178 H. HICKS ON THE SUCCESSION OF 



beds are nearly vertical, and the thickness of the series is about 500 

 feet. Two of the tuff-beds attain to a considerable thickness, while 

 others are merely narrow lines. The thickest one, which occurs 

 directly at the base of the series, is in part a conglomerate ; 

 but its upper portion has been pulverized and rearranged by the 

 action of water, and made to assume a flaggy appearance. The 

 fossil Didymograptus MurcMsoni was discovered in the beds of this 

 series at Aber-eiddy as far back as the year 1841, by Prof. Ramsay ; 

 and the locality has ever since been known as a favourite spot for 

 this graptolite and for Diplograptus foliaceus, which occur here 

 most plentifully and in a well-preserved state. It is only during 

 our researches of late years, however, that any thing like a fauna 

 has been discovered in the series ; and amongst the forms recently 

 added to it are species of Dendrograptus and Ptilograptus, also the 

 Trilobites Ogygia, Calymene, Trinucleus, and JEglina, and the Bra- 

 chiopods Lingida and SipJwnotreta. The fauna is now a tolerably 

 complete one, and it is distinct from that in the underlying and 

 overlying series. 



Middle Llandeilo. — The beds of this series, as shown in Section 

 Y. (fig. 6), rest conformably on the last, and consist of black calcareous 

 shales and flags, and at the upper part of tolerably compact lime- 

 stone. The beds occur at a high angle, with a "N.W. dip. The 

 thickness of the series, as shown on the coast, is about 800 feet ; 

 but there is every indication of a fault in the line of the Aber-eiddy 

 valley, along the sides of which the beds occur, and that a portion of 

 the thickness has been therefore cut off. The calcareous nature of 

 the beds enables one easily to recognize them from the series below 

 and above, and it also gives them a good lithological distinctness in 

 the section. Besides this section at Aber-eiddy, they occur also in 

 several other well-known places in Pembrokeshire, as in Musclewick 

 Bay, on the south of St. Bride's Bay ; and in the neighbourhood of 

 Lampeter Yelfrey and Llandewi Yelfrey, in east Pembrokeshire, 

 where they underlie Caradoc or Bala beds ; and they are everywhere 

 characterized by the well-known Trilobites Asaphus tyrannus, 

 Trinucleus Lloydii, Trinucleus favus, Calymene cambrensis, &c. The 

 series is therefore palceontologically and lithologically well marked 

 and easily recognized wherever exposed. 



The Upper Llandeilo rocks occur as black argillaceous slates, flags, 

 and flaggy sandstones ; and they rest conformably on the Upper Lime- 

 stone beds of the Middle Llandeilo on the north coast of Aber-eiddy Bay, 

 as seen in Section Y. (fig. 6). The beds also lie at a high angle ; and 

 the dip is about JN".1N~.W. The slate-quarries in Aber-eiddy Bay are near 

 thebase of the series; and the sudden change from the calcareous rock 

 to the true argillaceous slate is very marked at this spot. "When the 

 beds of this series were deposited the conditions must have been more 

 like those which prevailed during the Lower-Llandeilo period than 

 in that immediately preceding ; and there is evidence also in the 

 presence of interbedded traps of a return of volcanic activity. It is 

 difficult to know the exact thickness of the series in this section ; 

 for after attaining a thickness of over 1000 feet in natural succes- 



