334 DR. E. GREENLY ON THE SUCCESSION AND [vol. lxxix, 



14. Further Researches on the Succession and Metamor- 

 phism in the Mona Complex of Anglesey. By Edward 

 Greenly, F.G-.S. (Eead March 28th, 1923.) 



Contents. 



Page 



I. The Age of the Mona Complex 334 



II. The GwnaBeds 335 



III. The Fydlyn Beds 336 



IV. The Penmynydd Zone of Metamorphism 340 



V. The Age of the Gneisses 341 



VL The Origin of the Basic Gneisses 343 



VII. Chronology of the Gneisses 349 



VIII. Recapitulation 350 



Since my return to North Wales, the proximity of Anglesey has 

 made it possible (in intervals of my survey of the country about 

 Bangor and Carnarvon) to seek further light on the unsolved 

 problems of the Mona Complex ; and some of the results are 

 embodied in the present paper. 1 



I. The Age or the Mona Complex. 



Pebbles derived from the Mona Complex had already been 

 found in Cambrian rocks 2 ranging from the Bron-llwyd Grit 

 down to the basement conglomerates of Carnarvonshire. Frag- 

 ments have now been obtained from much lower horizons. For 

 the massive conglomerate of Bangor cannot be later than Cambrian, 

 and it is certain that, at any rate, the lower portions of the Bangor 

 Volcanic Series (with which portions alone we are concerned in 

 this paper) must be much older than that conglomerate. 3 The 

 Volcanic Series being essentially pyroclastic, fragments other than 

 those of the contemporaneous volcanic rocks are rare. The agglo- 

 merates, however, have yielded a few fragments of Gwna quartzite 

 and jasper. These fragments are not schistose. But in a slide 

 [E 1539],* cut from an agglomerate between the Workhouse and 

 Hendre-wen Lane, there is a beautiful oval fragment (fig. 1, p. 335), 

 about g inch long, of undoubted Penmynj'dd-Zone mica-schist, 

 well-foliated and holocrystalline, with sphene, zircon, and a pale 



1 The Geological Survey Memoir entitled ' The Geology of Anglesey,' 1919, 

 will be referred to in this paper as ' G. of A.' 



2 ' G. of A.' pp. 246-52. 



8 I have now mapped most of the Bangor area; but, until the whole of that 

 area is surveyed, it would be premature to discuss the question of uncon- 

 formity between the conglomerate and the Volcanic Series. It may, how- 

 ever, be said that evidence only available lately has revealed unsuspected 

 complications. 



4 The slide-numbers quoted are those of the Geological Survey Collection. 



