GEOLOGY OF ANGLESEY. 



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semblance, it must be confessed, is not very close. Both formations 

 are largely of volcanic origin ; but the felspathic materials of the 

 Anglesey rocks are derived from a more distant source, while the 

 frequency of quartzite pebbles indicates the proximity of land not 

 covered by volcanic products. 



The Gneissic series would appear to be closely related to the 

 granitoid rock of Twt Hill. As the normal position of the dark 

 schists is below the granitoidite, there seems to be no reason to 

 doubt that the green schists on the Anglesey side of the Menai 

 Straits pass conformably beneath the Twt-Hill beds. The post- 

 Carboniferous fault on the west side of the Twt-Hill ridge might 

 not materially affect the true relations of the older subdivisions. 



(2) St. David's. 



Slaty Series. — The lithological resemblances between this group 

 and the St. David's Pebidian are numerous and striking. The 

 varieties whose Pebidian facies is most marked are the pale- 

 green, purple, and yellow felspathic shales of Ehosbeirio, the 

 felspathic breccias north of Llanrhyddlad and east of Bodorgan 

 station, the tough green rock at Cerrig Ceinwen and Cerrig-engan 

 fawr, porcellanitic bands near Llangristiolus, and hornstones of 

 many localities. The slaty beds are less distinctively of St. David's 

 type. 



Gneissic Series. — The Anglesey granitoidite has been compared 

 by Messrs. Bonney and Hicks with the St. David's Dimetian. If 

 this correlation be conceded, the entire Anglesey series, from the 

 halleflinta upwards, must go with the Dimetian. But it must be 

 admitted that the two. groups present important differences. At St. 

 David's the Dimetian is represented by a great thickness of grani- 

 toidite and quartzite ; whereas in Anglesey the similar band is much 

 thinner, and is underlain by a considerable thickness of gneiss and 

 other schists. 



(3) Shropshire. 



The felspathic slaty beds of Lilleshall Hill are hardly distin- 

 guishable from some of the Anglesey rocks. The Lilleshall group 

 is probably higher than the Wrekin volcanic rocks, which have 

 many points of resemblance to the fragmental group south of Bangor. 

 The purplish ash of the Wrekin, for example, is very similar to the 

 grit at Bryn llwyd. Nothing certain, of course, can be inferred 

 from such facts ; but it is at least worthy of consideration, and the 

 suggestion may be a guide in further inquiry, whether the Anglesey 

 Pebidian may not represent a higher horizon than the Bangor group. 

 Some of the gritty and slaty rocks of the Precambrian ridges east 

 of Church Stretton also display affinities with the Anglesey series. 



It is worthy of note that the Charlton-Hill conglomerate, an 

 undoubted part of the Wrekin group, contains, amongst enclosures 

 from the Malvern series, pebbles of rocks which more closely 

 resemble some of the gneissic and quartzose types of Anglesey. 



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