206 PROF. fT. G. BONNEY ON ROCK-SPECIMENS COLLECTED BY 
rock is in close association with indubitable coarse gneisses of the 
oldest type, and seems to be inseparable from them. 
(4) That the Anglesey granitoid rocks and some at least of the 
schists had assumed in all important respects their present mineral 
condition when the conglomerate of Llanfaelog (which, if not Cam- 
brian, must at least be extremely low in the Ordovician*) was 
formed, and that some fragments in the basal Cambrian conglo- 
merate of the Llyn Padarn district were derived from a rock sub- 
stantially identical with that of Twt Hill, where also a conglomerate, 
certainly not newer than basal Cambrian, rests on the granitoid 
rock itself~. The only conceivable alternative hypothesis (which 
is inapplicable to the last instance) is that these granitoid blocks 
were portions of fundamental rock ejected among the ash and scoria 
by very early Cambrian volcanos; and this, I think, could hardly 
be seriously entertained by any one well acquainted with the cir- 
cumstances. 
(5) That when the conglomerates so often referred to by Dr. 
Hicks, Professor Hughes, myself, and others, were formed, there 
existed in the neighbourhood either (a) a tract or tracts of Archean 
rocks upon which groups of volcanos ejecting rhyolite had been built 
up; or (6) that an ancient volcanic district had previously been in 
places so deeply gashed by agents of denudation that the granitic 
cores of some of its extinct craters were already exposed; and ona 
review of all the circumstances, I think the former far the more 
probable, for the latter takes no account of the schists which have 
contributed to the conglomerates of Anglesey and, to some extent, 
of Twt Hill. But in either case we are forced to the conclusion 
that between the commencement of the sequence of sediments which 
all agree in calling Cambrian, and the formation of certain iavas and 
schists and granitoid rocks, there was a distinct interval of time, 
which, in the case of some of these, must have been a very long one. 
Hence it follows that we are fully justified in asserting the existence 
of Archean rocks in North Wales, even if we admit that the rocks 
immediately below the conglomerate, taken by Dr. Hicks and Prof. 
Hughes as the base of the Cambrian, are not separated from it by 
so wide an interval as, perhaps, some authorities would demand; 
for, if the granitoid rock at Twt Hill and all at Llanfaelog be granite, 
it must certainly be Pre-Cambrian in date; and if a metamorphic 
rock, it must be referred to the oldest known Archzans—the 
‘“‘ Fundamental Gneisses.” 
* Tt is coloured Lower Silurian on the Survey Map; but this was issued 
prior to Prof. Hughes’s identification of Tremadoc and Arenig rocks consider- 
ably above the conglomerates of Anglesey. Quart. Journ. Geol. Soe. vol. xxxyvi. 
p. 239. 
; + The one which I erroneously referred to the Archzan series, entertaining 
opinions as to the action of selective metamorphism which I now see cannot be 
so readily assumed. I think, however, the exact positicn of the conglomerate 
is open to question, but that at any rate “ basal Cambrian” is the latest possible 
date that can be assigned to it. See Geol. Mag. dee. ii. vol. ix. p. 18. 
