636 MR. C. A. MATLEY ON THE [Aug. 1 899, 



general dip to the north or north-east, and consist mainly of green 

 chloritic schists and grits, and greenish-grey and purple phyllites, 

 slates, and shales. They are everywhere hounded on the south by 

 black slates and shales of acknowledged Ordovician age. This 

 boundary -line forms a curve running from coast to coast from near 

 Carmel Head on the west through Llanfflewiu to Forth y Corwgl on 

 the east ; it is shown on the 1-inch Survey map, and is considered 

 by several geologists (among others, Eamsay, Hicks, Callaway, and 

 J. E. Blake) as a line of dislocation — an ordinary fault according 

 to Eamsay,^ or according to later writers as a thrust whereby 

 the Northern District rocks have been pushed over the Ordo- 

 vician beds. Eamsay ^ believed this northern area to be mainly 

 altered Cambrian strata, with some Lower Silurian (Ordovician) 

 beds in the northernmost parts near Cemaes,^ though he could 

 not here separate the rocks of the two systems.* Dr. Hicks ^ con- 

 sidered the beds north of the curved fault to be pre-Cambrian 

 (Pebidian) ; Dr. Callaway ^ also classified the beds as pre-Cambrian 

 of the slaty (Pebidian) type. Prof. Blake '' concurring in the pre- 

 Cambrian view, grouped the rocks into two sets : — the ' Sedi- 

 mentary Series ' and the ' Disturbed Volcanic Group,' both of which 

 he assigned to the middle part of his ' Monian System ' ; he also 

 separated from them certain conglomerates and black shales in the 

 northernmost part of the district as ' undoubted Ordovician.' 



On the other hand. Prof. Hughes ^ has long held the opinion that 

 the supposed fault is either non-existent or of little throw, and that 

 the barren green and gnarled strata of the Northern District are 

 intermediate in age, as they are in position, between the black 

 Ordovician slates of the south and those of the north. Dr. Eoberts ^ 

 supported this view, and in 1891 Sir A. Geikie also expressed his 

 opinion ^° that the whole of the Northern District strata were of Bala 

 age. This view finds expression in the recently-published Index 

 Map of England and Wales (Sheet 7). 



1 Mem. Geol. Surv. ' GeoL N. Wales,' 2nd ed. (1881) p. 236 & fig. 88. 



- Ibid. pp. 234 & 242 ; Geol. Surv. Map, Sheet 78. 



^ This is the spelling adopted by the inhabitants and the postal authorities, 

 but the village is called Oemmaes on the Ordnance maps. For all other 

 names I follow the spelling of the 6-inch Ordnance Survey map. 



* Geol. Surv. Eng. & Wales, Horiz. Sec. Sheet 40. 



^ ' Pre-Cambrian (Dimetian, Arvonian, & Pebidian) in Caernarvonshire & 

 Anglesey,' Quart. Journ. Geol. Soc. vol. xxxv (1879) p. 295. 



^ ' Archgean Geol. of Anglesey,' ibid. vol. xxxvii (1881) p. 210 ; ' Archcean & 

 Lower Palaeozoic of Anglesey,' ibid. vol. xl (1884) p. 567. 



'' ' Monian System of Rocks,' ibid. vol. xliv (1888) p. 514. 



'^ ' Altered Rocks of Anglesey,' Proc. Cambr. Phil. Soc. vol. iii (1880) 

 pp. 346-347; 'Geol. of Anglesey,' Quart. Journ. Geol. Soc. vol. xxxvi (1880) 

 p. 239. 



9 Geol. Mag. 1881, p. 573. 



1° Pres. Addr. Quart. Journ. Geol. Soc. vol. xlvii (1891) Proc. p. 132. 



