GEOLOGY OF ANGLESEY. 



213 



rous, the lower gneiss being brought up to the surface at many 

 points. Probably the entire series is exposed west of Gaerwen, 

 where the total thickness can hardly reach 1000 feet. 



An isolated mass of schist occurs south-west of Pentraeth. It is 

 about a mile long by half a mile broad, and is bounded by faults on 

 all sides. Carboniferous Limestone surrounds it on three sides. On 

 the east it is separated from the main mass by a faulted strip of the 

 formation now to be described. 



b. Slaty Series, 



This wedge is composed of rocks of a newer Archaean group. 

 At Tan y graig, south-west of Pentraeth, is a greenish, coarse, 

 slaty variety. In the fields to the north-west, a jaspery rock 

 projects in small knolls through the turf ; but its relation to 

 the bedded type is obscure. Crossing a hollow to the east, we 

 come to a ridge, the western slope of which is occupied by 

 massive breccias. The fragments and the matrix are similar, and are 

 composed of a grey and purplish dolomite (Note 40, p. 235). Further 

 to the north-east, above Wugan bach, is a light-purple slaty rock, 

 which in places is literally smashed. Between this point and Plas 

 gwyn, along the ridge, the prevailing type is a purplish ashy rock, 

 sometimes brecciated, and in places altered almost to a hornstone. 

 The dip is high to the north-west. The breccias of this ridge are 

 obviously connected with faulting. North of Phis gwyn, on the 

 same strike, is a puzzling rock, which, under the microscope (Note 

 41, p. 236) appears to be a limestone rendered impure by volcanic 

 mud. Slaty beds are again seen to the north-east of Pentraeth, 

 dipping south-east. 



Por the greater part of its length this mass is separated from the 

 Menai anticline by a faulted strip of Palaeozoic shale ; but north- 

 east of Rhiwlas it comes up against the gneiss, the fault between 

 the two formations running along a hollow. The heavy breccias 

 which fringe both groups point to excessive fracture and crushing. 



II. Llaxgefxi " Syncline." 



Crossing the Carboniferous strata of Malldraeth Marsh, we come 

 to an altered and contorted group of rocks which bear a superficial 

 resemblance, especially in their colour, to the Menai schists, and 

 have hitherto been regarded as a part of the same group. But on 

 a careful examination of the lithology of this series, fundamental 

 differences are observable. Even my first day's work in the district 

 convinced me of the existence of Pebidian rocks of the St. -David's 

 type ; and fuller working-out of the area brought to light analogies 

 with other Pebidian localities. 



Slaty rocks occupy only the south-eastern side of the apparent 

 syncline lying between the Carboniferous on the south-east and 

 the granitoid band on the north-west, the north-western side being 

 composed of gneissic strata. The two groups are brought together 

 by a fault or faults : but as they dip towards each other, an appa- 

 rent basin is formed (see section near Aberffraw, PI. VIII. fig. 2). 



Q. J. G. S. No. 146. a 



