangford Lough. 



The southern part rises to some 600 feet near Saintfield, and 

 falls away gradually towards the south with many minor hills, both 

 of drift and of solid rock, diversifying its surface. These minor 

 hills, drumlins, and roches moutonnees give to the county 

 its characteristic hummocky surface, which, locally, is likened to a 

 basket of eggs. 



(4) The Igneous Areas of Mourue, Slieve Croob, 

 and Carlingford. 



The Silurian rocks of the southern part of County Down are 

 invaded by two masses of granite, the older and northernmost 

 occupying a belt of country which stretches some 6 miles from 

 north to south, and extends from Castlewellan on the east to 

 Newry and thence to the neighbourhood of Camlough in County 



