_ 808 oS. V. Elsden—Igneous Rocks of Lleyn, N. Wales. 
We now come to the so-called serpentine of Porth Dinlleyn, 
described by Prof. Bonney in Q.J.G.S. vol. xxxvii. p. 48, and iden- 
tified as altered diabase-tuffs, decomposed diabases and basalts. His 
specimens were selected exclusively from the eastern side of the 
peninsula. I therefore proceed to the west side of the promontory. 
Near the ‘‘ Ancient Fortress” the rock appears to be a true diabase, 
and shows under the microscope a network of felspar, too clouded 
for determination of extinction angles. Strings of serpentinous 
matter and a good many viridite patches are scattered through the 
slide. Pale brownish augite occurs in small crystalline grains still 
polarising brilliantly. The opaque bodies are not abundant, and 
seem to consist entirely of magnetite and some pyrites. A little 
further south the rock is very similar to the above. About half the 
slide consists of prisms of felspar, sometimes polarising brilliantly, 
but generally decomposed. The extinctions are at a somewhat high 
angle to the trace of the twinning plane. Some of the felspars are 
porphyritic, with broken outlines. Augite is sparingly represented 
in detached grains and crystals, rather deeply coloured, and showing 
marked pleochroism. A good deal of greenish decomposition product 
occurs between the felspar crystals, and magnetite, ilmenite and 
some pyrites are scattered throughout. Apatite needles and a little 
biotite also occur here. Some of the felspars show a tendency to 
to saussuritic decomposition, giving aggregate polarisation, and 
greenish alteration products with some epidote. 
Still further south the rock becomes more serpentinous in appear- 
ance, and under the microscope more than half the slide is seen to 
consist of bright green serpentine, the remainder being an opaque, 
greyish substance, interpenetrated everywhere by serpentinous 
matter. Patches of calcite abound, and threads of the same mineral 
traverse the serpentine. Innumerable bands of dark granular 
matter, in more or less wavy parallel lines, occur throughout. These 
under higher powers transmit feeble brownish colours, and have the 
appearance of picotite. This is so far as I know the only instance 
of the occurrence of a rock approaching the character of true serpen- 
tine in Lleyn. The arrangement of the opaque bands is not such 
as to recall the “maschen structur” of peridotic serpentines, but 
possibly as my specimen was taken from the junction of the rock, 
we may have here the results of contact metamorphism of the sur- 
rounding schists. 
_ The greenstone dykes of Porth-wen appear to be chiefly dolerite, 
Under the microscope the rock appears to be crowded with lath- 
shaped crystals of felspar, no longer polarising in very bright colours. 
A good deal of viridite and some biotite are present. Olivine crystals 
are almost entirely altered, but are still recognizable. Augite also 
remains, but generally very opaque, from iron separation. A good 
deal of secondary calcite is seen, as well as magnetite and pyrites. 
