: 
EE. B. Tawney — Woodwardian Laboratory Notes. 211 
intrusion of the olivine-diabase. This seems suggested by their 
relations in the promontory described: another fact tending in the 
same direction is the occurrence of a black rock, poor in felspar, in 
the ridge where the word Careg llefain stands on the map (a micro- 
scopic analysis is given below). It is, from its position, apparently 
intrusive ; I did not delay to trace its boundary, and had no 
opportunity of returning to it. [P. 84-5] are labelled “under 
Penarfynydd.” A similar rock [P. 81-3] is labelled “ W. end of 
Hell’s Mouth.” This may be taken to mean Penarfynydd ridge, 
or possibly near Rhiw; in the latter case it would show a second 
intrusion of this rock. 
Fossils were found in an opening at the back of Penarfynydd 
farm, the beds being a black softish pencil slate, with small specks 
of mica; their appearance was suggestive of Arenig. Determinable 
were Trinucleus Etheridgii, Hicks, Didymograptus bifidus, Hall; 
there was also a small" Obolella most like O. sagittalis, badly pre- 
served, and a tail of Barrandea with such a wide margin that I 
cannot refer it to any described species. The above fossils show the 
date of intrusion of the igneous rocks to be subsequent to Upper 
Arenig times. The dark shales around Llanfaelrhys are noted on 
the Survey Map as fossiliferous, and in Prof. Ramsay’s Survey 
Memoir (p. 173, 256-7) are cited from Llanfaelrhys “in flagey beds 
which resemble the Tremadoc slate” Ogygia Selwynii, Graptolites 
sagittarius, Obolella 2 sp.. One of the latter (ib. pl. 12, fig. 6) is 
probably the same as the one found at Penarfynydd. 
Olivine-diabase from Penarfynydd.—The specimen which I had 
sliced was from the top of the ridge; it has felspar, but less 
serpentine and olivine, differing thus from Sedgwick’s specimen 
numbered [P. 85]. The hand-specimen reminds one of Baste rock, 
being dark in colour, and the bright cleavage planes of the bisilicates, 
interrupted by dull portions of the ground obtruding through them ; 
generally greenish to brownish-black, with a few white spots of 
felspathic matter (in another specimen in local patches, the serpentine 
and bisilicates almost exclude other constituents): no effervescence 
with acids: it is identical with [P. 84]. 
Microscopic examination.—The rock consists largely of olivine 
crystals of rounded outline, and inclosed as separate crystals in 
felspar, pyroxene or hornblende; they are mostly very fresh, but 
some of them pass into serpentine in the usual manner with 
magnetite segregation, they always show the characteristic fissures, 
the first resort of serpentinous decay. The most abundant constituent 
perhaps is a pale-yellowish augite, much like the Radauthal one in 
appearance, with rather distant cleavage; it forms large sheets and 
smaller detached portions; some basal sections are seen; it is 
frequently intruded on by the olivines, which indent it or are 
inclosed within it. When undergoing decomposition it seems to be 
first crowded with magnetite grains, and then passes to a pale green 
or colourless substance, with very feeble doubly refractive power ; 
this is occasionally surrounded by zeolitic fibrous crystals. Horn- 
blende is present, forming a few large sheets of most strongly 
