306 J. V. Elsden—Igneous Rocks of Lleyn, N. Wales. 
Immediately around Meyllteyrn we find the whole rock consist- 
ing of a network of felspar prisms, generally rather turbid, the 
interspaces being filled with augite in every stage of decomposition 
into viridite. In one specimen not a fragment of unaltered augite 
remains, while in others some crystals are still very fresh, polarising 
brilliantly and extinguishing perfectly. One large patch of viridite 
has at its two extreme ends two small portions of still unaltered 
augite, which extinguish simultaneously, as if originally portions of 
one and the same crystal. Magnetite is not very abundant, and 
generally of secondary origin, forming imperfect skeletons of augite 
crystals. There is a fair quantity of ilmenite, but much decomposed 
into leucoxene, and pyrites is sparingly present. 
It now remains, before leaving this tract of igneous rock, to men- 
tion the so-called Rhos Hirwaun syenite, which is mapped by the 
Survey as occurring in two isolated patches. It has been shown by 
Mr. Tawney that the boundaries of these patches are incorrectly 
mapped, for he describes specimens from Pen-y-gopa and Penllech 
(within the greenstone area) which should belong to the so-called 
syenite, while the rock of Clip-y-Cilfinhir is in reality a diabase.' 
The rock of Pen-y-gopa differs from that of Ty-mawr, described by 
Prof. Bonney,” in containing hornblende and but little mica, and is 
described as hornblendic gneiss. The specimen about to be described 
is taken from quite the other side of the main mass of this rock, 
from near Pen-y-bont, close to Llangwnadl. In the hand specimen 
this rock is an exceedingly tough fine-grained dark green hornblendic 
rock, traversed by fine veins of lighter green and speckled with 
small grains of quartz and felspar. Under the microscope the rock 
is seen to consist chiefly of dark greenish-brown hornblende, with 
characteristic cleavage and strongly dichroic where fresh, but the 
dichroism decreases rapidly with alteration, and it ultimately passes 
into a chloritic pseudomorph. The felspar is generally turbid, but 
here and there shows the characteristic twinning of plagioclase. 
Some orthoclase is also recognizable. The quartz is almost indis- 
tinguishable from the felspar except in polarised light, and is 
generally turbid with a multitude of minute cavities. A few crystals 
of magnetite and some pyrites are present. 
On the whole this rock bears a much closer resemblance to the 
Pen-y-gopa specimen than to that of Ty-mawr, but there seems to be 
scarcely sufficient trace of foliation to entitle it to be classed as 
hornblendic gneiss. 
Between Llanengan and Llangian are two small greenstone areas 
not touched in Mr. Tawney’s paper. At Pen-y-gaer quite half this 
rock appears to consist of felspar, but much decomposed. The 
felspars form a network, filled up by augite, which it also penetrates 
deeply. ‘The augite is brownish, with much iron separation along 
the cleavage cracks, and sometimes the whole crystal is rendered 
opaque from this cause. Scarcely any of the augite extinguishes 
1 GrotocicaL Macazine, Vol. X. p. 68. 
2 Q.J.G.S. vol. xxxv. p. 306. 
