Dr. H. Hicks—Pre-Cambrian Rocks of Wales. 099 
crushed, others assuming a schistose or gneissose appearance as the 
result of pressure and metamorphism. I have readily admitted in 
previous papers that since these rocks have been proved to be of 
igneous origin they cannot have the same value for purposes of 
classification as when they were supposed to be metamorphosed 
sediments; but on the other hand it is clear that they are older than 
the quartzites and other sediments of Pre-Cambrian age which 
frequently contain fragments derived from them. At the meeting of 
the British Association in 1878, I announced for the first time that 
Thad obtained evidence to show “that the so-called intrusive granites 
in Anglesey and the whole of the area marked as altered Cambrian 
in that island” were of Pre-Cambrian age. I classed the rocks there 
under three names as follows:— 
Druert1an, to include the granitoid rocks and gneiss. 
ArvontANn, the schistose and compact quartz rocks and some 
felsites and porphyries. 
PEBIDIAN, green and purple agglomerates and breccias, green 
chloritic schists, with massive greenstone bands, talcose schists, ete. 
In the previous year, 1877, Prof. Hughes and I, in separate 
papers, had pointed out that there were masses of granitoid rocks 
and of quartz felsites in Carnarvonshire, which had every appearance 
of being older than any of the Cambrian rocks. It was stated that 
they resembled in a marked degree some of the Pre-Cambrian rocks 
of Pembrokeshire, and that as in that area they were flanked by un- 
altered Cambrian sediments. The volcanic series in Pembrokeshire, 
in Carnarvonshire, and in Anglesey we placed as the highest group, 
and we showed that the overlying Cambrian conglomerates reposed 
transgressively on, and contained large pebbles derived from, beds 
belonging to that series. We have since shown that the Cambrian 
conglomerates in reality are made up of fragments derived by 
denudation from the three series referred to, and that an important 
interval must have elapsed between the upturning of the beds, even 
of the latest of the Pre-Cambrian series, and the deposition upon 
them of the basal Cambrian conglomerates. 
In the paper already referred to Sir Archibald Geikie says that 
the schists, quartzites and limestones in Anglesey present a close 
resemblance to the Dalradian series of Scotland and Ireland “and 
that the coarse gneiss”” may be compared, in general character, with 
parts of the Lewisian rocks, so that we seem to have here, as in 
Ireland, two groups of schistose rocks, and both of these much older 
than the unaltered Cambrian strata which lie above them. I have 
already stated that the oldest group, “coarse gneiss,” above referred 
to was mapped by the Geological Surveyors as granite intrusive in 
Cambrian and Silurian strata, and that no suspicion that it was of 
Pre-Cambrian age was entertained until the year 1878, when I pointed 
out that it resembled in a marked degree the “ Dimetian” of St. 
Davids, and stated that I had obtained evidence to show that it was 
not intrusive in the surrounding rocks. At first I associated the 
““quartzose rocks” with the “ Dimetian,” but afterwards placed them 
in a newer group “ Arvonian,” and in this group I included the 
