338 



THE BRECON ANTICLINAL OF SILURIAN ROCKS. 



by the transverse breaks before alluded to, three of which are parallel, occupied 

 by the streams Yscir-fechan, Yscir-fawr, and Honddhw, tributaries of the Usk. 

 Wherever the Silurian strata compose a distinct anticlinal ridge, as at Mynidd-bach, 

 near Corn-y-fan, PI. 33. f. 8., they are symmetrically arranged on each side of the axis, 

 — the Upper Ludlow rocks graduating on both flanks into the Old Red Sandstone, 

 through fissile, yellowish sandstones, which represent the beds of Downton Castle, p. 197. 

 Similar strata of Upper Ludlow Rock occur in the dome-shaped hill of Castell Rhiwan- 

 nest, though their passage upwards is well seen only on its north-western slopes, the 

 Old Red Sandstone on its western face being rendered unconformable by a powerful 

 fault. 



The hills of Castel Madoc on the left bank of the Honddhu present a fine develop- 

 ment of the Upper Silurian Rocks, expanded over a width of about two and a half miles. 

 On first viewing the outline of these rocks, the geologist would imagine that the lower 

 and dome-shaped masses in the centre must necessarily be composed of underlying 

 strata, like the centre of the Ludlow promontory (PI. 31. f. 5.), or still more like the 

 Valley of Woolhope near Hereford, hereafter to be described (PI. 36. figs. 9 and 10.), 

 where the central dome consists of Caradoc sandstone. But, on examination, we dis- 

 cover two parallel lines of elevation, by which strata of the same age are repeated in 

 separate anticlinal forms on both sides of a central depression, dipping inwards as well 

 as outwards, and thus rendering the low inner domes*, portions of the youngest, and not 

 of the oldest strata as at Ludlow and Woolhope. (See PI. 31. f. 6. and this wood-cut.) 



Allt-fawr. 63. 



a. Lower Ludlow Rock. b. Middle Ludlow Rock, including a calcareous band with Terebratula Navicula. 



c. Upper Ludlow Rock. d. Old Red Sandstone. 



As these undulations are at some points rapid, the strata being disposed at high 

 angles so as to meet in sharp ridges and at others form domes, and as on the outer 

 flanks the beds pass beneath the Old Red Sandstone at gentle angles, we have abundant 

 facilities for determining the complete succession of the strata. The external faces of 

 the outer ridges are composed of the true Upper Ludlow Rock, and contain its cha- 

 racteristic fossils, including Leptama lata, Cypricardia amygdalina, with Orthocerata, &c. 



These Upper Ludlow Rocks, which are of considerable thickness, pass upwards into 

 ordinary marls and tilestones of the Old Red System. They are underlaid (particularly 

 along the crests of the sharp ridge of Allt-fawr) by a thin band of impure limestone, 

 made up almost exclusively of the small Terebratula navicula, which forms so constantly 

 the roof of the limestone of Aymestry. This small shell is, therefore, a most valuable 

 guide in marking the place of this subdivision or central part of the formation, even where 



