Cotz & CunnincHam—On Certain Rocks styled ‘‘Felstones.”’ 317 
the magma, as is often the case in modern rocks of the same kind.! 
Numerous steam-vesicles occur, partly filled up by intrusion of 
the glassy groundmass during cooling, and partly by secondary 
calcite. Pyrite has developed abundantly in these cavities, and, 
in smaller cubes, reddened by alteration, even within the serpen- 
tinised olivines. 
The groundmass of the rock contains spherulitic sheaves of 
felspar microlites; many of these minute crystals are repeatedly 
twinned, though retaining the residual glassy band up the centre 
of the prism, which is so characteristic of the imperfectly developed 
plagioclase in tachylytes. Their optical characters place them 
between andesine and labradorite. 
The brown glass becomes more prominent, and at the same 
time more vesicular, near the selvage, and is here crowded with 
little cumulitic patches. The ferro-magnesian constituent has not 
developed ; but the alliance of the rock with the olivine-basalts is 
manifest. It is the kind of material that might have developed 
good variolitic selvages, had it occurred on a more massive scale. 
The larger of the two dykes near Laghy has a more ordinary 
groundmass of minute crystals of plagioclase and very pale brown 
monoclinic pyroxene. The prisms of pyroxene are, however, very 
abundant, and it is improbable that the two dykes are direct off- 
shoots of the same mass. Olivine is represented in this case by 
green and black pseudomorphs, and is by no means so prominent 
as in the smaller dyke. In one instance, the olivine crystal has 
formed about a pre-existing needle of felspar, which runs through 
its length, and projects at either end. 
This compact basalt contains inclusions of a greyer olivine- 
basalt, of a somewhat unusual trachytic aspect. ‘These have under- 
gone partial absorption at their surfaces. Both the types of rock 
thus associated contain many small crystals of magnetite and 
pyrite. 
The two dykes north of Laghy must, then, be removed from the 
field-class of “ felstones” ; but they cannot with certainty be 
correlated with the products of the Cainozoic eruptions. They 
are post-Carboniferous, and may perhaps represent some early 
1 Compare G. Cole, ‘‘ The Variolite of Ceryg Gwladys, Anglesey,’’ Sci. Proc. Roy. 
Dubl. Soc., vol. vii. (1891), p. 117. 
