692 PROF. T. MSKENNY HUGHES ON THE PRE-CAMBRIAN 
dip is seen, however, at the 8.W. end of the quarry, and the long 
face of rock fronting the 8.E. is very nearly the bedding-plane. 
The conglomerate is much like that seen in the quarry shown in 
fig. 10, except that almost all the pebbles are white quartz. It is 
exceedingly tough. The finer portions are much like the upper 
conglomerate seen at Garth Point (see fig. 8, a*), and also resemble 
some of the conglomerate in the Hendrewen quarry (see fig. 9). 
The overlying beds consist of a grey grit and micaceous purple 
flaggy sandstone. This is much weathered, and it is not clear how 
far the colour may be due to weathering. 
The conglomerate is again seen on the 8.E. of Twt Hill in front 
of the new church. The dip was not clear, but it appeared to be 
S.E. (fig. 1 
Thus we find the conglomerate and grit, which must all be referred 
to the base of the Cambrian, flanking the Pre-Cambrian rocks all the 
way from Twt Hill, Caernarvon, to Garth Point, Bangor, dropped in 
and repeated by faults here and there, and lapping round the ends 
of all the principal masses of which the range is made up. 
The character of the beds varies according to the rocks on which 
they rest, and from which they were principally derived. Near 
Caernarvon the conglomerate contains chiefly quartz; near Lland- 
deiniolen it is composed of pebbles of felsitic rocks in a fine matrix, 
also like felsitic rock. Near Bangor, though the pebbles seem to be 
chiefly derived from felsitic rocks, the matrix, especially those 
portions which are furthest from the felsitic rocks, 1s often a grit, 
and generally is much more open and porous, probably being largely 
derived from the clastic volcanic series; and this accounts for that 
portion of the conglomerate being most decomposed. 
DESCRIPTION OF PLATE XXXVI. 
Map of the district between Bangor and Caernarvon. 
Discussion. 
Dr. Hicxs remarked that the points of difference between himself 
and the author of this paper were now very small, as it was quite 
clear that the author had abandoned the theory in which he attri- 
buted an igneous origin to the Dimetian (Twt-Hill) series; and he 
hailed the appearance of this memoir also as showing that Prof. 
Hughes was gradually coming round to his views, and accepting his 
threefold division of the Pre-Cambrian rocks. 
Mr. Tawnny stated that the specimen alluded to by Dr. Hicks 
as from a mineral vein was from a bed in the quarry near Twt Hill 
(admitted by every one who had been there to be a conglomerate) ; 
the only question was as to its age. Concerning this, two fairly 
divergent opinions were before us, viz. one recorded by Prof. 
Bonney, that it was a bed in the metamorphic Twt series ; the 
other by Prof. Hughes, that it was lying unconformably against the 
