120 C. A. Matley — The Arenig Rocks near Aherdaron. 



The base of the succession is not seen. A specimen of Tetragraptus 

 serra, Brongn. ( = T. hryonoides, Hall), was found some 300 feet or 

 so below the summit of (1). 



Further north, in a small black-shale quarry near Aberdaron, 

 a number of graptolites were obtained, probably Didymograptus 

 extensus, Hall, and D. nitidus, Hall. Loose blocks found in the 

 neighbourhood also yielded graptolites of the D. extensus type, 

 though the actual species to which these forms belong have not 

 been made out with certainty. They are very probably of Lower 

 Arenig age. 



Where the western boundary fault reaches the sea at Parwyd 

 a mass of black shale with numerous flaggy bands of sandstone is 

 wedged in between the pre-Cambrian rocks and a dyke-like mass 

 of picrite which separates it from zone (1) of the above table. 

 These Parwyd beds yielded Azygograptus suecicus, Moberg, and 

 Didymograptus, sp., perhaps Mrundo, and they represent a higher 

 horizon than the beds with the ' extensiform ' graptolites at 

 Aberdaron. 



The beds of the Aberdaron tract appear, therefore, to comprise rocks 

 ranging from the Lower to the Middle and perhaps Upper Arenig. 

 All the graptolites discovered are forms found in the " Middle 

 Skiddaw Slates " ^ of the English Lake District. 



Eastern or Zlanfaelrhys Tract. 

 The rocks dip easterly, and consist of sandy and argillaceous shales, 

 often containing flaggy and thick bands of sandstone. At various 

 horizons in the series are injections of basic igneous rock, usually 

 dolerite sills. Figs. 3 and 4 are sections across part of this area. 



The succession is best observed along the coast, and may be stated 

 in descending order as under : — 



[Laccolite of Mynydd Penarfynydd, a mass of hornblende -picrite and horn- 

 blende -dolerite.] 

 (6) Penarfynydd shales with Didymograptus bifidus, Hall. 



[Dolerite sill near Forth Llawenan.] 

 (5) About 6 feet of dark shale containing layers crowded with Bidymograptus 

 hifidus, Hall. 

 [One or more dolerite sills ; coast obscured by drift.] 

 (4) Ironstone and manganese zone of Nant y Gadwen, comprising also con- 

 cretionary blue mudstone and dark shales. 

 (3) Dark shales of Nant y Gadwen, characterized by B. Mrundo, Salter. In 

 these beds is a band that has been worked for manganese. 

 [Maen Gwenonwy dolerite sill.] 

 (2) Flaggy sandstones and sandy shales. 



[Gallt y Mor dolerite sill.] 

 (1) A thick series of dark slaty beds, often sandy, and containing, especially in 

 their lower part, courses of sandstone. 



The base of (1) is not seen, being faulted against a strip of pre- 

 Cambrian rocks. 



Geologists are indebted to the late Mr. Tawney - for the first 



1 As defined by Miss EUes, Quart. Journ. Geol. Soc, vol. liv (1898), pp. 528 

 and 530. 



* Geol. Mag., 1880, p. 211. Fossils, including graptolites, were also found in the 

 neighbourhood of Llanfaelrhys by the Geological Survey ; see Eamsay, Geol. North 

 "Wales, 2nd ed. (1881), pp. 215 and 377. 



