336 [B.N.F.O. 



THE CARNMONEY CHALCEDONY, ITS OCCURRENCE 

 AND ORIGIN, (WITH A GENERAL NOTE ON THE 

 FORMATION OF "SECONDARY" SILICEOUS 

 MINERALS IN VOLCANIC LAVAS). 



BY JAMES STRACHAN. 



i. Carnmoney Hill, with its steep escarpment sloping 

 sharply towards the Belfast Lough, is a prominent and 

 picturesque feature in the landscape of the country lying to the 

 North of Belfast. The attention of the geologist is attracted 

 both by the peculiar shape and the comparatively isolated 

 position of this Hill, which represents the site of an ancient 

 volcano, from whose throat, in Tertiary times, poured forth part 

 of the Upper Basaltic Lava. On the south side of the hill, the 

 denuded 'neck' or 'plug' of this old volcano may be traced, 

 cutting through the Lower Basalt, the Cretaceous, and older 

 strata. The Upper Basalt and part of the Lower have been re- 

 moved by denudation, leaving the plugged-up vent, which is 

 almost one quarter of a mile in diameter. The material of the 

 neck is a vesicular lava similar in appearance to that of the 

 doleritic dykes found in various parts of Co. Antrim. In a 

 recent Survey Memoir, the following penological analysis of 

 this rock is given : — "Under the microscope the rock is a fairly 

 coarsely crystalline dolerite, containing much magnetite in the 

 form of opaque, black, and unaltered crystals. The chief 

 constituents are pale brownish augites and fresh plagioclase 

 laths (labradorite) intergrown ophitically with one another. 

 There occurs also here and there in the sections a more com- 

 pact or finely crystalline material of similar composition, but 

 with a large proportion of glass. It is chiefly in this part that 

 the mineral 'hullite' occurs as a brownish or greenish-brown 

 translucent substance, somewhat like palagonite in appearance, 



