T. m'KENNY hughes Olf THE GEOLOGY OP ANGLESEY. 23'! 



15. Oil the Geology of Anglesey. By T. M^Kenny Hugh:es, M.A., 

 r.G.S., Woodwardian Professor of Geology, Cambridge. (Eead 

 February 25, 1880.). 



The rocks to which these notes chiefly refer were described by Prof. 

 Henslow under the heads of Quartz Kock, Chlorite-schist and Grau- 

 wacke, in a paper read before the Cambridge Philosophical Society in 

 1821. He evidently considered them to be distinguishable members of 

 one group, drawing attention to the passage from the quartzite into the 

 schists, and to the generally coinciding dip of the clay slate and schists, 

 even where, as at Porth Corwg, there is a small discordancy between 

 the two, caused by a fault. He points out that green slates and 

 brecciated conglomerates occur in the upper part of the black-slate 

 group; but he records no fossils from which the geological horizon of 

 the beds described can be determined, and his sections do not give 

 much more than the geographical succession of the rocks over the 

 areas referred to in this paper. 



Prof. Eamsay (Mem. Geol. Surv. vol. iii. p. 175) describes the 

 Cambrian and Silurian Eocks of Anglesey as in great part resembling 

 those of Caernarvonshire. The Cambrian rocks, he further remarks, 

 are, as a rule, highly metamorphic, like those in the promontory of Lleyn 

 (Cambrian in his nomenclature includes only the Harlech group). 

 The Silurian (i. e. the Lingula-flags and all above them), he goes on to 

 say, are also sometimes metamorphosed into mica-schists and gneiss, 

 while in other places they are rich in Caradoc or Bala and Llandeilo 

 fossils. Unfortunately the genera and species of the Graptolites are 

 not recorded except from one locality mentioned in the appendix, 

 where it is said that Graptolitlms, sp., and Diiilograpsus teretiuscidus 

 occur near Glanygors. Probably the localities rich in Bala fossils are 

 those referred to by Salter, in the Appendix, p. 258, where he says, 

 " The multitudes of a dwarf variety of Orthis calUgramma give a 

 Caradoc aspect to the grits." 



It is clear that the four great masses of gnarled schists, the 

 Holyhead mass, the ximlwch mass, the Llangefni mass, and the 

 Meuai mass, were considered by Prof. Eamsay to be metamorphosed 

 Cambrian, i. e. Harlech beds, with the exception of certain beds near 

 Cemmaes and some in the central axis from near Llanfaelog to the 

 north end of Llaneilian mountain, which are spoken of as altered 

 Silurian. 



By others since the great masses of gnarled shale have been con- 

 sidered to be Pre-Cambrian, and some of the conglomerates have been 

 referred to a Pre-Cambrian part of the period still earlier than the 

 gnarled shale. 



Before we can with safety speculate upon the age of the gnarled 

 series, it is desirable to endeavour to clear up the evidence with regard 

 to the age of the fossiliferous grauwacke ; and the following shoit 

 communication deals chiefly with this question, the conclusion arrived 

 at being that the beds hitherto referred to the Caradoc are Tremadoc, 

 that they are succeeded by Arenig, and that there are higher slates 

 among which we shall by and by probably identify the Lower Bala. 



