196 H. HICKS ON CAMBRIAN CONGLOMERATES IN 
at p. 22, that the Llanerchymedd conglomerates vary “according to 
the character of the underlying rocks,” and contain ‘* bands composed 
chiefly of white quartz with occasional fragments of jasper, quart- 
zite, schists, &c., and some beds of large felsite pebbles.” Sir A. 
Ramsay also, at p. 247 of vol. iii. of the Memoirs of the Geological 
Survey, gives a list of fragments found in the conglomerates in the 
north-west of Anglesey; but immediately afterwards says that they 
could not have been derived from the Anglesey rocks themselves, as 
it would be evident on reflection that such could not be the ease, 
‘the metamorphism of the Anglesey rocks having taken place after 
the deposition of the Lower Silurian strata.” Our contention, on 
the other hand, is that the fragments of granitoid and compact 
quartzose rocks, and the chloritic and mica-schists &c., are identical 
with rocks in Anglesey which now underlie the conglomerates in 
which they are included ; and hence that these rocks, instead of being 
metamorphosed “ after the deposition of the Lower Silurian strata,” 
were in their present condition even before the Cambrian rocks. 
were deposited, and are therefore clearly to be classed as of Pre- 
Cambrian age. The average test, as applied to the conglomerates 
of St. David’s by Prof. Geikie, could be used here greatly to our 
advantage; for instead of quartz pebbles and quartzites forming, as 
he claimed they did there, over 90 per cent., we may safely say that 
rocks of undoubted local origin form here more than 90 per cent. of 
the fragments. It must be understood, however, that as we consider 
the basal conglomerates to have been deposited along old shores, 
the percentage of any special rocks must vary much even within 
very limited distances. This must always be the case with basal 
beds where the rocks which are being denuded vary in their 
character, and when currents and other influences affect the drift 
of the shingle. Nothing can be more fallacious, therefore, than 
to select pieces of conglomerates from one or two points, and make 
the average of the pebbles in these applicable to a whole district. 
Bangor and Caernarvon. 
The conglomerates in contact with and lying directly along the 
east side of the ridge, composed of granitoid rocks and quartz felsites, 
which extends from Caernarvon to Bangor, have been so frequently 
referred to in papers by Prof. Hughes and Prof. Bonney that very 
little need be added concerning them. The evidence as to their 
having been derived by denudation from those rocks has been 
accumulating every year since the first papers on these rocks by 
Prof. Hughes and myself were read. An unusually good exposure, 
however, of the quartz-felsite conglomerate at the east entrance of 
the west tunnel, is now being made in consequence of excavations 
for widening the siding room. The pebbles here are of very large 
size, and particularly well rounded, and all the varieties exhibited by 
the underlying quartz-felsites may be found*. Fragments of the 
* These quartz-felsites have been shown by Prof. Bonney to be true rhyolites, 
and were so indicated by him in his appendix to Prof. Hughess paper read 
Dec. 1877. 
