Vol. 54.] GRITS AND SHALES IN NORTHERN ANGLESEY. 375 



is to the north, or a little east of north, and there is a clear change 

 in the nature of the sediment as we go northwards. The lowest 

 group is highly quartzose and gritty. In the middle, around Llan- 

 fechell, there is a considerable admixture of softer beds, which 1 have 

 described as hypometamorphic shales; while at the summit 

 of the series the shaly strata predominate, with gritty seams in 

 subordinate proportion. The almost uniform northerly dip might of 

 course have been produced by excessive pressure causing overfolding 

 or overthrust, but of this there is no evidence, and it may be fairly 

 concluded that the series is regular and ascending from south to 

 north. This, however, is a minor point, and has no material 

 bearing on my main contention. 



II. The Metamorphism as seen in the Field. 



The highest beds are well exposed in quarries and natural sections 

 around Rhosbeirio. They consist of purple, yellow, and pale-green 

 shales,^ with intercalated bands of grit. The dip is at moderate 

 angles, and there are no signs of contortion or dislocation. A slight 

 glistening on lamination-surfaces indicates some alteration, and 

 suggests the use of the term ' hypometamorphic' 



Beds of an intermediate character form the underlying horizon. 

 They can be studied around Llanfechell. The grits do not greatly 

 differ from those at Rhosbeirio, but the softer seams display a distinct 

 micaceous or chloritic lustre. Similar beds are exposed along the 

 easterly strike at Nant-y-Cyntin. Farther east, a short distance 

 south of Rhosbeirio, rocks resembling the Rhosbeirio Shales and 

 Grits are seen in a quarry at Bodewryd ISTewydd. They are slightly 

 contorted in places, and these contorted seams are more schist-like 

 than tlie shales. 



The lowest beds of the series are conspicuously gritty. Indeed, 

 they were originally described by Ramsay^ as 'foliated grits.' 

 Intercalated with the coarser seams are bands of a well-foliated 

 micaceous or chloritic schist, the micaceous constituent evidently 

 predominating. Near Cas Clock, for example, the schist is highly 

 foliated, and displays a silvery lustre. jN^earh^ everywhere in these 

 gritty rocks the contortion is strongly marked. 



Excellent sections are exposed at Pant-y-Glo, and these may be 

 taken as typical of the lowest series. They are also of critical im- 

 portance, as bearing on the question of metamorphism. The strata 

 consist predominantly of hard green grits, showing a laminated 

 structure, and evidently much compressed. That they were once 

 ordinar}^ sediments will be shown by microscopic evidence. Inter- 

 calated with them are seams of jierfectly foliated glistening schist. 

 So closely associated are these schists with the grits that a common 

 origin must be predicated for both. 



^ Tlie pai'tially volcanic origin of the Anglesey Pebidians was noticed in iny 

 paper in this Journal, vol, xxxvii (1881), and subsequently by the Rev. J. F. 

 Blake, in 1888. 



2 Mem. Geol. Surv. vol. iii (1866) ' Geol. of North Wales,' p. 184. 



