204 PROF. T. G. BONNEY ON ROCK-SPECIMENS COLLECTED BY 
(12) Pebble from Cambrian Conglomerate, Moel Tryfaen.—This is 
not in acondition very favourable for examination, but it appears to 
be one of the above-described granitoid rocks, with its felspar 
much decomposed, which may have been crushed and recemented 
before it was broken away from the parent rock. 
(13) and (14). Pebbles from Cambrian Conglomerate by shore of 
Menai Straits, west of Garth—These specimens are from rounded 
pebbles, each rather more than 2 inches long. They do not differ 
greatly to the eye, and might be readily taken for a granitoid rock, 
but are really a rather spherulitic quartz-felsite, containing, por- 
phyritically, quartz and felspar. The former sometimes occurs in 
rounded grains, sometimes shows moderately perfect crystal angles. 
Inlets and enclosures of the devitrified matrix also occur. Cavities 
are fairly numerous and of moderate size, the majority containing 
bubbles, which occupy about one eighth of their volume. The fel- 
spar is rather decomposed, but orthoclase and a plagioclase, with 
rather small extinction-angles, may be recognized. The spherulites, 
which occupy the greater part of the mass, are not very sharply de- 
fined, and appear to consist mainly of felspar. The black cross is 
very indistinct, but they exhibit a structure which, so far as my ex- 
perience goes, is rather rare, namely, that considerable portions of a 
spherulite give, with the nicols, a nearly uniform tint, as though 
the usual “sheaf” of crystallites had been replaced by a single 
crystal. Possibly this is analogous to the common change in calcite, 
when a mass originally cryptocrystalline assumes an ordinary crys- 
talline structure. The only markedly spherulitic rock which I know 
to occur in situ in the district is at Tan-y-Maes, near Port Dinorwig™*, 
where is one which is quite as good an example as the well-known 
instance at the School House, St. David’s. To this, macroscopically, 
these pebbles bear a great resemblance, but in the Tan-y-Maes rock 
the radial structure of the spherulites is more perfectly developed, 
and the fluid-cavities are fewer and smaller. Still I have little 
doubt that these pebbles have been connected with the great masses 
of rhyolitic lava from which the conglomerates of the district have 
derived so much of their materials; though from what I know of 
spherulitic structure, I should think it more probable that they (like 
the rock at Tan-y-Maes) have been formed from a dyke or a part of 
a flow, cooled before reaching the surface of the ground. 
To the above description of Dr. Hicks’s specimens I shall venture 
to append afew remarks as to the nature of the evidence for the 
existence of representatives of the Archean series in North Wales; 
because, as it appears to me, some geologists are unable to realize 
how strong it is, and regard the question as a still open one. This 
perhaps is due to the fact that, partly owing to the novelty of the 
investigation and partly to the nature of the case, the evidence can 
only be appreciated by those who have devoted themselves to its 
* Geol. Mag. dee. ii. vol. vii. p. 301. 
