1909-1910.] 



291 



Intrusive Masses and Necks, a division upheld by petrological 

 examination, for the former were Basalts and the latter Dolerites. 

 The numerous volcanic dykes, with their varying texture and 

 structures, provided a complete connection between the two 

 classes, shewing a gradual and complete transition from hypo- 

 crystalline to holocrystalline types. Further classification of the 

 basaltic rocks of Co. Antrim had been made by dividing them 

 into " fiuxional," " ophitic," and " granulitic " types, but Mr. 

 Strachan, who has devoted considerable time to the study of the 

 Basalt, suggested a sub-classification based primarily on the 

 varying basicity of the rocks, as follows : — 



I. Basalts without Olivine. 



(a.) Flow Type. The fine-grained basalt without olivine 

 found near Spanish Bay, Giant's Causeway. 



(b.) Intrusive Type. The dolerite (basaltic andesite) of 

 the neck at Carnmoney Hill, Belfast. 



II. Olivine Basalts. 



(a.) Flow Type. The common basalt of the neighbour- 

 hood, shewing " fiuxional," " ophitic," and other 

 structures. The olivine increases gradually from 

 minute scattered grains to numerous porphyritic 

 crystals. 



(b.) Intrusive Type. The common dolerite of the district, 

 generally ophitic, but also shewing granulitic structure. 

 Olivine variable as in the basalt ; the neck at Scawt 

 Hill, Ballygally Head, &c. 



III. Basalts Rich in Olivine. 



(a.) Flow Type. Composed for the greater part of olivine 

 phenocrysts, with minimum of augite and felspar ; 

 colourless interstitial glass ; N. side of Carnmoney 

 Hill. 



