TRANSACTIONS OP SECTION C. G71 



4. On the Origin of certain Quartz Dykes in the Ide of Man, 

 By J. LoMAS, A.R.C.S., F.G.S. 



About Foxdale, I.O.M., the ground is strewn with blocks of quartz, and 

 numerous quartz veina traverse the altered slates which form the structural 

 anticline of the island. In nearly all cases these veins are accompanied by acid 

 dykes having the same general trend. 



Where the quartz traverses the granite mass at Foxdale, it is locally developed 

 into a coarse pegmatite with idiomorphic crystals of felspar 3 inches long and 

 mica over 1 inch in diameter. 



Away from the granite the quartz is clear or milky, rudely columnar in struc- 

 ture, and on the joint faces sericite is common. 



It is suggested that the quartz is a true injection resulting from an overplus of 

 silica in the magma from which the granite crystallised. 



5. Supi^lementary List of Minerals occurring in Ireland. 

 By Henry J. Seymour, B.A., F.G.S. 



The following species may be added to the list of Irish minerals published last 

 year in the Report of the Belfast Meeting of the Association: Minium, Wad, 

 Xanthosiderite, Strontianite.Nephelite, Scapolite,Orthite, Brewsterite, Zinnwaldite, 

 liipidolite, Halloysite, Mimetite, Betinite. 



6. The Average Composition of the Igneous Rocks, 

 By F. P. Mennell, F.G.S. 



The author has calculated the average composition of the igneous rocks 

 occurring in a given district — that surrounding Bulawayo — taking into considera- 

 tion the bulk of each type of rock present. Owing to the great preponderance in 

 bulk of the granitic intrusions, despite their inferiority in number to those of other 

 rocks, the result obtained for the average composition shows a silica percentage 

 of 69-88, a number sensibly equal to that assumed for the granite (70 per cent.) 

 The author considers that the same would certainly hold good for the whole of 

 Rhodesia, and probably in general for any large representative area, and draws 

 the conclusion that granite represents substantially the magma from which even 

 the moit basic rocks have been developed by some process of differentiation. 



