part 3] LOWER PALAEOZOIC OF ARTHOU-DOLGELLEl\ 263 



(iii) The Lower Pencil-Slates. — Following on the Dictyo- 

 neina Band, the slates become less cleaved and less rusty as they 

 pass upwards into a hard dark-grey mudstone. The ill-cleaved 

 mudstone then passes upwards in the course of some 40 feet into 

 blue slates, which usually take on a distinct ' needle ' or ' pencil ' 

 cleavage. These latter beds sometimes contain in about the 

 centre, of the group partings and thin layers of quite coarse- 

 grained gritty material. 



(iv) The Asaphellus Flags and Shales. — The lower pencil- 

 cleaved slates are succeeded by a group of hard flagg}' beds more 

 greyish and sometimes striped, though not markedly so, owing to 

 the incoming of a certain amount of sandy material. Other 

 bands are rather -bluer and highly micaceous. Nodules of cone- 

 in-cone ironstone are occasionally to be found. Where there is 

 not sufficient sandy material present to render the beds actually 

 flaggy, then the rocks cleave into lenticular fragments, causing 

 crags and weathered blocks to present a characteristic rubbly 

 appearance. The rocks tend to weather white, but frequently are 

 strongly iron-stained, often with iridescent films. Not infrequently 

 also very black films may develop. This group of beds gives rise 

 to a prominent feature (marked d on PI. XVI) which runs almost 

 continuously from one end of the district to the other. 



Numerous fossils are obtainable from thinhy-bedded micaceous 

 flags or shales belonging to the upper part of the group exposed in 

 trial-levels just beyond the north-western end of Llyn Wylfa. 

 For a few feet the beds are here remarkably fossiliferous, rivalling 

 the famous locality of Penmorfa ; yet in levels only a few yards 

 away from these highly prolific beds very few fossils are to be 

 found. The fossils include : — 



Asaphellus liomphrayi Salter. Conularia homphrayi Salter. 



Agnostics calvus Lake. Centrotheca cuspidata Salter. 



Ormetopus prsenuntiits Salter. Theca sp. 



Lingulocaris sp. Obolella. 



(v) The Upper Dictyonema Band. — Above the flaggy 

 Asaphellus Beds occurs a second band rich in Dictyonema. 



The best and most easily accessible fossil locality is that on the 

 western bank of the Gwynant, about 80 yards south of Pont Ty- 

 gwyn. The D icty one m a -be&ving- strata here occupy the bed of the 

 tributary rivulet (often dry), and they also give rise to screes 

 on the steep bank above the roadway. Cleavage and bedding 

 almost coincide, and Dictyonema is so extremely abundant that 

 large slabs with surfaces entirely covered by the fossil may be 

 obtained. Less commonly, specimens of Gallograptus are to be 

 found. The fossil occurs in grey-blue, somewhat flaggy slates that 

 weather rusty at first, finally becoming greyish-white. 



Fragments of Dictyonnna were also obtained at a slightly lower 

 horizon from baked beds on the north side of the diabase at Pont 

 Ty-gwyn ; therefore the fossil tends to occur through a considerable 



