516 Geological Society. 



Head on the north. At Potter's Slade, a little north-west of Llan- 

 huadain, a conglomerate dips to the northward, and is traceable 

 westward to Ford, and eastward towards Llangan, where a sandstone 

 conglomerate occurs containing Trilobites and shells. Proceeding 

 on the line of section, the conglomerate is succeeded first by sand- 

 stone and sandstone shales, and then at Clarbeston by limestone with 

 carboniferous shales, dipping northward, and containing Graptolites 

 and casts of shells. Similar carbonaceous shales exist on the west 

 of Clarbeston, at St. Catharine's Bridge, near Camrose ; also at 

 Rudbaxton, and on the east at Long Ford, near Llandysilio. They 

 have in some localities been unsuccessfully worked for coal. Grap- 

 tolites have likewise been found in calcareous shales at Robleston, 

 about a mile north-west of Camrose. At Llys-y-fran, north of Clar- 

 beston, the carbonaceous shales are succeeded by roofing-slates, 

 which at Mynydd CasteU-bythe (Castell-y-furoch, Ord. Map) and 

 Morfel alternate with trap. On the summit of Mynydd Pontfaen, 

 sandstone with coarse slates occurs, and between the summit and 

 Pontfaen, trap again alternates with slates. The summit of Mynydd 

 Llanllawer consists of coarse-grained, rudely columnar greenstone, 

 flanked on the northern declivity of the mountain by coarse sand- 

 stone of trappean aspect. This rock is overlaid by roofing-slates, 

 which extend nearly to Dinas Head, where a hard conglomerate 

 sandstone, containing crinoidal remains, is exhibited. All these 

 strata are represented in a section as dipping towards the north. 



In Aberreiddy Bay, about twelve and a half miles to the south- 

 west of Dinas, slaty beds with a northwardly dip, and apparently 

 prolongations of the schists on the line of section, contain the Grap- 

 tolithus Murchisonii and G. foliaceus of the Llandeilo flags, also nu- 

 merous casts of an Euomphalus, resembling the E. perturbatus of 

 that formation, and a species of Lingula. Although these slates 

 differ in lithological characters from the Llandeilo flags, yet Mr. 

 Maclauchlan is of opinion that this difl"erence may have been pro- 

 duced by the masses of trap which are associated with the slates. 



The conglomerate sandstone of Dinas Head, which occurs also in 

 Newport Bay and at Trewyddel, near Cardigan, is stated to resem- 

 ble one of the conglomerates of the Caradoc sandstone described by 

 Mr. Murchison ; and the crinoidal stems which it contains, to agree 

 with analogous remains found by Mr. Murchison in the Caradoc 

 sandstone at Little London, May Hill*. 



In addition to the phsenomena which occur in the immediate vi- 

 cinity of the section, Mr. INIaclauchlan alludes to indications of anti- 

 clinal lines near Narberth and at Camrose, in Southern Pembroke- 

 shire ; also at Solfach, south-east of St. David's, and at Porthllisky, 

 to the south-west of that city. At the latter village the dip changes 

 to the westward, and continuing to alter, assumes in Whitesand Bay 

 a northwardly direction. This dip also prevails at St. Laurence, 

 thirteen miles east of St. David's Head ; at Leweston, three miles 

 south of St. Laurence, and at Long Ford, about two and a half miles 

 south of Llandysilio. Trappean ash also is stated to occur near green- 



* Silur. Syst., pi. xx. fig. 19- 



