On Tridymite-Quartztrachyte, and on Olivinegabbro, 31 
The sanidine is a very typical one, and as it is quite free from 
interstices of a triclinic felspar, it is a very decisive argument 
that the orthoclases also really embrace two modifications ; often 
appearing in isomorphous mixtures; a kali or potash orthoclase, 
and a natron or soda orthoclase. The proportion of mixture in 
the sanidine of Tardree Mountain is potash: soda—1: 1. 
It seems, therefore, to be fairly proved, that the name chosen 
by me for the Rock of Tardree Mountain—“ Tridymite Quartz- 
trachyte,” is justly according to its petrological characters. 
II. 
Olivinegabbro of Carlingford Mountain. 
Another remarkable rock, of which I am indebted for specimens 
to Professor Hull, is that forming the ridge of the Carlingford 
Mountains, rising in the south of the Carlingford Lough, (9. Down. 
Mr. Hull in his recently published excellent work—* The Phy- 
sical Geology and Geography of Ireland,” gives (pages 143-45) a 
description of the geological occurrence of rth rock. He says — 
“The whole of this group of hills is formed of felspathic and 
pyroxenic rocks of several forms and varieties graduating into 
-one another, which the officers of the geological survey are fully 
persuaded are the representatives in time of those of the Mourne 
Mountains on the opposite side of the Carlingford Lough. Thus 
we may classify the two sets as under: 
Pyroxenic Group (the more ancient) consisting of micaceous 
dolerite of Slieve Gullion, diorite of Trumpet Hill, dolerite and 
anorthite hypersthene rock of Barnavene and Slieve Foy. 
Felspathic Growp (the more recent) consisting of varieties of 
felstone, porphyry and syenite of the Carlingford Mountains, and 
representing the granite of Mourne.” 
It is the “ anorthite-hypersthene” Rock of Barnavene, that caps 
the syenite rock in the rugged ridge of the above named mountain, 
of which I will give some petrdlaniend details, the results of my 
studies on thin slices of that rock. 
The rock is largely crystalline granular, composed of a triclinic 
felspar, which Rev. 8. Haughton has justly defined as an anorthite, 
with adark green pyroxenic mineral, forming irregular crystalloids 
between the better developed felspars, of greenish greasy grains 
of altered olivine and metallic-like grains of magnetite, 
