886 
[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 old<_r 
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 petrological 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, 
