182 TILESTONES IN CAERMARTHENSHIRE, BRECKNOCKSHIRE AND RADNORSHIRE.

In this district, however, these lower red and yellowish beds, or “bur stones,” areseldom so fissile as the “tile stones” described in South Wales. They occasionallycontain a few organic remains, such as Avicula, and a small Lingula, both of newspecies, which will presently be described. In the Shropshire beds the remains of fishesprevail more than those of mollusca, including the Dipterus macrolepidotus (Sedgwickand Murchison), ichthyodorulites of the genus Onchus, and small bufonites the remainsof palates of fishes. (See PI. 1. fig. 2, 2 a. PL 3. and subsequent description of the shellsand fishes of the Old Red System.)

In the southern parts of Caermarthenshire and in Pembrokeshire, the tilestones can-not he traced as a persistent zone, and the triple subdivision of the system can nolonger be observed. Thus, in following the escarpment of the carboniferous limestonesof Caermarthenshire to the low hills near the coast, we gradually lose the distinct tracesof the red conglomerate below it. Beds of cornstone are very rarely to be detectedin the central masses, and the tilestone of Middleton Park (between Llandeilo and Caer-marthen) are the last well-defined examples of that variety of stone. At Black Pool andCastel Goylan near the mouth of the Towey the lower beds do not afford tilestones, but,on the contrary, are thick-bedded, slightly conglomerated, mottled, quartzose sandstones.

The previous sketch, however, of the lower member of the Old Red has been de-rived from numberless transverse sections, made between Pont-ar-lleche on the south-west and the environs of Ludlow on the north-east, being a distance of near ninety miles.

Reverting to the section at Pont-ar-lleche (PI. 34. fig. 5.), the shale and cornstone are there under-laid by alternations of red and green sandstone, some of the lowest of which are the “ Tilestones ”which give the name to the bridge. Below these are other bands of a quartzose, deep red sandstone,indurated shale, and slightly conglomerate pux-plish brown sandstone, containing small pebbles ofquartz, which gradually disappearing, the beds pass into the underlying grey Silurian rocks of thedistrict. These conglomerate beds, though not seen at the base of the Old Red System in any otherparts of its course between Caermarthenshire and Ludlow, are found in the same position in Pem-brokeshire and atThornbury, Gloucestershire . (See the chapters on Pembrokeshire and Tortworth.)Casts of Orthoceratites and other fossils occur in the finely laminated beds associated with the tile-stones at Pont-ar-lleche, similar to those of other localities.

The transverse section of these beds afforded by the valley of the Cwm Dwr, be-tween Trecastle and Llandovery, is of high interest. (PL 34. figs. 1 and 3.) Tilestonesare here quarried rising at an angle of sixty degrees from beneath the marly and sandybeds of the cornstone group, the lower tilestones graduating downwards into theequivalent of the Ludlow rock. The uppermost beds are of a dark purple colour, theirsurface being covered by large plates of a grey mica, and here and there indented withcertain impressions resembling those in the Old Red Sandstone of Scotland called“ Kelpie’s feet 1 .” The lower beds, as worked on the steep acclivity west of the meeting-

1 It is highly probable that the Arbroath pavement beds of the Old Red Sandstone of Scotland , will he foundto correspond with the tilestone formation of South Wales.