340 



UPPER SILURIAN ROCKS NEAR BUILTH. 



By inspecting the map it will be seen that this dislocated spur of elevation is pre- 

 cisely coincident with the axis of the Ludlow promontory, the point of which is forty-one 

 miles distant from Com-y-fan, while at Old Radnor, midway between these extreme 

 points of the line of elevation, is a ridge of volcanic rock, the eruption of which enables 

 us satisfactorily to account for the remarkable phenomena above described, and to con- 

 nect them with those detailed in the nineteenth and twenty-fifth chapters. 



Returning to the gorges of the Wye, whence we traced the anticlinal, the Ludlow 

 rocks on the right bank of that river exhibit partial dislocations, frequently diverted 

 from their true bearing, though at low angles of inclination ; but quitting that valley, 

 and following them as they fold round the Old Red Sandstone of the Mynidd Epynt, 

 we first find the strata inclined to the S.S.W. in the hills of Gwendwr and Pant-y-llyn. 

 They thence bend round by Pant-y-garreg and Cwm-nant-gwyn, until they regain 

 the prevailing north-east and south-westerly strike, and dip below the Old Red Sand- 

 stone. From the hill of Moel-fre, near Builth, they begin to strike persistently to the 

 south-west, and constitute that extraordinarily straight escarpment, the north-western 

 boundary of the wilds of Mynidd Epynt and Mynidd Bwlch-y-groes, which is prolonged 

 into Caermarthenshire. In all this range the Upper Ludlow Rock is clearly defined, by 

 its gray colour and characteristic fossils, and it is overlaid conformably at many points 

 by the old red tilestones. There are, however, few places where the mineral character 

 or fossil contents of the underlying strata enable us to separate them into sub-formations 

 with the same distinctness as in parts of the Brecon anticlinal. 



Of the many transverse sections which may be made from the junction of these strata with the 

 Old Red Sandstone of the Mynidd Epynt to the base of the Upper Silurian Rocks, I will cite only 

 two cases ; first, that on the sides of the high road from Brecon to Builth in descending from 

 Llang-y-nog into the valley of the Wye through the Cwm-nant-gwyn; and secondly, that in the 

 Cwm-craig-dhu, exposed in the new road from Brecon to the valley of Yrfon, because they can be 

 well seen by the traveller. In the first, the following strata occur in descending order : 



Hard, indigo grey, slightly calcareous and micaceous flagstone of good quality, containing orthocera and other organic 

 remains of the Upper Ludlow Rock. 



Sandy beds, of dirty green colours, finely laminated. 



Bluish grey, slightly calcareous rock, surface on weathering friable and porous, with ferruginous casts of fossils, including 

 Calyrnene BlumenbacTiii, Serpuloides longissima, Orthocera, Avicula reticulata, Pleurotamaria, and Turbo corallii, the coral 

 investing these two shells being of the same species as at Ludlow, distant between forty and fifty miles. 



Thicker bedded, indigo grey, hard stone, with flattened orthocera, some apparently of the pear-shaped species, 0, piri- 

 formis. 



Beds with the Terebratula navicula, marking the place of the Aymestry limestone as in the Brecon anticlinal. 



The above subdivision is exposed in successive layers about 500 feet thick, and represents the Upper and Middle Ludlow 

 Rocks. The beds dip 16° to 18° south-east. They are underlaid by shale disposed in large concretionary masses, and from 

 beneath these rise other beds of strong flags, somewhat calcareous and of bluish grey colours, which mount into the hill of 

 Rhiw-frenin, and contain many flattened orthocera and other fossils of the Lower Ludlow Rock. The latter beds occupy 

 a considerable thickness in the escarpment of Rhiw-frenin, and at the bridge they are underlaid by stratified masses of 

 shale and schist, occasionally putting on a very imperfect slaty cleavage, and weathering into numberless small angular 



