876 Proceedings of' the British Association. 



ferous series, while western Derry and Donegal arc mostly pri- 

 mary. No district could therefore be conceived more favoura- 

 ble for observing the phenomena of transportation ; and we ac- 

 cordingly find that there is a marked distinction in the nature 

 of the detritus in different parts. Over all the basaltic area it 

 consists chiefly of various kinds of trap, as we sliould naturally 

 expect; but mixed with it there are fragments of rock which 

 could only have come from the west, and several o? which have 

 actually been identified with the rocks of Donegal and Western 

 Derry. Passing into the latter district, we find in the gravel 

 beds no fragments of the rocks of Antrim or Eastern Derry, all 

 their contents still point to a western origin. The fragments are 

 of much larger size along the western border of the trap district 

 than towards the centre or eastern part, proving that the cur- 

 rents diminished in force as they rolled to the eastward. If this 

 transport took place subsequent to the formation of the Derry 

 mountains, we must admit that the currents were strong enougli 

 to carry large blocks up inclined planes of a height varying from 

 SOO to 1500 feet. On the transition tract of the county of 

 Down, the most abundant rock in the detritus is of course grey- 

 wacke, but there occur also rolled masses of trap in great plenty, 

 of chalk, of chalk flints, and greensand, so far to the south-east 

 as Portaferry and Downpatrick, these being rocks which tould 

 only have come from the trap district, which is well known to 

 be every where encircled by chalk and greensand, lias and sand- 

 stone. In the north-east angle of Down, large pieces of chalk, 

 with its characteristic fossils, are frequent ; and seem to point to 

 x\\e formation of Belfast being at a period subsequent to the 

 transport of these masses. In the diluvial ridges in the valley of 

 the Lagan, the detritus consists of the secondary neptunian and 

 trap rocks of Antrim and Derry, and many varieties of primarv 

 rocks : no greywacke is found there, though it exists over all 

 the country to the south and south-west ; nor have we any in- 

 dications of the limestone of Fermanagh and Armagh, which 

 lies to the west, and the whole phenomena seem clearly to pcint 

 to a nortli-west current in these parts, and, therefore, to the pos- 

 terior origin of Longh Neagh, as it lies exactly in that direc- 

 tion, and as we cannot suppose that any current could possess 

 sufficient force to transport rocky materials through so deep a 

 basin. The direction of the diluvial r!dtres over all the area 



