On Tridymite-Quartztrachyte, and on Olivinegabbro. 33 
is always covered by magnetite and therefore is not visible at 
the surface of the rock-specimens, It becomes entirely colourless 
in the sections, its borders are of a green yellow colour, a com- 
plete net-work of magnetite is spread through it and makes it 
partly non-pellucid. It contains glassy interstices, and many 
very little opaque needles, all disposed in a parallel direction. 
But that direction is parallel to the line of maximum extinction 
of light, and that is very distinctive as against diallage, where 
the direction of maximum extinction is oblique to the position 
of the interstices. The olivine appears partly altered to a green 
fibrous matter, polarizing like serpentine. After this description 
of the mineral constituents of the rock of Carlingford Mountains 
there cannot be any doubt of its classification. It is not an 
hypersthene rock, because it does not contain the orthorhombic 
pyroxene: it is not asyenite because it contains neither amphibol 
nor orthoclase, but it is a very typical Gabbro of the group 
characterised by the presence of olivine, and is not very different 
from the rocks described by Mr. Pettersen* from Store 
Bekkafjord,in Norway. From a cursory examination it seems 
probable to me also that one or another of the dolerites from 
Killala Bay, Co. Mayo, may be placed in the same group of rocks. 
* N. Jahrb. f. Min. 1876, p. 174. 
ScreN. Proc. R.D.3., Vou. 11., Pr. t. D 
