Geological Society of London. 239 
Cambrian formations ;—the “Dimetian,” consisting of crystalline, 
gneissic, and granitoid rocks; the “ Arvonian,” formed of felsites, 
quartz-porphyries, hilleflintas, and other highly silicated rocks ; and 
the “Pebidian,” composed of tuffs, voleanic breccias, and basic lavas. 
He regards the “Arvonian” as later than, and unconformable to the 
« Dimetian,” and the Pebidian as younger than, and unconformable 
to both; and he asserts that the basement conglomerate of the Cam- 
brian system lies quite unconformably on all these rocks, and is in 
great part made up out of their waste. 
Taking up each of these groups in the order of sequence assigned 
to them, the author maintained that the “ Dimetian group” is an 
eruptive granite, which has disrupted and altered the Cambrian 
strata, even above the horizon of the supposed basal conglomerate. 
He described a series of natural sections where this relation is 
exposed, particularly one on the coast at Ogof-Llesugn, where the 
conglomerate has been torn off and involved in the granite, and has 
been intensely indurated, so as to become a kind of pebbly quartzite. 
No other rock occurs within the granite mass except dykes of 
diabase, which rise through all the rocks of the district, but are 
especially abundant in the granite. The veins of finer granite, so 
general in granite areas, are conspicuous here. In short, whether 
studied in hand-specimens or on the ground, the rock is so unmis- 
takably an eruptive mass that the author could not understand how 
this view, which was that expressed on the Geological Survey maps, 
should ever have been called in question. The manner in which it 
has risen across the bedding of successive horizons in the Cambrian 
series proves that, instead of being a Pre-Cambrian gneiss, it must 
be much younger than all the Cambrian rocks of the district. 
The “Arvonian group” consists of quartziferous porphyries, or 
elvans, associated with the granite, and of the metamorphosed strata 
in their vicinity. Reference was made to natural sections where the 
actual intrusion of the elvans across the bedding of the rocks could 
be seen. 
The “Pebidian group” comprises a series of volcanic tufis and 
breccias, with interstratified and intrusive lavas. The author main- 
tained that this group forms an integral part of the Cambrian system 
as developed at St. David’s. It has been broken through by the granite 
and porphyries, and is therefore of older date. Instead of being 
covered unconformably by the Cambrian conglomerate, as asserted 
by Dr. Hicks, the volcanic group is covered quite conformably by 
that rock ; and seams of tuff are interstratified with the conglomerate 
and occur on various horizons above it. The conglomerate, instead 
of being mainly composed of fragments of the rocks beneath it, 
consists entirely of quartz and quartzite, only four per cent. of frag- 
ments having been found to have been derived from some of the 
projecting lava-islands underneath it. 
From the evidence now brought forward, the author contended 
that as the names “ Dimetian,” “ Arvonian” and ‘ Pebidian,” had 
been founded on error of observation, they ought to be dropped out 
of geolovical literature. 
