386 



OLD RED SANDSTONE, CONGLOMERATES, ETC. 



those who had not worked out the relations of the system to conclude, that these strata belonged 

 either to some lower portion of the unproductive coal-field or to the Silurian System 1 5 but close 

 examination has convinced me, that these beds are an integral portion of the system of Old Red Sand- 

 stone. 1st. They contain at intervals masses of true red rab which alternate with them until the 

 whole passes conformably into the lower limestone shale. 2nd. They are not only lithologically 

 distinct from any Silurian rocks but they never contain a trace of the organic remains so abundant 

 in that system. 3rd. They pass down into certain greenish grey, hard, flaglike, micaceous sandstone, 

 which has been shown to constitute the base of the Old Red Sandstone over large tracts in the Clyro 

 and Begwm Hills, Radnorshire, Herefordshire, &c. (p. 181.) 4th. Although such yellow sandstones 

 are uncommon in the Old Red Sandstone, their occurrence has been pointed out in other counties, 

 particularly at Prescot Bridge near Cleobury, Salop (see p. 174.), whilst some of the grey and flaglike 

 beds with fragments of plants are precisely similar to the strata near Bromyard, (see p. 177«) 



The colour of the great mass of the formation in Pembrokeshire, particularly in the central and 

 lower portions, is red \ the rocks consisting of thick and thin-bedded red rab, with occasional 

 courses of good building sandstone. Strata sufficiently calcareous to represent cornstone are very 

 rare, though a few occur among the variegated red and green marls near Wollaston Cross, near 

 Pwll-y-crochan, south of Milford Haven 5 also in the mottled marly beds alternating with sandstone, 

 in the deep ravines between Narbeth and Tavern Spite, particularly in that of Cilrhiw 2 . One variety 

 of this concretionary rock consists of bluish green shale, with yellowish spots of calcareous matter, 

 which upon decomposing give it a cellular appearance. Few, perhaps none of these cornstones are 

 sufficiently calcareous to be burnt for lime. 



Instructive sections of the lowest beds of the formation are visible in the deep and narrow combs 

 which furrow the great dome-shaped mass of Old Red Sandstone of the parish of Cynic (east of 

 Tavern Spite). In several of these, especially on the north side of the new road to Caermarthen, the 

 uppermost beds of the Silurian System crop out from beneath the Old Red Sandstone and define its 

 base precisely. Here, dull green and yellow, flaglike beds, alternate with red rab, and are succeeded by 

 mottled red and green, sandy, argillaceous strata, of concretionary structure, which even in the freshly 

 quarried faces present a honey-combed and rotten aspect, a variety of the Old Red Sandstone which 

 is almost peculiar to Pembrokeshire, and in conjunction with beds of quartzose conglomerate having 

 a greenish base, reappears frequently throughout the range of the formation to the westward. The 

 cavernous structure arises generally from the decomposition of the less tenacious concretions of 

 marl, though there are also cases of half concretionary, half conglomerate beds, from which quartz 

 pebbles fall out and contribute to give this aspect. Commencing at Pont-ar-llechau in Caermarthen- 

 shire, where such a structure was first noticed, the inferior members of the strata at intermediate 

 places have been observed frequently to contain hard quartzose conglomerate. A clear section of 

 this occurs in Canaston Wood south-west of Narbeth. 



a. Thick-bedded, mottled and porous red rab, without mica. 



I, Flags of greenish grey grit, faces covered with plates of white mica and traversed by veins of white quartz, passing into 



c. Hard (greijwacke-]ike) grit, in parts almost a conglomerate of grey and greenish grey quartz pebbles, fragments of 

 slate and Silurian rocks. Beds one and a half to three feet thick with way-boards of yellowish green shale, in all twenty 

 feet thick. This stone, being the hardest in the neighbourhood, has been largely extracted in constructing the new roads. 



d. Thick-bedded, cavernous, mottled, concretionary, red and green rab, in parts calcareous and resembling impure 

 cornstone. <?. Red rab, passing into 



1 Formerly mapped as greywacke. 



2 The residence of Mr. Baugh Allen. 



