ON Tllli: AltCir.F.VN UO( KS OF (inKAT niUTAlN. 



.■)41 



nil lioins,' 



Pebbles 



llocalilies 



vertical extent of this formation (hero is still a difforenoo of opinion. 

 I rcgiird it us consisting of tlio series of green argillites and breccias, 

 well exhibited in llimgor mountain, and of a lowiT series of argillites, 

 •"irits, and peculiar breccias. The latter, however, are regarded by Professor 

 ilughes as repetitions of the JJangor series and of the Cambrian eon- 

 "lonierato by faults ; but to myself the zones appear to bo tOo well marked 

 and traceablo over too large aieas for this to \w possible. On the existence 

 of a volcanic scries between the rhyolite and the Cambrian conglomerate, 

 all of the ' newer school ' are agreeil. 



Schists, of a type resembling some of those described below, are said 

 to occur in the Lloyn peninsnla on the west side, but further information 

 is needed. From my persnnal knowledge, I can only say that the rock 

 mapped at Porthdinlleyn as serpentine lisis no claim to the name.' 



As regards the island of Anglesey, it is admitted on all hands that a 

 portion of the shore at the southern end of the ^lenai Straits, like the 

 opposite maiidand, is fringed by Carboniferous limestone, and that there 

 is a tract of similar rock at the north-east angle, not far from Beaumaris. 

 Carboniferous limestone, with overlying Coal-measures, and possibly 

 Permian in one part, extends across the island from slightly north of the 

 above-named tract to the western shore near !Maeldroth Marsh. To the 

 north of this is a thin strip of ' Lower Silurian,' and yet further north a 

 considerable tract which extends diagonally across the island, and sends 

 oifa prolongation at right angles to the northern shore.'- Fringing the 

 part just named on the south, and extending to the western shore, is a 

 strip of 'granite,' and the rest of the island is coloured on the Geological 

 Survey map as metamorphosed Cambrian, &c. As to the unaltered 

 PaliDO/oic rocks there is sulistaiitial agreement, except that the ago of the 

 lower portion is nncertiiiii, Professor Hughes believing that he has 

 iduutitied Tremadoc beds in Anglesey, which is doubted by Dr. Callaway. 

 The Survey view may be stated in the words of the Memoir:^ 'The 

 Cambrian strata of Anglesc^y heiiig wholly metamorphic, and the Silurian 

 rocks being metamorphosed in part, tliere is reason to believe that their 

 metamorphism was contemporaneous and of Lower Silurian date, being 

 connected with the presence of granite, pi'obably of the same age with 

 the imperfectly granitic rock and quartz porphyry on the opposite side of 

 the Straits ' (as has been shown above, both these rocks underlie the 

 (Jambrian). There is no doubt much in the very complicated and diffi- 

 cult geology of Anglesey which must still be regarded as unsettled, but I 

 think that there is a general concurrence of all who have studied the 

 subject, both in the field and ■with the microscope, as to the following 

 points : — 



(a] That this region of * metamorphic Cambrian and Lower Silurian ' is 

 so|>arable into two, one strictly speaking metamorphic — schists, micaceous 

 and ehloritic, fine-grained gneisses, quartzites, etc. : tho other hypometa- 

 iiiDrphic, as it has been termed by Dr. Callaway, consi.sting of slaty or 

 schistose rocks, sometimes apparently of volcanic origin. Further, tho 

 so-called granite consists in part of coarse gneisses and micaceous or 

 hornblendic schists, and it is doubtful (as at Twt Hill and the Ercal) 

 whether even the most granitoid beds arc a true granite. Including this 

 coarsely crystalline group with the schists, tho metamorphic rocks of 



' (J. J. a. X, vol. xxxvii. p. 40. 



'-' 'I'liero is also a littlu liowcr Silmi.in neiir Hoaumaris. 



■' Mvm. Giol, Siirrci/, vol. iii. \k 177. 





