374 DE. C. CAULAWAT OX THE METAMOEPH153I OF "Aug. 1898, 



2S. On the !META2J0EPHisir of a Seeie? of Geiis and Shales in 



XORTHEEX AX&LESET. Bv ChAELES CaLLAWAY, M.A.. D.Sc., 



F.G.S. (Eead May 18th, 1S9S.) 



Ix a recent paper ^ read before this Society, I endeavoured to show 

 that certain gneisses in Southern Anglesey were formed out of 

 plutonic rocks. Fragments of these gneisses have been identified^ 

 in younger pre-Cambrian strata, lying to the north, and these, in 

 their turn, yield an abundance of rounded derivatives^ to the basal 

 Palceozoic conglomerates of the island. These newer pre-Cambrians, 

 which I have provisionally referred to the Pebidian system.,^ display 

 great variations in their degree of crystallization. Hence arises one 

 of the chief difficulties in unravelling the geology of Anglesey. It 

 has become increasingly evident that the questions still in dispute in 

 this complicated area can be, settled only by (1^ carefully mapping 

 the ground in minute detail, and {'2) determining the nature and 

 degree of alteration which many of the rocks have undergone. 

 Mr. Edward Greenly has devoted himself with great enthusiasm to 

 the former task, and the first-fruits ' of his work promise highly 

 satisfactory results. The present paper is offered as a further 

 contribution to the series of problems arising under the latter 

 head. It is here proposed to examine a series of grits and shales 

 in Northern Anglesey, and to show that they are transformed into 

 true crystalline schists as we follow them in a southerly direction. 



I. The Aeea desceibed. 



!N"orth and north-west of Ajaalwch the ground is much disturbed 

 and mixed up with faulted fragments of Palaeozoic strata. To 

 avoid controversial topics.^ I shall confine myself to a patch about 

 3 miles square, lying south-west of Amlwch, extending north and 

 south from Llanfechell and Ehosbeirio to the boundary-fault near 

 Mehn Pant-y-Gwydd and Pant-y-Glo. and west and east from 

 Mynyd Mechell to Bodewryd. Over this area the rocks appear to 

 be unbroken by faults, and exposures are fairly abundant, 



I originally" divided the rocks of this district, taking them from 

 south to north, into (1) Chloritic Schists of Mynydd Mechell, (2) 

 Danfechell Grits, (3) Ehosbeirio Shales ; aud I am disposed to 

 conclude that the strata form an ascending series. The usual dip 



^ Quart. Journ. Geol. Soc. toI. liii tl897i p. 349. 



=* Ibid. vol. xxrrii 1 188D p. 235. 3 Ibid. vol. d (1884) pp. 568. 571. 578. 



■* Ibid. vol. sxxA-ii (1881) p. 232. '' Rid. toI. Hi (1896) p. 618. 



[^ I refer especiaRv to the connexion alleged by Prof. Hushes to exist between 

 these rocks and the fossiliferous strata at Porthwen (•Geol. of Anglesey,' 

 Quart. Journ. Geol. Soc. vol. sxxvi. 1880. p. 237. & voL sxxriii, 1882, p, 16). 

 and to the subsequent criticisms of the Rev. J. F. Elate (Quart. Journ. Geol. 

 Soc. vol. xliT, 1888. p. 521). See also Hughes in Eep. Brit. Assoc. (York) 1881, 

 p. 644.] 



" Quart. Journ. Geol. Soc. vol. xrxrii (1881) p. 223. 



