Igneous Rocks. 97 



common are blue-green beryl, topaz, and colourless amethyst, 

 much sought after by mineralogists and collectors. 



Along the coast, to the south of Newcastle, numerous 

 dykes of varied composition occur in uptilted Ordovician 

 strata. Noted examples are the ones of eurite, just north of 

 Bloody Bridge, at Green Harbour,andat Glasdrumman Point. 

 Many of these dykes are composite. The one at Glasdrum- 

 man has been described by Professor Cole.^ In this example 

 we find an original dyke of basaltic andesite, with an intru- 

 sion of eurite. Veins of eurite penetrate the basalt ; in 

 places the basalt has been re-fused, and allowed the floating 

 of crystals of quartz and orthoclase into the basalt. Lumps 

 of the basalt, containing red orthoclase floated in, have been 

 floated into the eurite along margins. The width of this 

 dyke is given at one place as composed of two margins of 

 basalt 17 feet and 4 feet respectively, with a central intru- 

 sion of eurite 37 feet wide. In another dyke south of 

 Bloody Bridge the eurite approaches a rhyolite in flow- 

 structure. 



The Newrv Granites. 



In the granite of the Newry district we have a rock of 

 greater antiquity than the Mourne granite, though not 

 yielding such picturesque scenery. The rock is later than 

 the Silurian but older than the Carboniferous limestone; it 

 represents the core of an uplift of lower Old Red Sandstone 

 times, and is thus of same age as the granites of the 

 Leinster district. The Newry granite is a grey, fine-grained 

 rock, free from the large porphyritic crystals of felspar and 

 from the large flakes of mica that characterize the Leinster 

 granite. 



The Acid Rocks of Antrim. 



The exposures of rhyolite at Tardree present many types 

 of rocks, from the massive pink lithoidal rhyolite to the 

 black glassy obsidian.-' The area occupied by the rhyolite 

 in the Tardree district is about ten square miles, but 

 sections are not well developed except in a few places. 



1 Scientific Trans. Roy. Dub. Soc, vol. v. (series ii), August, 1894. 



2 Pkof. Cole. — The Rhyolites of the County Antrim. Sci. Trans. 

 Royal Dub. Soc, vol. vi (series ii), May, 1896. 



