200 PROF. T. G. BONNEY ON ROCK-SPECIMENS COLLECTED BY 
12. On some Rocx-sprctmens collected by Dr. Hicks in ANGLESEY 
and N.W. CaERNARVONSHIRE. By T.G. Bonney, D.Sc., F.R.S., 
Sec.G.8., Professor of Geology in University College, London, 
and Fellow of St. John’s College, Cambridge. (Read December 5, 
1883.) 
TuIs series of specimens, which has been entrusted to me by Dr. 
Hicks for description, appears to me of the very first importance as 
throwing light upon the relations of the earlier rocks of North 
Wales. By his fortunate discovery he has welded, if the phrase be 
permitted, the last link in the chain of evidence which demonstrates 
the existence of Archean rocks in that region. I propose, then, 
in addition to giving a description of the microscopic structure of 
the slides which he has had prepared, to offer a few remarks as to 
the significance of these structures and their bearing on the general 
question. But as some of the slides closely resemble specimens 
which have already been described at length by Mr. T. Davies and. 
myself*, I shall not feel it necessary In every case to enter into 
very minute details, unless they appear to me of some special im- 
portance. 
(1) Railway-cutting east of Llanfaelog.—The rock-specimen is a 
very characteristic representative of the pale-coloured granitoidite T 
from this neighbourhood. Its peculiar subcompact aspect distin- 
guishes it readily from a normal granite, the quartz and the felspar 
appearing to the eye as if imperfectly separated one from the other. 
Parts of the specimen have a greenish hue, as if tinted by a mineral 
which varies from a yellowish to a pale dull green. Small scales of 
silvery mica and crystals of pyrite are visible. Under the micro- 
scope the slide is seen to consist mainly of quartz and felspar, the 
former predominating. There is no marked foliation in the strict 
sense of the word; but in the irregular, ill-defined outline of the 
felspar, the aggregation of the quartzes, and what I may call the 
confused mottled look of the slide when the polarizing apparatus is 
used, the rock presents more resemblance to a gneiss than to a 
normal granite. But as it evidently has been much crushed and 
subsequently recemented by secondary quartz and a minute scaly 
colourless mineral (probably a hydrous mica), it is difficult to decide 
what value may be attached to its present aspect. All that it is 
safe to say, with our present knowledge, is that the onus probands 
lies on those who affirm it to be a granite. In general aspect it 
corresponds with the more granitoid specimens of such ancient 
* In the pages of this Journal and the Geol. Mag. at various times since 
1878. ii 
+ For definition of this term see Quart. Journ. Geol. Soc. vol. xxxv. p. 322. 
I admit its inelegance, but, as no one proposes a better, must continue to use it. 
