13° TROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



than is given in the Survey maps and Memoirs ; and as I have not 

 had an opportunity of visiting it myself, I pass it over for the 

 present. In the Breidden Hills, and again in the country west of 

 Corndon, andesitic tuffs and agglomerates, interstratified with the 

 Bala beds, point to a distinct volcanic district still farther to the 

 east *. 



Let me now ask your attention to another part of the country, 

 about which much has been written and keen controversy has arisen. 

 In the centre of Anglesey, among the rocks grouped together by the 

 Geological Survey as " altered Cambrian," there occur masses of agglo- 

 merate, the probable volcanic origin of which was, so far as I know, 

 first clearly recognized by Professor Hughes t- Dr. Callaway regards 

 them as pre-Cambrian, while Professor Blake places them in his 

 " Monian system." 



The agglomerates, well seen near Llangefni, contain abundant 

 blocks of reddish quartzite, pieces of various felsites, and of finely 

 amjgdaloidal andesites. The matrix is a greenish material, such as 

 might be derived from altered diabase detritus. Everywhere the 

 rock presents evidence of having undergone enormous crushing and 

 shearing. Its thin intercalated seams of quartzite have been broken 

 down, and pieces of them have been carried along and isolated, so 

 as to look like stones of the agglomerate. Bedding, though hardly 

 traceable in the coarser parts, is easily made out elsewhere to be 

 vertical b}^ the intercalation of more solid strata and purple shaly 

 mterstratifications. It becomes more and more decided as the rocks 

 are followed towards the north-west. Layers of red, purple, and 

 greenish slate and pebbly grit intercalated in the volcanic conglo- 

 merates bear a close resemblance to parts of the volcanic series of 

 Llyn Padarn and Bangor. A further point of similarity is supplied 

 by the occurrence of bands of pale, compact, flinty tuffs like those 

 that play so important a part in the Bangor group. 



The relation of these rocks to the Lower-Silurian strata which 

 immediately follow them to the north-west could not be satis- 

 factorily determined in the traverses made by Mr. Peach and 

 myself ; but I could see no reason for believing them to belong to 

 a different system. The kind of crushing which has so seriously 



* See W. W. Watts on the igneous and associated rocks of the Breidden Hills, 

 Quart. Journ. Geol. Soc. vol. xli. (1885) p. 532. The Arenigand Bala volcanic 

 zones of the Oorndon area have been worked out by Professor Lapworth, but his 

 detailed account is not yet published. 



t Proc. Camb. Phil. Soc. vol. iii. (1880) p. 347. 



