18 EE. B. Tawney— Woodwardian Laboratory Notes. 
ground is seen to consist chiefly of decomposing felspar crystals. 
The small felspars are much decomposed, they are often margined 
by decomposition products stained ferruginously. There is a little 
augite in a fresh condition, its extinction angle being 37°, but horn- 
blende has entirely disappeared ; parallel bars of ferric oxide perhaps 
indicate its pseudomorphs, the interior being entirely rearranged 
and unrecognizable, while chloritic matter and epidote, scattered 
abundantly through the ground, may also be considered indications 
of its presence. ‘The rock may be classed as epidiorite, rather than 
porphyrite, since amorphous matter seems to be absent from among 
the decomposing elements of the ground. 
A specimen taken from the new set-quarries on the N. side of Girn 
Ddu was also microscopically examined. It is a grey rock, in 
appearance like that of Bwlch Mawr, but the same quarry yields 
within a few yards quite light varieties, with a pinkish or slightly 
greenish hue. The rock is not yet deeply quarried, and under the 
microscope decomposition is seen at work. The large felspars are 
chiefly plagioclase, but are much attacked. No hornblende preserved ; 
scattered augite crystals are preserved, but are seen changing into 
a green substance with aggregate polarization; specks of this are 
scattered through the ground. Magnetite is present, connected with 
the chloritic tracts which also inclose apatite rather frequently. The 
ground is chiefly crystalline, consisting of squarish felspar crystals, 
and occasionally a small quartz grain, with some cryptocrystalline 
tracts. The smaller felspars may possibly be orthoclase, they mostly 
show no building up of hemitrope laminz, some few extinguish 
parallel toe their length, others donot. It is the augite and magnetite 
which give the dark colour to the rock in this part of the quarry, but 
the felspathic element of the ground is far in excess. ‘The rock may 
be classed as a variety of the epidiorite-group, though augite-syenite- 
porphyry would come also near to its constitution, if monoclinic 
felspar predominated. 
A little further south of these hills are other igneous masses 
marked on the Geological Survey Map in an almost parallel line to 
the above. ‘T'wo of these form the hills Penygaer and Moel bron-y- 
miod. Penygaer is mapped as greenstone and described as diorite 
in the text (l.c. p. 219). On ascending to the encampment at the 
top of the hill, I found it apparently a felsite, so that some error has 
crept into the colouring of the map here. 
This rock is light grey, with a more compact ground than have 
the preceding rocks, but with similar felspars about one-eighth of an 
inch in length scattered through it. A microscopic examination 
shows that it differs somewhat from the preceding; the ground is 
microcrystalline, a minutely granular mixture of quartz and felspar 
apparently, the felspathic portions much decomposed and a micro- 
pegmatitic structure is set up in spots. Chloritic or viridite matter 
in aggregates is scattered about, but a fragment of augite left unde- 
composed in the centre of one of the patches points to what the green 
mineral originated from. Apatite is abundant. Some of: the larger 
felspars are again plagioclase. In the minute quartz of the ground 
