LOWER SILURIAN ROCKS— BRECKNOCKSHIRE, 



341 



shivery fragments. In their prolongation to the north-east, at the Hennalt and New Hall, these beds are quarried for tiles 

 and coarse slates. They are again underlaid by other masses of dull grey, non-micaceous perishable shale, which has a 

 tendency to spheroidal or large concretionary structure, and these range over the undulating grounds south-east of Builth. 

 This lower portion of the Upper Silurian Rocks dips at a less angle than the higher, seldom exceeding 8° or 10° to the 

 S.S.E. 



The transverse section of Cwm-craig- dhu, descending from the Mynidd Epynt to the banks of 

 the Yrfon, is in some respects better than that of Cwm-nant-gwyn. The bottom beds of the Old 

 Red Sandstone, consisting of red and green marls with micaceous flags (tile stones), and dipping 

 30° to the south-east, pass downwards into the intermediate grey rock, the " Downton Castle 

 building stone," Beneath is a full succession of the Upper Ludlow strata, charged with abun- 

 dant casts of fossils, overlying, indigo-coloured calcareous flagstones, like those of Rhiw frenin. 

 These are similarly underlaid by sandy shale and schist, weathering into rotten and fissile fragments, 

 containing the Cardiola of the Lower Ludlow Rock, with some beds of harder flagstone, slightly 

 micaceous and calcareous. The inclination of the lower beds gradually increases from 40° to 60°, 

 and they repose upon black shale of great thickness, weathering to small shivery fragments (rotch), 

 but no clear section is obtained till the banks of the Yrfon are reached, when the shale is highly 

 inclined, vertical, and dislocated. (See PI. 33. f. 6.) 



Few of these lower beds have sufficient cohesiveness to afford building materials, but in the bed 

 of the Yrfon, between Tafarn-y-pridd and Llangammarch, the schists contain a few concretionary 

 courses of dark grey calcareous flags, with thin beds of conglomerate. 



cc Lower Silurian Rocks." 



The underlying or Lower Silurian Rocks are ill developed in Brecknockshire, for as 

 soon as the protruded mass of trap rocks described in the last chapter, disappear near 

 Builth, these older strata are no longer seen rising from beneath the cover of Upper 

 Silurian Rocks. A few beds of Caradoc sandstone accompany, it is true, the black 

 Llandeilo flag upon the "Wye, and occupy the low hillocks between its right bank and 

 the Park Wells, the relations of which have been pointed out in the last chapter. 

 With this exception, all the district between the escarpment of the Upper Silurian 

 Rocks and the boundary line of the slaty rocks of the Cambrian System, is occupied 

 by undulating hills of perishable schist and shale, with very rarely a course of sand- 

 stone and grit. The latter, however, occur in the hill of Garth, forming strong-bedded 

 quartzose conglomerate, grit, and sandstone in nearly vertical strata, extending for 

 about two miles to the N.N.E. ; and again, at Dol-aeron, are beds of bluish grey, flag- 

 like building-stone, dipping north-east 20°. In these quartzose rocks I could detect 

 no fossils. They may, indeed, belong to the Lower Silurian division, but from their 

 mineral characters I should be disposed to place them in the Cambrian System. The 

 subsoil of all the sterile tract watered by the Chwefrw, the Cammarch, the Dulas, and 

 the Cwm-dwr (tributaries of the Yrfon and Wye), consists of rotten shale with scarcely 

 the vestige of a solid bed of stone ; so that this part of Brecknockshire offers a strong 



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