TRANSACTIONS OF SECTION C. 605 



axis, so far as they have been examined, are found to be full of fragmeutal quartz 

 and felspar, which is undoubtedly derived from a granitoid rock ; some beds being- 

 made up of little else. No rock of this character, so far as I am aware, is exposed 

 in this part of Wales, but a ridge of granitoid rock extends from the town of Car- 

 narvon to the neighbourhood of Port Dinorwig. Through this, apparently, the 

 great felstone masses which occupy considerable tracts on the northern margin of 

 the hills between Carnarvon Bay and the valley of the Ogwen have been erupted, 

 and over this comes a series of grits, slates and conglomeratic or agglomeratic beds, 

 overlain ultimately by the basal conglomerate of the undoubted Cambrian series. 

 It was formerly maintained that these felstones were only lower beds of the Cam- 

 brian metamorphosed — practically fused by some 'metapeptic' process. This 

 notion, however, was quickly dispelled by microscopic examination. The overlyinj; 

 conglomerate is often crowded with pebbles, identical in all important respects with 

 the felstone itself, which also presents many characteristics of a lava flow as 

 opposed to an intrusive mass, and is no doubt an ancient rhyolite now devitrified. 

 There is some difference of opinion among the geologists who have worked in this 

 district as to the exact correlation of various gritty, conglomeratic or agglomeratic 

 beds which succeed the felstone, as is only natural where disturbances are many, 

 and continuous outcrops generally few. But all agree on the existence of a series, 

 into which volcanic materials enter largely, between the above-named basal Cam- 

 brian conglomerate and the felstone. In this, then, and in the basal conglomerate 

 we have again and again more or less rounded fragments of old rhyolitic lavas. 

 "VVe have numerous and varied lapilli, probably of like chemical composition. We 

 have grits which are largely composed of quartz and felspar, resembling that in the 

 granitoid rock, together with fine-grained quartzose schists and bits of rhyolite, all 

 mingled together. We have also occasionally, as in the Cambrian conglomerate 

 near Llyn-Padarn, pebbles of the granitoid rock. Further, the basal conglomerate, 

 as near Moel Tryfaen, is sometimes crowded with fragments of gritty argillites. 

 Fine-grained schists, as will be noted, seem to be rare in this district, but, as such 

 rocks occur in situ in the Lleyn peninsula, they will probably be discovered 

 more abundantly when the Cambrian conglomerate is examined further in that 

 direction. 



Fine-grained micaceous, chloritic, and other schists occupy a considerable portion 

 of Anglesey, and in the neighbourhood of Ty Croes there is an important outcroj.i 

 of granitoid rock. The former were once regarded as metamorphosed Cambrian, the 

 latter as granite which aided in the metamorphism at the end of the Ordovician 

 period. In Anglesey the earlier Palaeozoic rocks are not generally rich in fossils, 

 so that it is sometimes difficult to settle their precise position. The oldest beds 

 which have been thus identified have been placed in the Cambrian (Tremadoc), but 

 some experts have doubted whether quite so low a position can be assigned to them. 

 Hence the exact age of the oldest Palfeozoic beds in this island is uncertain, and 

 whether the basal conglomerates near Ty Croes are of the same age as those in 

 Carnarvonshire. This, however, is certain, that some of the Anglesey grits above 

 the basal conglomerate are largely made up of quartz and felspar derived from a 

 granitoid rock. Others include numerous fragments of very tine-grained schists, 

 like those so abundant in the island, and the conglomerate contains pebbles some- 

 times full two inches in diameter, absolutely identical with the rocks in the adjacent 

 granitoid ridge (the foliated structure distinctive of some parts of it having been 

 even then assumed), together with various metamorphic rocks, some green schistose 

 slaty rocks, and some reddish slates. The last two were, I doubt not, cleaved be- 

 fore they became fragments ; probably these were derived from the hypometamor- 

 phic series, which Dr. Callaway has described, and which also contains pebbles of 

 the granitoid rocks. Fragments of the characteristic fine-grained schists are, so 

 far as I at present know, less common among the Anglesey grits and conglo- 

 merates than one would expect, perhaps owing to their comparative destructi- 

 bility ; but I have found them occasionally and suspected their presence more often. 

 Hence there can be no doubt that older crystalline rocks have very largely con- 

 tributed to the formation of at least the coarser members of the lower Palaeozoics 

 of Anglesey. 



