part 1] IGNEOUS AND ASSOCIATED ROCKS OF LLANWRTYD. 21 



(d) The Hardened Mudstones, etc. 



The spilites are succeeded by a somewhat variable group of 

 sediments. The main part of the division seems to consist of 

 hardened mudstones with several bands of flinty ' hornstone ' 

 and ashes. Murchison states that the 



' schist ... is silicified or in the state of hornstone, highly translucent at the 

 edges, of a scaly fracture and dark-grey colour with cloudy streaks, as if 

 formed by an imperfect separation of hornblende. Other varieties are black 

 Lydian stones, ringing under the hammer, and splitting with a fine conchoidal 

 fracture ; some of them containing a number of bright metallic spots, probably 

 of oxide of iron.' (' The Silurian System ' vol. i, 1839, p. 344.) 



The softer sediments are so rarely seen that one is apt to ignore 

 their existence, especially as the bands of 'hornstone ' and ash are 

 often well exposed. On the north several such bands can be traced 

 and mapped, but the arrangement is more irregular in the south. 

 Hard grit-bands also occur, especially in the south. A feature of 

 great interest is the occurrence, about 30 or 40 feet from the top 

 of the group, of an ashy impure limestone, which weathers on the 

 •surface to a typical ' rotten-stone ', such as one sees in the Llan- 

 •deilian near Llandeilo. In the fresh rock it is practically 

 impossible to distinguish any fossils ; but in the weathered rock 

 •abundant traces of fragmentary brachiopods, crinoid-ossicles, and 

 other remains are visible. The only determinations which have 

 been at all possible include 



Orthis elegantula (?) Dalman. Cystid-plates. 



■Orthis vespertilio (?) J. de C. Sowerby. Bryozoa. 

 Rafinesquina sp. (P). 



Exposures may be typically seen at the point marked E on the 

 "map (PL II), and blocks occur scattered over the surface in a 

 'variety of situations. 



The graptolitic horizons which occur both below and above 

 this group indicate an age equivalent to that of the Mydrim- 

 Limestone division of the Dicranograptus Shales of South Wales. 

 In this connexion the occurrence of a rottenstone so far north is 

 interesting, as the whole horizon is a calcareous one in the south. 



(e) The Upper Ashes. 



Most of the hard bands in the succession form marked features, 

 and this is especially true of the Upper Ashes. The lower part of 

 the division consists in the south of a bed of coarse ash some 

 40 feet thick. On the west it dips away from the centre of the 

 anticline at an angle of about 45°, and forms a conspicuous crag 

 north of the River Irfon, about 300 yards south-east of Llanwrtyd 

 Church. It can also be traced south of the river, as a tree-covered 

 escarpment, which gradually becomes less conspicuous as the dip 

 decreases and one approaches the centre of the strongly-pitching 

 anticline. A considerable area of the summit and eastern slopes 

 of the southern range of hills (Garn Dwad) consists of these ashes. 

 Actually the slopes of the hill are largely the dip-slopes of the 



