DR. HICKS IN ANGLESEY AND N.W. CAERNARVONSHIRE. 203 
sent, which has been broken across twice and, as it were, *‘ faulted ” 
by subsequent pressure. 
(9) Cambrian Conglomerate from Pen-y-Gaer, Pont Rothell, Caer- 
narvonshire.—Pebbles well rounded, about as big as peas, many of 
them evidently the usual quartz-felsite (old rhyolite), a few paler 
and rather granitoid in aspect, matrix dull green. One of the latter 
pebbles is included in the slide. Itis a crystalline mixture of quartz 
and felspar; the former occurs in larger irregularly outlined grains, 
with a rather corroded look at the edge; the smaller are rounder in 
outline, and interrupt the felspar crystals. This mineral is rather 
decomposed, but orthoclase and microcline are recognizable. The 
structure is a perplexing one; certainly it is not that of a normal 
granite, but it is not unlike that of some vein-granites, so that at 
present we must remain uncertain as to the true nature of the rock. 
In structure it is more nearly allied to the specimens from Twt Hill 
than from Llanfaelog, the former rock, in one or two respects, 
approaching a little nearer to the characteristics of a true granite. 
Besides this the slide shows well-rolled fragments (a) of a rhyolite, 
exactly resembling that so often described in my papers on the N.W. 
of Wales, (6) of a rock of the same class, but with a less clearly marked 
fluidal structure, and rather numerous blackish belonites; both 
contain the usual crystals of quartz and felspar. The smaller frag- 
ments, so far as they can be identified, consist of numerous bits of 
rhyolites (some black and slaggy-looking, some containing abundant 
elongated crystallites of felspar), together with broken quartz and 
felspar. Among these, often as a kind of setting, and sometimes 
penetrating into cracks in the fragments, is a green chloritic mineral, 
feebly dichroic, and with but little depolarizing power, probably 
the altered detritus of a pyroxenic mineral. One of the fragments 
contains a patch of two of it. 
(10) A pebble about 1} inch long, from Cambrian Conglomerate, 
Dinas Dinorwig.—The slide shows this to be a granitoid rock, con- 
sisting of quartz, felspar, and a green mineral. The usual inclusions 
are noted in the quartz ; the felspar is rather decomposed, but 
orthoclase, microcline, and albite (?) are present. Partof the green 
mineral is hornblende, the rest is a chloritic mineral, probably 
replacing it. The remarks made about the granitoid rock in (9) 
apply to this specimen, which only differs in the larger proportion 
of the green mineral. 
(11) Cambrian Conglomerate from Dinas Dinorwig, containing a 
well-rolled pebble about 1 inch diameter, similar to the last. The 
microscope shows the apparent identity to be real. The matrix is 
largely composed of rhyolite in rolled fragments, one of them exhi- 
biting minute spherulites with irregular edges; quartz, felspar, a 
chioritic mineral, and an iron peroxide are also present. 
The contents of these three slides prove that either a district of 
Archean rock supported the rhyolitic voleanos, or the latter had been 
gashed by denudation to their crystalline cores when the conglo- 
merate was formed. 
