. ( ; C 



m 



542 



itKPoiti— ia84. 



Anglesey occupy three areas — one, a strip roughly parallel with tlio 

 Menai Straits, extending from the western to the eastern shore, consist- 

 ing mainly of micaceous or chloritic schists, with some rather deciuedly 

 crystalline hornblendic schists; another (in. which the gran-ioid rocks 

 are included), extending fi'om tiie western shore at Porth Nobla, nciir 

 Ty Croes and LLinfaelog, about two-thirds way across the island, with 

 sundry outcropping patches to the north-east which show that an Archieaii 

 floor underlies the Palu'o/.oif rock right up to the eastern shore ; and. 

 lastly,a rudely triangular patch, cousistiiig of the island of Holyhead, and 

 a still larger tract on the adjacent mainland. 



(i) That at the base of the indubitably Pakeo/oic scries, there is in 

 many places a conglomerate which contains fragments of tlie granitoid 

 rock, liie schists, and the schistose or slaty beds of the ' metamorpliic 

 Cambrian ar . Lower Silurian region' of the Survey. Hence, that whctluT 

 this conglomerate is Cambrian, as is the opinion of Professor Hughes nvi] 

 Dr. Hicks, or very low down in tlie Ordovician, as seems to bo the view nl' 

 Dr. Callaway,' there is a very important break between it and the ' nu'f;i- 

 morphic series,' in Avhich also the upper member contains pebbles of the 

 lower. 



((■) The greater antiquity of the granitoid series and its non-intrnsive 

 character is placed beyond doubt by the foUov^ing section : - Near Llaii- 

 faelog the gi'anitoid series and the so-called Lower Silurian can be .seen 

 iii close proximity "^ and traced for a considerable distance. Not only uiv 

 grits in the latter formed of debris closely resembling the constituents o!' 

 the former, but also there are occasionally bands of well-rounded peblil">, 

 sometimes more than four inches in diameter, which are absolutely iiulis- 

 tinguishable macroscopically and microscopically from the varieties of tin' 

 granitoid rocks of the Llanfaclog ridge. 



When we come to the classification of the truly metamorpliic scrit'>, 

 there is at present considerabl'.i discordance of opinion, and it will doubt- 

 less take much careful work, liotli in the field and with the micro. 

 scope, before the subject can be regarded as settled. Dr. Hicks L;ivcs 

 the following succession in ascending order : — (1) Gr.anitoid scries. 

 (2) '^ompact quartzose rocks. (."!) (^)nartz-schist, and chloritic or mica- 

 ceous, hist Cthe most alnindant rock in the metamorpliic serios of 

 Anglesej'). Tlie lowest series he correlated with the Dimetian, the ' liiillc- 

 flinta ' with the Arvonian, and the ' cliloritic series ' with the Pebidian. In 

 my opinion, over much importance has been assigned to the ' hiilletlinta' 

 zone, which is made up ])artly of some compact quartzomicaccous nr 

 quartzochloritic rocks not very distinctly foliated (for which it i< 

 difficult to find a satisfactory name), ])artly of some felstones, aliuost 

 certainly intrusive, and so giving no help in classification. Further, tlio 

 great ' cliloritic ' series cannot be correlated with the Pebidian (i.r., tlie 

 series nnderlying the Cambrian conglomerate 't St. David's and near 

 Bangor), without setting aside all the conclusions to which we are led 

 by the use of the micrcscope in studying tho British rocks. Dr. ('iillii- 

 way gives the following succession in ascending order : — (1) Htilletiinta 

 (2) Quartz -schist. (;?) Crey gneiss. (I) Dark schist. ("») Granitoiilii<' 

 The dark schist (4) i'lclude.-s the dull lead-coloured or greenish, cdiloritit' 



' Q. J. G. X. vol. xl. p. .->(;:. 



-' llieks, Q. J. a. S., vi;l. xl. |>. 187. 



^ .\bout liOviinUat the nearest point •■ Hiekt;, mijim. [k ]'XJ. 



