ANNIVERSARY ADDRESS OF THE PRESIDENT. 85 



tions, while tho sheared portions pass into cpidiorites and true 

 hornblende-scliists. Some of these bands are no doubt intrusive 

 sills,but others may possibly be intercalated contemporaneous sheets. 

 They occur across tho -whole breadth of tlie island from the Mcnai 

 Strait to the shores of Holyhead. IBcsides these undoubtedly igneous 

 rocks, the green chloritic slates deserve notice. They are well- 

 bedded strata, consisting of alternations of foliated fine grit or sand- 

 stone, with layers more largely made up of schistose chlorite. The 

 gritty bands sometimes contain pebbles of blue quartz, and evidently 

 represent original layers of sandy sediment, but with an admixture of 

 chloritic material. The manner in which this green chloritic consti- 

 tuent is diffused through the whole succession of strata, and likewise 

 aggregated into bands with comparatively little quartzose sediment, 

 reminds one of the " green schists " of the Central Highlands and 

 Donegal, and suggests a similar explanation. Taken in connexion 

 with the associated basic igneous rocks, these chloritic schists seem 

 to me to represent a thick group of volcanic tuffs and iuterstratified 

 sandy and clayey layers. If this inference is well founded, and if 

 we are justified in grouping these Anglesey rocks with the Dalradian 

 schists of Scotland and Ireland, a striking picture is presented to the 

 mind of the wide extent and persistent activity of the volcanoes of 

 that primeval period in Britain. 



In the third place, to the north of a curving fault drawn upon the 

 Survey map of the north of Anglesey from Carmel's Point on the 

 west to Porth-j'-Corwg on the east, a tract of country is coloured as 

 altered Cambrian. In the Memoir on North Wales, however, Sir 

 Andrew Ramsay expressed his doubts as to how far some of this 

 area might not be Lower Silurian, for he showed that the strata 

 included bands of black slate containing graptolites, and he was 

 rather inclined to regard the whole series as Silurian *. This view 

 has been rejected by Dr. Callaway and Mr. Blake, who regard most 

 of the rocks north of the curved fault as pre-Cambrian f. On the 

 other hand, Professor Hughes has expressed his belief in the general 

 accuracy of Sir Andrew Eamsay's reference of the rocks to the Lower- 

 Silurian series J. 



I shall, however, have more to say respecting this part of Angle- 

 sey in a later section of my Address. 



* Op. cit. pp. 224, 236, 242. 



t Callaway, Quart. Journ. Geol. Soc. vol. xxxvii. (1881) p. 210; Blake, op. 

 cit. vol. xl. (1884) p. 403. 



+ Proc. Phil. Soc. Cambridge, vol. iii. p. 347. 



