Igficoiis Rocks. lOl 



general level. The dykes vary much in width, ranging from 

 a few inches to three or four feet. They occur straight, 

 sinuous, and occasionally branched. They run more or less 

 N.E. and S.W., and in this particular they differ from the 

 basalt and dolerite dykes of Tertiary age, which run in 

 directions practically at right angles to the dykes now under 

 consideration. These dykes are clearly later than the Upper 

 Silurian rocks, into which they are intrusive, and also later 

 ihan the period of the earth movements which tilted the 

 rocks on end. If the dykes may be considered to have their 

 origin from the same acid magma as the granites of the 

 Newry and Castlewellan range, then the conclusion is that 

 the dykes are of Lower Old Red Sandstone age. Two main 

 types are predominant — camptonite and kersantite — but 

 various intermediates occur. Special types are not confined 

 to distinct areas as a rule, but frequently the different varieties 

 alternate with one another over an extended tract of country 

 or shore line. Microscopical examination of the rocks 

 reveals the fact that the great majority are considerably 

 altered notwithstanding their fresh appearance in the field. 

 I'his is specially true of the felspathic constituent, which is 

 very often entirely replaced by calcareous and other second- 

 ary material. The coloured constituents are much less 

 altered and occasionally quite fresh. 



H. J. Seymour, f.g.s., in the Geoh\s;ica/ Mnoasine, June, 

 I goo, has described a dyke which occurs in South Bay, 

 three miles south-east of Portaferry, containing a secondary 

 blue amphibole (arfvedsonite), this being the first recorded 

 locality for the mineral in Ireland. The rock itself is a 

 hornblende kersantite, containing as essential minerals biotite 

 with subordinate green hornblende, plagioclases considerably 

 altered with various inclusions, and as accessory constituents 

 apatite, a blue amphibole, chlorite, and secondary calcite 

 replacing hornblende. The blue amphibole occurs mostly 

 as a secondary addition to, but partly also as a replacement 

 of the primary green hornblende, and has grown in crystal- 

 lographic continuity with it. 



County Antrim has not many representatives of the inter- 

 mediate rocks, but a fine dyke of camptonite at Rue Bane 

 Point, east of Murlough Bay, deserves to be noted. It is 



