386

OLD RED SANDSTONE, CONGLOMERATES, ETC.

those who had not worked out the relations of the system to conclude, that these strata belongedeither to some lower portion of the unproductive coal-field or to the Silurian System 1 ; but closeexamination 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 thewhole passes conformably into the lower limestone shale. 2nd. They are not only lithologicallydistinct from any Silurian rocks but they never contain a trace of the organic remains so abundantin 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 Clyroand Begwm Hills, Radnorshire, Herefordshire , &c. (p. 181.) 4th. Although such yellow sandstonesare 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 flaglikebeds 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 andlower portions, is red; the rocks consisting of thick and tliin-bedded red rab, with occasionalcourses of good building sandstone. Strata sufficiently calcareous to represent cornstone are veryrare, though a few occur among the variegated red and green marls near Wollaston Cross, nearPwll-y-crochan, south of Milford Haven; 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 varietyof 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 aresufficiently calcareous to be burnt for lime.

Instructive sections of the lowest beds of the formation are visible in the deep and narrow combswhich furrow the great dome-shaped mass of Old Red Sandstone of the parish of Cyffic (east ofTavern Spite). In several of these, especially on the north side of the new road to Caermarthen, theuppermost beds of the Silurian System crop out from beneath the Old Red Sandstone and define itsbase precisely. Here, dull green and yellow, flaglike beds, alternate with red rab, and are succeeded bymottled red and green, sandy, argillaceous strata, of concretionary structure, which even in the freshlyquarried faces present a honey-combed and rotten aspect, a variety of the Old Red Sandstone whichis almost peculiar to Pembrokeshire, and in conjunction with beds of quartzose conglomerate havinga greenish base, reappears frequently throughout the range of the formation to the westward. Thecavernous structure arises generally from the decomposition of the less tenacious concretions ofmarl, though there are also cases of half concretionary, half conglomerate beds, from which quartzpebbles 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 intermediateplaces have been observed frequently to contain hard quartzose conglomerate. A clear section ofthis occurs in Canaston Wood south-west of Narbeth.

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

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

c. Hard (greywacke-Vike) grit, in parts almost a conglomerate of grey and greenish grey quartz pebbles, fragments ofslate and Silurian rocks. Beds one and a half to three feet thick with way-boards of yellowish green shale, in all twentyfeet 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. e. Red rab, passing into

‘ Formerly mapped as greywacke.

The residence of Mr. Baugh Allen.