E. B. Tawney—Woodwardian Laboratory Notes. 455 
white incrustation. A little secondary quartz crystallized out in a 
few spots. Professor Ramsay (Mem. Geol. Surv. vol. ii. p. 173) 
speaks of it as a rock in which “the hornblende rather predominates 
over the felspar.” 
—[P.119.] Quarries, N. Base of Carn Fadrin, near Cefn Madrin.— 
Identical in appearance with the Gimblet Rock and [P. 58] : plentiful 
effervescence with acid. The microscopical examination shows only 
slight differences from the above-mentioned rocks. The augite has the 
same characters ; in some crystals bands of ferric-oxide-like inclosures 
made up of detached grains cross the prismatic cleavage irregularly. 
Besides the augite, there are flakes of a dichroic mineral, sometimes 
brown, sometimes pale green; when placed with the cleavage parallel 
to the short diagonal of the lower Nicol, it has all the outward 
appearance and prism angle of hornblende, but in each case the 
maximum extinction angle was 35° in a clinodiagonal section, which 
differs from normal hornblende; it must be probably derived from 
the augite; the green is apparently a further change from the 
brown, as the latter shades off into it in some patches. 
There are also a few flakes of brown mica strongly dichroic. The 
felspars are mostly decomposed interiorly into an opaque white 
matter. The decomposition products are the same as in the pre- 
ceding slide, viridite patches, calcareous specks, fibrous aggregates, 
ete. 
We may call the rock a proterobase, using the word in a petro- 
logical and not a geognostical sense. In the present rock the horn- 
blende is far inferior in amount to the augite ; in the preceding case 
it was absent, but in some of the following it predominates over the 
augite. These hornblendic diabases are now noted in several parts 
of the Lleyn promontory, viz. near Aberdaron, Nevin, Pwllheli, and 
Clynnog.—E. B. T. 
[P. 58.] Quarry in the Nevin Hills, used for building purposes. 
This rock, which is of a brownish colour, speckled with white, is 
identical in the hand-specimen with that of Gimblet Rock Pwllheli 
[ P. 71], which has been incidentally mentioned as a gabbro (Q.J.G.S. 
vol. xxxil. p. 145). Though to the eye like a gabbro, the microscope 
characters of the pyroxene are nearer to augite, and it is therefore 
classed as diabase : the felspathic portions effervesce with acid. 
Microscope.—This slide shows a coarsely crystalline aggregate of 
intercrystallized plagioclase, felspar and pyroxene. The plagioclase, 
though still showing in some places its characteristic twinning, is 
generally much decomposed, being kaolinized in parts. The pyroxene 
oceurs in large irregular masses, whose edges are generally defined 
by plagioclase crystals, which often protrude into and are contained 
within it: though much cracked, it is well preserved and gives rich 
tints with polarized light. A third substance is also present, con- 
sisting apparently of aggregated chloritic and serpentinous minerals : 
from the external shape of these pseudomorphs one would conjecture 
them to have replaced pyroxene, but if so two varieties of that 
mineral of different durability must have been formerly present. 
Irregular grains of iron peroxide: are scattered throughout the slide, 
