E. B. Tawney—Woodwardian Laboratory Notes. 69 
much decomposed, being full of secondary products, scales and 
needles in groups at irregular angles, calcareous specks, etc. ; most of 
them show remains of repeated twinning. The ground is micro- 
crystalline, consisting largely of minute grains of quartz, the rest 
being felspathic, of indistinct outline, with a little pale-greenish granu- 
lated matter in the interstices optically almost inert. Hornblende in 
remnants only, pale brownish-yellow, feebly dichroic, and mostly 
converted to a yellowish-green chloritic substance, which is feebly 
doubly refracting and sometimes fibrous. Apatite is abundant, piere- 
ing both the felspars and the chloritic portions. Black oxide in 
hexagonal form is abundant (ilmenite). A few bright crystals of 
secondary epidote have been formed. From the character of the 
felspars, the rock takes its place among the porphyrites, or more 
particularly quartziferous porphyrite. ‘The hill is mapped as sur- 
rounded by shale, said to be much covered by drift. In my cursory 
examination I did not succeed in finding any opening to show a 
junction. 
Immediately south of this hill—distant about a quarter of a mile 
—is another hill, Carn Fach, or Carn Fadrin of the map. It is 
similar to that of the Llanbedrog mass, but of a pale, quite light- 
grey colour, and compact ground; ferruginous spots in it are signs 
of decomposition. It is impossible to obtain a perfectly unweathered 
specimen ; with the hand lens some of the felspars are seen to be 
triclinic. 
The shales round this hill are noticed as being fossiliferous 
[Survey Memoirs, vol. iii. (ed. 2), p. 218]. Under the microscope 
the ground is found to be microcrystalline, consisting of minute 
quartz and turbid felspar grains. It is difficult to be certain that there 
is no amorphous matter among the granulated products of felspar 
decomposition, but there can be but little. The separate felspar 
crystals are chiefly plagioclase, often in groups set at various angles 
to each other, and much decomposed. They are fairly riddled 
frequently with branching veins of ferric oxide ramifying through 
them; larger ferric patches are abundant in the ground, and the 
veins often proceed from them, but minute specks are scattered 
throughout the ground; some opaque patches are perhaps limonite. 
Calcite occurs as decomposition product. Apatite is not abundant, 
but is inclosed in felspar. Notwithstanding the quantity of plagio- 
clase, we may put this rock among the felsites; bisilicates seem 
absent. 
East of these two detached hills is a large area of igneous rocks, 
also with them mapped as Arvonian by Dr. Hicks. ‘Two specimens 
from the Nanhoron quarries in this tract, collected by Dr. Hicks, 
have been already described by Professor Bonney [l.c. p. 306] as 
intrusive types. From a look at the area between Llanbedrog and 
Llanfihangel Bachellaeth I find the rock to be orthoclase-felsite for 
the most part. At the top of Y Foel Fawr the rock has a darkish- 
grey ground with not many separate felspars ; at Mynydd Manetho 
it is much the same as at Llanbedrog, viz. a greyish felsite with 
abundant felspars, with ferruginous spots from decomposition. 
