JO Guide to Belfast. 



schists are pre-Devonian, as pebbles of the schist and 

 numerous pebbles of quartz, apparently derived from the 

 veins that traverse the schists, are to be found in the con- 

 glomerates of Old Red Sandstone age. That these pebbles 

 are much rounded, and that so large a percentage of them 

 are composed of hard quartz, would lead us to infer that 

 a very considerable time elapsed from the formation of the 

 schist series to the deposition of the rocks of the Old Red 

 Sandstone period. 



It is only by correlating the schists with the exposures of 

 lithologically similar rocks occurring in the .Mull of Cantyre, 

 that we can gain any light as to their age ; and in doing so, 

 we are led to place our series among the "Archaean," 

 between which and the overlying Devonian conglomerates 

 we find in the north-east of Antrim no representatives of the 

 Cambrian, Ordovician, or Silurian strata. 



The Silurian Rocks of the District. 



Passing from the early rocks of the north-east of Antrim, 

 the next representatives of the geological series found in our 

 district are the Ordovician rocks of the County Down. 

 Mr. Swanston, f.g.s., and Professor Lapworth have pub- 

 lished the results of their investigations in the appendix 

 to Proceedings of the B.N.F.C. for the year 1887; more 

 recently, officers of the Geological Survey have gone over 

 the ground, and the results arrived at are summarized in a 

 Guide to the Collection of Rocks and Fossils beloftging to the 

 Geological Survey of Ireland, 1895. 



These rocks are found widely distributed throughout the 

 whole of County Down, and occupy the entire of the lovv- 

 lands. Only on the slopes of the Mourne mountains are 

 they to be found at any elevation, and their elevation there 

 is due to the intrusion of a large laccolite of granite of much 

 later age. 



They extend beyond the limits of the county into Armagh, 

 Monaghan, Longford, and Cavan, also into Louth and Meath 

 on the south. Tracing the line of strike to the east, we find 

 the same rocks occurring in the lowlands of Scotland ; and 

 in Mr. Swanston's paper above referred to the beds or zones 

 in these two districts have been closely correlated. 



