278 Scientific Proceedings, Royal Dublin Society. 



Down. 



The area within the limits is nearly solely occupied by 

 Ordovicians, which towards the south are in part metamorphosed,, 

 having associated with them Granitic rocks of different ages — Or- 

 dovician, Triassic (?), and Eocene (?). At the'extreme south of the 

 county, also in the neighbourhood of Castle Espie, N.W. of Strang- 

 ford Lough, are very small tracts of Carboniferous Limestones. 

 On the shore of Belfast Lough is a small exposure of dolomyte,. 

 having fossils of Permian types ; while in the valley of the 

 Lagan, to the N.W. and W. of the county, the Trias is capped 

 with Cretaceous and Eocene (?) rocks. 



In the Cainozoic rocks are thin, valueless beds' of lignyte, and 

 in the Trias gypsum, but in too thin veins to be valuable. 



In the Ordovicians are numerous small veins and indications 

 of lead and copper, but only in a few places have they been found 

 rich enough to work. A few thin beds of anthracyte have been 

 noted in the vicinity of Strangford Lough. 



In early times iron was mined in the Slieve Croob district ;. 

 and a few years ago good hematite was discovered at Deehommed,. 

 south of Banbridge : this mine has still to be developed. 



In exogenous veins in the Triassic (?) Granyte of the Mourne 

 Mountains topazs and beryls have been procured ; the localities 

 being the N.W. side of the small lake on Bingian, on Slieve- 

 Havila, and the Chimney Rock Hill. 



Dublin. 



In this county the prevailing rocks are Carboniferous' of the- 

 Calp type. At Howth and Bray are small tracts of Cambrians. 

 West of the latter is the N.E. extension of the Leinster Granyte 

 range, flanked westward by Ordovicians, in part met amorphic; 

 while at Portraine and Balbriggan there are small exposures of 



OrdQVh'iCAS. 



The county is poor in mines, the lodes found, being principally 

 lead and copper. Id the Granyte at Dalkey, tin is also recorded 

 hy Griffith. 



