68 E. B. Tawney—Woodwardian Laboratory Notes. 
to chloritoid or viridite substance. Ferruginous and chloritic strings 
traverse the rock. There is no parallelism in the arrangement of 
the constituents in the hand specimen to determine its being called 
gneiss. The difference between this type of rock and granite is a 
difficult subject. Some distinctions have been already mentioned 
by Professor Bonney [l.c. p. 806]. It may be remarked that in 
igneous rocks, so far as I have observed, triclinic felspars and quartz 
do not occur together in nearly equal abundance. 
As mentioned by Dr. Hicks [l.c. p. 800], the boundaries on the 
Survey Map are not quite correct here, since these last two places 
are within the greenstone area on the map. 
West of Mynydd Rhiw, near Sarn, is a patch of so-called 
syenite from which Sedgwick, in 1831, collected what he terms in his 
MS. Journal “ gneiss,” remarking that it was the only place in Lleyn 
where gneiss occurred. He found it in a roadside quarry near the 
back of Meillionydd House, about three miles N.E. of Aberdaron. 
To the eye it is a mixture of rectangular portions of opaque white 
felspar, with milky quartz, and in places much blackish-brown mica, 
the mica irregularly distributed usually, but occasionally, in layers. 
[P. 52.] Under the microscope the quartz, which forms large 
mosaic patches, is seen to be fairly clear, the outlines usually only 
visible in polarized light; inclusions sometimes in lines; where 
bubbles are seen, they are often spontaneously moveable. The 
felspar seems chiefly plagioclase; it is much attacked by decom- 
position, which results in thorough change of its nature. 
Of all the areas of rock in the Lleyn promontory which Dr. 
Hicks claims as Pre-Cambrian, this patch of metamorphic rocks 
near Sarn seems to be the only one with any claim. It is, however, 
unfortunate that we have not been able to find any fragments of it 
inclosed in Cambrian beds. 
The range of hills east of this area called Mynydd Rhiw, consists 
of greenstone. The summit of Mynydd Rhiw is a more or less 
cellular diabase, rather fine-grained; some vesicles are filled with 
calcite, some with delessite. The conical hill by Clip y Cilfinhir of 
the map, is also of this rock, though mapped as within the syenite 
boundary. Castell Carron, a boss to the north of Mynydd Rhiw, 
is quarried, and the solid diabase is well exposed here. 
The microscope shows large augites, which are penetrated by the 
narrow plagioclases ; many tracts of secondary chloritoid matter 
result from decomposition of the augite; secondary quartz grains 
also occur in the same connection. Magnetite is abundant in single 
and branched forms. 
All observers admit these greenstones to be igneous. 
We may notice next the rock of the finely-shaped hill of Carn 
Madryn, some four miles §.S.W. of Nevin. In Dr. Hicks’s map it 
stands as Arvonian. It isa rock of the Bwlch Mawr type (noticed 
previously); the ground is darker grey than most of the Nevin 
porphyries, but the greenish-white felspars are of about the same 
size averaging 38-5 mm.; many can be seen with a hand lens to be 
triclinic. Under the microscope it is seen that the larger felspars are 
