304 J. V. Elsden—Igneous Rocks of Lleyn, N. Wales. 
oxide, both magnetite and ilmenite.1 His specimens were selected 
from Pen-y-cil, Aberdaron quarry and Tynrhedyn, S. of Llanfaelrhys, 
and to his description of these rocks I find nothing to add, with the 
exception that one of my rocks taken from Pen-y-dre, close to 
Aberdaron, contains a much larger quantity of augite, in well-defined 
crystals. 
The small isolated patches, lying to the North of Aberdaron, and 
coloured as serpentine on the Survey map, are not described in Mr. 
Tawney’s paper. I obtained specimens of these so-called serpentines 
from Hendrefor, Ty-hen and Methlan. The rock from the last- 
mentioned locality appears under the microscope to consist of a net- 
work of plagioclase crystals, sometimes polarising brilliantly, but 
generally clouded with minute enclosures and decomposition pro- 
ducts. The felspars are seen penetrating plates of augite, with 
brilliant polarisation colours. ‘The whole rock is traversed by veins 
of serpentinous substance, and there are many patches of viridite. 
Biotite is sparingly represented and probably of secondary origin. 
Titaniferous iron, mostly decomposed into leucoxene, is present, and 
some bronze crystals of pyrite are seen by reflected light. Although 
there is far more serpentinous and viriditic matter than in the rocks 
of Aberdaron, iron separation has been less extensive. 
The specimen from Ty-hen is finer-grained, and the felspars much 
more decomposed. The interspaces are chiefly filled with green 
fibrous viridite, with aggregate polarisation. ‘'There is but little fresh 
augite, and a good deal of titaniferous iron generally altered into 
grey opaque leucoxene. Some threads of serpentinous substance 
also occur, and a little secondary biotite. ‘Two specimens taken from 
the extreme margins of these small bosses are interesting examples 
of contact metamorphism. The felspar crystals become very minute, 
probably owing to more rapid consolidation, and portions of the sur- 
rounding sedimentary rock appear entangled in a confused way with 
the igneous matter. 
Macroscopically these rocks are greenish-looking and sufficiently 
soft and serpentinous to account for the Survey colouring. They do 
not differ much, however, from the diabases around Aberdaron, 
except in their more pronounced viriditic decomposition. 
We come now to the large mass of igneous rock extending from 
Mynydd Penarfynydd on the south to Mellteyrn on the north. The 
rocks of this area are especially interesting on account of their variety. 
Thus the western slopes of the Penarfynydd ridge, as shown by Mr. 
Tawney, are of olivine diabase, which he considered to be a dyke 
intrusive in the hornblendic diabase of which the rest of the ridge 
is composed. It must be mentioned also that Mr. Tawney was 
unsuccessful in his attempt to discover the locality from which 
Sedgwick procured his specimen of hornblende picrite in this vicinity. 
The specimens of hornblende diabase described in “‘ Woodwardian 
Laboratory Notes”? were taken one from Careg Llefain, and the 
other from near Plas Rhiw. — 
My first specimen, in connexion with this area, was taken from 
1 GroLocicaL Magazine, 1880, Decade II. Vol. VII. p. 214. 
