H. HICKS ON CAMBRIAN CONGLOMERATES IN ANGLESEY ETC. 187 
11. On the Camprian Cone@LoMERATES resting upon and in the 
vicinity of some Pre-CamBrtan Rocks (the so-called INTRUSIVE 
Masszs) in AwneLEsrY and CamrnarvonsHire. By HENRY 
Hicxs, M.D., F.G.S. (Read December 5, 1883.) 
Iy papers which I communicated to the Geological Society in 1877 
and 1879, I brought forward evidence to show that some rocks in 
Caernarvonshire and Anglesey which were indicated on the maps of 
the Geological Survey as great eruptive masses intruded into strata 
of Cambrian and Silurian age, were really rocks of Pre-Cambrian 
age. I pointed out that there was the clearest evidence to show 
that the lowest Cambrian conglomerates known in those areas, 
instead of having been altered by these so-called eruptive masses, 
as would be inferred from the maps of the Geological Survey, 
repose unconformably upon the rocks composing these masses, and 
were mainly built up of materials derived from them by denu- 
dation. In the paper of 1877 I stated that the pebbles in the . 
conglomerates were ‘‘usually distinctly rounded and generally im- 
bedded in either an unaltered or semicrystalline matrix, from which 
they can be easily removed,” and that “they were evidently in their 
present state, as regards consolidation, before they were cemented 
together to form the conglomerates, and must have been derived 
from rocks highly metamorphosed at that time, such rocks, in- 
deed, as now occur immediately under them, and which, we ven- 
ture to believe, belong to a Pre-Cambrian series.’ In the same 
paper I mentioned that the false appearance of being intrusive 
masses exhibited by these rocks and “the passage by gradual 
alteration mentioned by various observers, are mainly due to the 
fact that the matrix in the conglomerates has been derived from 
rocks immediately below or from similar ones, and from a slight 
subsequent change in the matrix, due, probably, to proximity to the 
intrusive dykes, aided by a readiness perhaps in the material to 
assume this change. This is clearly observed by watching the 
weathering of these conglomerates even when in direct contact with 
the porphyritic series; for any apparent melting-away of the hard 
pebble is shown not to be a fact, since on very slight weathering 
the pebble becomes easily separable from the matrix, and its outline 
is as perfect as on the day it became cemented in the mass.” 
Professor Bonney’s very important paper (Quart. Journ. Geol. 
Soc. vol. xxxv. p. 309) added the necessary evidence to make these 
facts certain, and in it he showed also the rhyolitic character of some 
of the Pre-Cambrian rocks. Professor Hughes, Professor Bonney, 
Dr. Callaway, and Dr. Roberts have also from time to time, in differ- 
ent papers read before this Society and elsewhere, called attention to 
the same facts as are referred to above; and I should hardly have 
thought it necessary to bring forward this further evidence, highly 
conclusive as it may be, were it not that the last edition of the 
Geological Survey memoir relating to these areas, published in 
