204 G. L. Elks — Gmptolitc Zones in Arenig Rocks. 



lower betls are uncleaved fissile blue-black sLales with some 

 calcareous flaggy bands. Numerous graptolites can be obtained 

 at this point, and are particularly abundant in the more shaly 

 bands, especially in a band close below the limit of staining. 

 I have collected the following fossils from this locality : — 



I>i(hjmograptHs extcnsus, Hall. Tctrnrjraptm serra, Brong. 



J). Nichohoni, Lapw. T. Amii, Lapw. 



J), nitidus, Hall. Caryocaris, sp. 



B. gibberulus, Nich. Lingulella, sp. 



i). cf. ««v/on«w, Elles & "Wdod. Trilobite (imlet.). 



Bidymograptus extensus is an abundant form, and the beds seem 

 therefore to belong to the zone of D. extensus, though I consider 

 the presence of Tetragrapti indicates that the beds are somewhat 

 lower down in the zone than those of the Seiont Eiver. 



Lleyn Peninsula. 



In the neighbourhood of Aberdaron Matley has described several 

 localities where Arenig rocks may be found (loc. cit., p. 118). 

 He groups these into two districts, the eastern and the western. 



(a) Eastern District. 



The succession and mutual relation of the beds is best indicated 

 in the eastern district near Llanfaelrhys. 



As Matley (and previously Tawney) have indicated, the Penarfynydd 

 shales contain numerous and excellent specimens of Didymograptus 

 hifidus, as well as some trilobites and other fossils; these, therefore, 

 represent the zone of B. hifidus. They are well exposed on the north 

 side of Mynydd Penarfynydd, south and south-east of Penarfynydd 

 Farm, and Matley has shown that the same shales are again seen 

 on the coast between Porth Llawenan and Porth Alwm. They 

 occur here between two dolerite sills, and seem to be faulted slightly 

 out of their normal position. Further west excellent exposures 

 of a series of slaty shales, mudstones, and ironstones are seen in 

 the little gorge of Nant y Gadwen, and are termed by Matley the 

 Nant y Gadwen shales. These shales belong to the zone of B. hirundo 

 and the zone of B. extensus. The upper, more slaty beds, which 

 are now best seen on the east side of the gorge and contain 

 Bidymograjjtits hirundo, B. nitidus, and Azygograptns suecicus, constitute 

 the zone of B. hirundo ; while the lower, more sandy beds of the 

 extreme west side of the gorge seem to belong to the zone of 

 B. extensus, since they have yielded the following graptolites : — 



Bidymograptus extensus, Hall. Tctragraptus Amii, Lapw. 



B. nitidus, Hall. T. cf. Headi, Hall. 



B. gibberulus, Nich. Trinucleus Gibhsi, Salt. ? 



B. miifor/iiis, Elles & Wood. Calymene, sp. 



These lower beds have a fauna almost identical with that of the 

 Arenig beds of Menai Straits. 



The fauna of the beds of the manganese-works of Benallt, near 

 Ehiw, seems to be that of the zone of B. hirundo. 



