ON THE OLDER ROCKS OF ANGLESEY. 367 



Report of the Committee, consisting of Professor T. Gr. Bonney, 

 Mr. J. J. H. Teall, and Professor J. F. Blake {Secretary), 

 appointed to investigate the Microscopic Structure of the Older 

 Rocks of Anglesey. {Drawn up by the Secretary.) ^ 



[PLATES II.-V.] 



The rocks which form the subject of the present report have already 

 received the attention of geologists, and some eighty or ninety of them 

 have been described by Professor Bonney, mostly in appendices to various 

 stratigraphical papers by other authors.^ They have also been examined 

 and used for stratigraphical purposes by the Secretary.^ 



The present object, however, is to study them for the information they 

 may give on the nature and cause of the metamorphism to which many 

 have been subjected, and to determine how far their microscopic structure 

 may throw light upon their origin. Their special interest lies in the fact 

 that the series includes both altered and unaltered rocks, as also foliated 

 and non-foliated, and hence gives promise of throwing some light on tha 

 connection between them. 



Although the work already done upon the rocks of Anglesey has not 

 been of a systematic character from this point of view, being limited to 

 the determination of the jaarticular natni'e of individual rocks, yet, as 

 could not fail to be the case, many valuable observations of general 

 interest have been included in these descriptions. Moreover, the rocks 

 at St. Davids, which are in part the equivalents of these, and to some 

 extent of the same character, have been dealt with from a general point 

 of view by Dr. A. Geikie,'* whose conclusions may be usefully compared 

 with those of this report. 



For the purposes of study the older rocks of Anglesey may be divided 

 as follows: — 



1. Rocks which on stratigraphical grounds are generally considered 

 sedimentary, including volcanic contributions. 



2. Rocks of special origin. 



3. Crystalline rocks which may be of igneous origin. 



In referring to the localities whence the rocks are derived, and in 

 indicating their general stratigraphical relations, those subdivisions will 

 be adopted which are laid down in the paper above referred to on the 

 Monian system. It is there shown that those rocks in Anglesey which 

 are at least older than the Ordovician lie in six distinct areas, which are 

 severally called the Western, Central, Eastern, and Northern Districts, 

 and the districts south of Traeth Dulas and north-east of Parys Mountain. 

 The series of rocks which lie to the south of the fault which divides 

 Holyhead Island is spoken of as the South Stack Series. The Central dis- 



' The other members of the Committee wish it to be understood that, as this 

 report deals with subjects on some of which there is much controversy, Professor 

 Blake is solely responsible for the statements therein contained. 



^ Quart. Journ. Geol. Soc. vol. xxxv. p. 305. lb. vol. xxxvii. p. 40 and p. 232. 

 h. vol. xxxix. p. 470. lb. vol. xl. p. 200 and p. 283. Geol. Mag. N.S. Dec. ir. vol. 

 vii. p. 125. 



' Quart. Journ. Geol. Sue. vol. xliv. p. 463 et seq. The further study of these 

 rocks has, in a few instances, led to a slight moditication of some of the petrc- 

 graphical statements in that paper. 



* QiMrt. Jmirn. Geol. Soc. vol. xxxix. p. 261 et seq. 



