20 LE. B. Tawney—Woodwardian Laboratory Notes. 
see that hornblende is present in small quantities; some of the 
bisilicate is, however, passing into chloritic matter; magnetite is 
connected especially with these tracts. 
In two cases hornblende crystals inclose an augite centre, though 
set with axes parallel. the value of the extinction angle is quite 
different at the centre and outside, the latter only being dichroic. 
The rock may be classed as granite-prophyry slightly hornblendie, 
rather than quartz-diorite, though many of the larger felspars are 
plagioclase. 
The mass is mapped as surrounded with shale; evidence of 
induration of the adjacent shales as if by heat may be seen near Tir 
Gwyn. 
The set-quarries at Moel Gwyn, between Pistyll and Nevin, yield 
a rock with dark grey ground and white felspar, much like that of 
Girn Ddu; no quartz is visible in the lens, but many of the felspars 
are seen to be triclinic. 
[P. 56], Sedgwick Collection, from hill one-half mile $.S.H. of 
Pistyll, would perhaps be from Moel Gwyn; it is a similar darkish 
grey porphyry. Microscopic examination shows a ground consisting 
chiefly of rectangular crystals of felspar ; most of the larger felspars 
are certainly triclinic. Those of the ground- are somewhat quad- 
rangular, but mostly the length is about twice that of the 
breadth. They mostly extinguish very nearly in the direction of 
their length, and are therefore probably chiefly oligoclase. Some 
are probably orthoclase, but the limits of error in the present case 
make it uncertain; these small felspars do not show twinning. Both 
hornblende and augite are present in small quantities, accompanied 
by decomposition products, chloritic matter and magnetite. Apatite 
is also abundant. ‘The augite is recognized both by basal sections 
showing cleavage and a high extinction angle in clinodiagonal 
sections. The hornblende has nearly all decomposed. The rock may . 
be classed as epidiorite. 
[P. 8], Sedgwick Collection, from sea-side between Pistyll and 
Nevin, one-half mile from Pistyll—This must be from the igneous 
mass of which Nevin occupies the centre; it has the appearance of 
a syenite-porphyry, the dark green patches being almost equal to - 
the felspathic portion of the rock. 
Microscopic examination shows that the separate felspars are chiefly 
plagioclase, which is more abundant than orthoclase. They are much 
decomposed. Quartz is abundantly crystallized out in the spaces 
between the felspars, and the ground often shows.a micropegmatitic 
structure. The hornblende is green and fibrous, and forms some- 
times groups of actinolite-like crystals, which are at times inclosed 
in quartz ; the further state of change is that of chloritic aggregates. 
The quartz is probably partly of secondary origin. The rock may 
be classed as quartz-diorite, rather than granite-porphyry, from the 
nature of the felspars. 
The Gwylfa rock, near by, is also described in the Survey Memoir 
[ed. 2, p. 219] as diorite. 
The rock of Mynydd Nevin, which is the hill nearest to Carn 
