208 



G. L. Elles — GraptoUte Zones in Arenig Rocks. 



(1) Western District. 



Hicks ^ and Hopkinson- long ago drew attention to the graptolite- 

 bearing beds of tlie St. Davids district, and recorded fossils from the 

 mainland and also from Eamsey Island, but while recent work 

 tends to confirni Hicks' view that the beds of Llanvirn and Porth- 

 Hayog were of Upper Arenig age, it tends to modify his conclusion? 

 respecting the age of the other graptolite-bearing beds. 



(a) Abereidchj Boy. 



The relation of the lowest Llandeik^ beds, forming the zone of 

 D. 31archisoni, to the uppermost Ai"enig slates, constituting tlie zone 

 of D. hifidus, is particularly clear in this western district and worth 

 notice. At the extreme south end of Abereiddy Bay a series 

 of much cleaved black slates, dipping a few degrees W. of N., is 

 seen crowded with Bidijmogrcvptus MurcMsoni ; ascending the cliff, 

 a well-marked band of felspathic ash separates these black slates 

 from an underlying series of lighter-coloured slates, which can be 

 observed for a considerable distance along the track leading from 

 the spring to Nant ; they also dip a few degrees W. of N. at 

 a high angle, and have been extensively quarried in the past. The 

 most fossilifei'ous beds, as indicated by Hicks, are those in the 

 middle quarry, and from tliis place and the 'tips' lying north-west 

 of it most of my fossils were obtained. 



The beds contain 



Didyinoc/raptxn h[fichis, Hall. 



B. nanus, Lapw. 



I). Nicfioho/ii, Lapw. 



7>. patulns. Hall. 



I), enodiis, Ijapw.^ 



('rijploiirtijit.iiii tricoriiu^, Carr. 

 G/os.s(if/rrrptiis ciliff/iis^ Emmons. 

 Diplogiitpfits diHtatus, Biong'. 

 Placoparia camhvensis, Hieki^. 



This light-coloured slate series therefore clearly represents the 

 zone of D. hifidus ; it is underlain by a succession of grits and flags^ 

 well exposed in the old quarry near Nant. 



(b) Eamsey Island. 



Beds with a fauna very similar to that just described, but differing 

 somewhat in their lithological characters, were found by Hicks and 

 Hopkinson (loc. cit.) on Karasey Island at the head of a little creek 

 on the south-west known as Porth-Hayog or Porth Llanog. The 

 beds here are soft black shaly mudstones, sometimes highly 

 micaceous ; they dip N.W., and have yielded an abundant fauna, 

 including the following graptolites : — 



Didymograptua bifidus, Hall. 

 D. stahilis, Elles & Wood. 

 I), artm, Elles & Wood. 

 D. Hicholsoni, Lapw. 

 D. affinis, J^ich. 



l)idymo(jr((ptiifi acittidiiis^ Lajjw. 

 J), patulns. Hall. 

 Diplograptus dentatus, Bi'oug'. 

 Climacoyraptus Scharenhcryi, J^apw. 

 (!J. conftrtiis, Lapw.-* 



1 Hicks, Brit. Assoc. Rep.. 1872, 1S73; Uuait. Jouni. Geo]. Soc, 187-5, p. 1G7. 



2 Hopkinson, Brit. Assoc. Rep., 187'2, 1873. 



■* Cf. lists of Hicks and Hopkinson, loc. cit., and Hopkinson and Lapwortb, Quart. 

 Journ. Geol. Soc, 187-'5, p. 631. 



