[ 25] 
IIL—ON THE TRIDYMITE-QUARTZTRACHYTE OF TAR- 
DREE MOUNTAIN anp ON THE OLIVINEGABBRO OF 
THE CARLINGFORD MOUNTAINS, sy A. von LASAULX, 
PRoFESsOR OF MINERALOGY IN THE UNIVERSITY OF BRESLAU. 
CoMMUNICATED BY ProFessor HULL, F.R.S._ 
[Read April 15, 1878.] 
E 
One of the most interesting rocks, which I collected during 
my visit to Ireland in the autumn of 1876, is the Quartztrachyte, 
of Tardree Mountain, Co. Antrim. This remarkable rock has been 
well described by Professor E. Hull in the Explanatory Memoir 
of the Geological Survey to accompany sheets 21, 28, and 29 of 
the maps. It was formerly described by Portlock, who called it 
“ porphyry of Sandy Brae,” also by Berger and Bryce, and has 
generally been taken for a Quartz-porphyry. It seems un- 
necessary to enter into a description of its geological occurrence ay 
Professor Hull gives sufficient data to show that this rock is 
penetrated at the Scolboa Hill by the more recent) basalt, and 
that at other points, for instance the Carnearny Hiil and the 
Tardree Mountain, it is always covered by basaltic flows. There- 
fore it seems to be the oldest rock of volcanic origin in the 
County of Antrim. 
The petrological characters are the same at the different places 
which I had occasion to visit. Differences are only based on its 
colouration and are more or less the consequence of decomposition. 
In a light gray, yellowish, or light violet paste, of the characteristic 
roughish condition that gives the name of “ Trachyte,” to these 
rocks, are enclosed some rather larye crystals of sanidine, more 
rarely small prisms of a clinoclase, with visible triclinic stria- 
tion; subangular grains of quartz, sometimes exhibiting more 
determined dihexaedrons, and rarely minute flakes of a black 
mica. The sanidine crystals, when fresh, are colourless and glassy ; 
they become white and opaque by decomposition and change 
into caolinic matter. The quartzis smoky, often almost black. In 
the very numerous little pores of the rock are to be found groups 
