ON THE ARUU/EAN ROCKS OF GREAT BRITAIN. 543 



or micaceous, schist, -which constitutes so large a portion of Anglesey ; 

 the quartz-schist (2) including the well-known ' quartzite ' of Holyhead 

 mountain and some other quartzose rocks. It is quite true that, near 

 Craig-yr-AUor, 1 the granitoid series appears to pass down into some 

 dark micaceous or hornblendic schists ; but I doubt the identity of these 

 with some rather similar bands low down in the ' dark schist ' series, and 

 I may say, with regard to the position assigned to the granitoid group, 

 that if it overlay (except by intrusion) rocks with the microscopic 

 structure of the Holyhead quartzite and the Menai and Holyhead schists, 

 it would be a succession so abnormal as to show that neither microscopic 

 structure nor metamorphic character could be of the slightest value as a 

 factor in rock classification. 



Briefly to state my own view, it is that at present it is safer to 

 regard the metamorphic rocks of Anglesey as belonging to two treat 

 groups— (a) the lower, a series of granitoid gneiss and highly 

 crystalline schists, in which there may be rocks indistinguishable from 

 granite, but of which, all are among the oldest Archaeans ; and (I) a 

 more modern series, consisting mainly of well-bedded schists and some 

 quartzites, the former being generally chloritic or micaceous, and all, as 

 a rule, composed of rather minute mineral constituents. These I should 

 conjecture to be decidedly more modern than the granitoid rocks, down 

 against which, near Llanfaelog and Ty Croes, they appear to be faulted, 

 but still to be decidedly more ancient than the volcanic series of the 

 mainland, with its great subjacent rhyolitic lavas, which is probably more 

 nearly of an age with the hypometamorphic series of Dr. Callaway, and 

 both of which may be provisionally named Pebidian. 



It may be well before passing across the Scottish border to recapitu- 

 late briefly the reasons for which the less altered members of the above 

 regions are assigned to the great Archaean series. I take it as proved that 

 there is a good base to the Cambrian in South Wales in the conglomerate 

 which underlies the flaggy beds, containing Lower Cambrian fossils, and 

 in North Wales in the great conglomerates of Carnarvonshire, even if 

 we differ as to some points of detail concerning these, and regard it 

 as still an open question whether the Anglesey conglomerate is to be 

 considered Cambrian or very low down in the Ordovician. Accepting then 

 the above conglomerate as a base, there is in Britain no evidence of vol- 

 canic activity on a scale of any importance during the Cambrian period. It 

 was apparently one of subsidence and quiet sedimentation, during which, 

 by the detrition of older rocks, large deposits of sediments, generally 

 rather fine in texture, were accumulated. But it was preceded in North 

 and South Wales, as periods of subsidence often are preceded, by one of 

 volcanic activity, and it was followed in the same districts by new 

 outbreaks of the volcanic forces, though perhaps from slightly different 

 foci. Again, the lavas of these two epochs — though agreeing in belong- 

 ing to the acid division, i.e., in having a high silica percentage — differ con- 

 siderably in their minor characteristics, so that in a great number of cases 

 no hesitation would be felt by a practised collector as to which group 

 a specimen should be referred. Now there is macroscopically no incon- 

 siderable resemblance between the ' old rhyolites ' of the Bangor-Carnarvon 

 area and those of the Wrekin ; and chemically the relationship is very 

 close. Again, the former have a close resemblance to the average ' felsites ' 

 of the St. David's district, putting aside certain exceptional varieties. 

 1 Callaway, Geol. Mag., Dec.ii., vol. vii. p, 11". 



