236 Cotr—On the Geology of Slieve Gallion, in the County of Londonderry. 
much altered. But an ophitie character is discernible, despite the zones of inter- 
action between the constituents. These zones sometimes consist of a colourless 
amphibole; and iron-ores are not at all prominent. Thus, whether pyroxenic or 
amphibolic, the rock stands well within the diorite group. The lustre of the 
schillerised pyroxenes has caused some of them to be referred to hypersthene by 
older authors; but the microscope shows that the mineral is not rhombic. The 
specific gravity of the gabbro in the north of Tullycall is 2:98. 
In the mass, the diallage masks the felspar, and the latter mineral may be at 
some points almost absent. Mr. Watts* thus describes diallage-rock among the 
coarse gabbros of Athenree and Termonmaguirk. The basic rocks of the area 
have been generally referred to as ‘‘ ophyte” by Mr. G. H. Kinahan.t+ 
Granite veins, passing into eurite, traverse them, reminding us of the slope of 
Mobuy.t This granite is poor in ferromagnesian constituents, and is sometimes 
devoid of them, except for derived hornblendes from the basic series. A micro- 
granular pink-brown eurite, in the dolerite of the south-east of Feegarran, has a 
specific gravity of 2°61, identical with that of the fine-grained aplitic granite in 
Carndaisy Glen. 
Occasionally veins of pure felspar occur, which are obviously extruded from the 
underlying granite, but which are not familiar to me in other areas. These are 
sometimes milk-white, sometimes pink, and the individual crystals may be 3 mm. 
across. A microscopic section across one of these veins, traversing ophitic horn- 
blende-diorite (altered gabbro) one mile north of Oritor, shows the felspar com- 
posing it to be untwinned. The specific gravity of this pink felspar is 2°60. ‘The 
white felspar of the veins in a diorite further west gives 2°59 to 2°64; in this case, 
the flame-reactions by Szabd’s method indicate the species as a soda-orthoclase. 
We have, then, actual veins of orthoclase-rock as part of this ‘ plutonic complex.” 
In the ease last cited, the vein contains a little derived hornblende, and one or 
two crystals of sphene. 
Immediately north of the mill at Oritor, under the gabbro-domes of Craigs, a 
fine quarry exposes a variety of igneous rocks. At the north end, a seemingly 
bedded series, dipping N.N.E., lies above a diorite, which is regarded as hyper- 
sthenite in the Survey Memoir; but the proximity of hornblende-schist, clearly 
derived from some basic rock by metamorphism, makes it likely that the appear- 
ance of bedding in any part of this quarry issuperinduced. ‘The soft grey talcose 
rock in which this feature is exhibited is probably itself a decomposed andesite. 
The main rock of the quarry is a diorite, passing into an amphibolite in some 
places, and in others into a form with much dull pink felspar, the ‘“‘syenite” of 
* McHenry and Watts, op. eit., p. 73. Compare Portlock, op. ett., pp. 519 and 549. 
} ‘ Economic Geology of Ireland,” Journ. R. Geol. Soc. Ireland, vol. vii. (1887), p. 160. 
t Compare Mem. sheet 26, p. 13. 
