ON i'iiJ; AU( U.KAN UOCKS OF GUEAT BllITAIN. 



5r.\ 



or micaceous, soLi.u, wliicjh constitutes so lai'ge a portion of Anglescj- ; 

 the quartz-schist ("J) including the well-known ' quartzito ' of Holyhead 

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

 Crai"'-yr-Allor,' the granitoid sciies appears to pass down into some 

 dark micaceous or hornblondic 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 

 sti'uctiire of the llolyliead quartzite and the Menai and Holyhead schists, 

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

 structure nor metamorpliic cha-acter could be of the slightest value as a 

 factor in rook classification. 



BrieOy to state my own view, it is that at present it is safei to 

 rcard the metamorpliic rocks of Anglesey as belonging to two great 

 "roups — (i() the lower, a series of granitoid gneiss and highly 

 crystalline schists, in which there may be rocks indistinguishable ii'oui 

 granite, but of which all are among the oldest Arclueans ; and (A; a 

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

 (inartzitcs, 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, 

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

 mainland, with its gr'"<t subjacent rhyolitic hivas, which is probably mure 

 nearly of an ago witii tiie 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 l)riefiy the reasons f(jr \vhich the less altered members of the above 

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

 there i.s a good base to the Cambrian in South V 't<Jes in the conglomerate 

 which underlies the ilaggy beds, cc-4aining Lower Cambrian fossils, and 

 In Xorth Wales in the great conglomerates of Camiarvonslilre, even if 

 we ditfer 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- 

 cfniic activity oiia scale of any importance during the Cambrian joeriod. It 

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

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

 rather tine in texture, were accumulated. iJut it Avas preceded in North 

 and South Wiles, as i)oriods 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 diiferent 

 foci. Again, the lavas of these two epochs — t! jugh agreeing in belong- 

 ing to tlie acid division, i.e., in having a high silica percentage — diU'er con- 

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

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

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

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

 area and those of the Wrekin ; and chemically the relationship is \ cry 

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

 of the St. David's district, puttirg aside certain exceptional varictio. 



' Callawiiy, O'vol. .V(7;) , O.c. ii, vel. \ii. p, 117, 



