Vol. 56.] GEOLOGY OF NORTHERN ANGLESEY. 255 



their course. They do not exhibit a continuous outcrop, but are 

 traceable at intervals along their line of strike. Followed in this 

 way, felsite is seen sometimes alone, sometimes accompanied by 

 basalt ; sometimes again there is basalt and no felsite. 



From a consideration of the appearances in the field, so far as I 

 have been able to observe them, it seems to me that both felsites 

 and basalts were intruded at a time, subsequent to the great period 

 of unrest in Anglesey, when minor movement was taking place and 

 new fissures were forming; further, that the felsites were injected 

 early while the Assuring was imperfect and incomplete, and the 

 basalts later when the fissures were more numerous and more 

 continuous. This will account for the less frequent occurrence and 

 the gaps in the continuity of the felsites ; for the later injection of 

 the basalts ; and for the predominance of the latter over the former. 

 How it is that both acid and basic materials were at hand is a 

 question into which I do not propose to enter. 



The basic dykes appear to be of the same character as those wjth 

 a general similar direction in other parts of Anglesey, such as the 

 south-east, where some are intrusive into the Carboniferous Lime- 

 stone. One 4 greenstone '-dyke is stated by Ramsay 1 to penetrate 

 the Coal Measures, but not to pass into overlying Permian strata. 

 This suggests a post-Carboniferous and pre-Permian age for the 

 post-movement basalts of Northern Anglesey — an age consistent 

 with the evidence of minor movement which they afford. 



I have to thank Prof. W. W. Watts, M.A., Sec.G.S., for his great 

 kindness in again undertaking the microscopical examination of the 

 rocks, and for allowing me to embody some of his notes in this 

 paper. 



EXPLANATION OF PLATES XIII & XIV. 

 Plate XII L 



Geological Map of the Northern Complex of Anglesey, on the scale of 

 4 inches to the mile. 



Plate XIV. 



Section 1. — Along the coast from Cemaes Bay to the Graptolitic Shales of 

 Porth Padrig (length = about A mile). 



Section 2. — Along the coast from Llanbadrig Point to near Llanbadrig 

 Church (length = about 350 yards). 



Section 3. — Along the eastern side of Porth Wen Bay, and a short distance 

 inland (length = about i. mile). 



Discussion. 



The Rev. J. F. Blake said that he had listened with much interest 

 to the paper, which contained many interesting observations that 

 had not been made before. He drew attention to the very numerous 

 1 faults ' which were introduced by the Author both in his map and 

 in his diagrams, and thought that they suggested an explosion from 



J Mem. Geol. Surv. < Geol. of N. Wales ' 2nd ed. (1881) p. 264. 



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