part 3] METAMOBPHISM IN THE MONA COMPLEX. 337 



more ferruginized, resemble those of the Fydlyn coast in every 

 particular. The massive rlvyolitic types with rounded quartz, and 

 the pyroclastic varieties, are both present. They are highly fel- 

 spathic, often weathering just as white, 1 and there is the same 

 sporadic silicification, while deformation is not excessive. 2 The 

 rhyolitic series first appears on the coast at the back of the eastern 

 cove of Padrig, whence (interrupted in Porth Gron by the Carmel- 

 Head thrust-plane) there are excellent sections as far as the western, 

 walls of that cove. Perhaps the finest is on the eastern crags of 

 Gron, where one might imagine oneself on Fydlyn south cliff; ; the 

 same white felspathic varieties with rounded quartz, and weathering 

 into cavities, being evidently rhyolites, while pyroclastic types 

 come on to the south of them. With microscopic examination all 

 doubt vanishes. The more massive types are composed of a micro- 

 felsitic matrix (micaeized and silicified) wherein are phenocrysts- 

 of quartz and felspar. The others — such as the ' massive grit ' 

 of Porth Gron ('G. of A.' p. 295)— are dust-rocks (highly 

 micaeized) with numerous irregular angular fragments of quartz,, 

 felspar, keratophyre, and rhyolite. Such felspars as yield good 

 optical reactions are albite. Finally, they contain the same opaque 

 yellowish-wdfite leucoxene as has now proved to be persistently 

 present at Fydlyn. 3 



Most important, however, is the fact that here, as at Fydlyn, 

 they adjoin the Gwna Beds, their gradation into which by alter- 

 nation with thin grits (now cut up into melange) is even better 

 exposed, and much more accessible. The zone of gradation, about 

 10 feet thick, is perfectly clear at a cave's mouth between the 

 coves of Padrig, on both sides of the western Padrig inlet, along 

 the shelf (below boulder-clay) which runs along the sea-cliff 

 thence to Porth Gron, and on the eastern crags of that inlet. 



Wylfa. — The suspicion that the rocks by the Lifeboat Station 

 might be the Fydlyn Beds (' G. of A.' p. 308) has been confirmed. 

 Although much ferruginized, they are quite white in many places. 

 The type containing rounded grains of quartz, like those of 

 rhyolites, can be identified. Other varieties have the characteristic 

 micaeized matrix, crowded with angular pyroclasts of quartz, 

 alkali-felspar (some of which is determinable as positive), kerato- 

 phyres, and rhyolites ; they are manifestly rhyolitic tuffs. Leuco- 

 xene is also present. Thin bands of grey shaly matter, which 

 resemble those of Fydlyn beach, though excessively shattered,, 

 preserve the bedding for a few inches at a time. 



1 The silica-percentage of a rhyolitic tuff from Fydlyn south cliff [E 12362],. 

 kindly estimated by Mr. Roberts, Lecturer in the University College of North 

 Wales, is 69-38. 



2 Fig. 2 (which should be compared with the 1-inch map, and with 

 ' G. of A.' folding-plate xiii) shows the alterations in the map which this 

 discovery has involved. 



3 Even these rhyolitic lavas thus partook of the generally high titanium- 

 content of the Mona Complex. 



