CASTELL CERRIG CENNEN.

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fig. 6.) The breadth of this valley between the escarpment of the carboniferous lime-stone on the south, and the Silurian rocks on the north, does not exceed two miles. Itwill hereafter be shown, that the full development of the Old Red Sandstone terminatesat Pont-ar-lleche, and as that system is not laid open by any transverse gorge in thevalley of the Cennen, no accurate opinion can be offered respecting all the strata ofwhich its chief and central masses may be composed. The uppermost beds, however,present a thin zone of conglomerate and grit, and the lowest tilestones are distinctlyexhibited between Cerrig Cennen and the vale of the Towy. The intermediate strata,upon some of which the limestone outlier of Cerrig Cennen is placed, are argillaceousand sandy; those on the north side of the castle being vertical and unconformable tothe bedding of the calcareous rock, while those at its southern base, though imperfectlyexhibited, are apparently twisted under the limestone.

The calcareous beds of “Cerrig Cennen” are well exhibited, and consist of the blackand grey varieties of the adjacent carboniferous limestone, inclosing the same fossils ;viz., Productus hemisphmricus , P. Martini, and large bunches of one of the corals pecu-liar to this formation, (Lithodendron sexdecimale, Phill., &c.)

The precipitous face or escarpment of the rock on which the castle stands, is exposedto the depth of about 60 feet, the summit of the hill being between 250 and 300 feetabove the Cennen. The remainder of the declivity from the base of the rock to theriver, is covered by a steep grassy slope, which conceals almost entirely the strata belowthe limestone ; but at one point the Cennen has worn away a small portion of the talus,and exposed a few feet of red sandstone, which appear to dip, as before said, unconform-ably under the limestone of the castle hill.

On the left bank of the Cennen the strata of Old Red Sandstone interposed betweenthe outlier and the main escarpment of the carboniferous limestone, are so concealed bydetritus, that traces of them are to be detected only in the bed of one small rivulet.On ascending, however, to the site of the great fault already described, or towards thebreak in the zone of limestone above alluded to, we find the upper member of the OldRed Sandstone plunging to the south beneath the limestone on one side of the ravine,at an angle of seventy degrees, and on the other of thirty or forty degrees to the south-east (sources of the Lwchwr). Seeing that the sudden break in the continuity of themain escarpment of the limestone is exactly opposite the outlier Cerrig Cennen, and thatthis dislocated mass would, as nearly as could be estimated, fill the portion of the gapwhich is between the spring head of the Lwchwr and the house of Cwrt-a-barddh, itmight at first be concluded, that the rock Cerrig Cennen had been torn from its parentseat, and precipitated down the shelving escarpment upon the surface of the Old RedSandstone; an inference which would not appear extravagant in this region of violentdisturbance. But though highly curious and different from that of Carreg-las alreadydescribed, or of any other detached mass along the frontier of the South Welsh coal-field, the dislocation of Cerrig Cennen may be simply explained. It is evident from