Vol. 52.] GEOLOGr OF THE NEIGHBOURHOOD OF CARMARTHEN. 529 



4. Didymograptus Murchisoni-shales. 



The outcrop of this series serves as a datum-line throughout the 

 district, owing to the fact that the fossils are characteristic and 

 abundant. The beds sweep across the country in a straight line 

 from Felin-wen on the east to beyond Xant-y-ci on the west. From 

 this point westward the outcrop takes a southerly bend, owing to 

 the closing in of the anticline, and exposures occur south of Melin 

 Ricket as described by the late Mr. Roberts. The general dip ot 

 the beds is magnetic north, at a greater or lesser angle with that 

 of the cleavage. No actual junction with the higher beds is seen 

 except in Pistyll-y-gwion quarry, where a yellow sandy series 

 overlies the shales. The sporadic appearance of this sandstone, and 

 the fact that we have here the higher beds of the Didymograptus 

 Jlurchisoni-zone, render it admissible to suppose that the sandy 

 rock is a local development of Llandeilo Limestone, otherwise 

 absent throughout the district. Just north of the Plough and 

 Harrow Inn higher beds also occur. So certain boundary-line can 

 at present be drawn between these beds and the underlying bifidus- 

 shales, as fossils are so exceedingly rare in the upper part of the 

 latter. 



An extensive exposure of the Didymograptus Jlurchisoni-be&s is 

 seen in the large quarry behind the Plough and Harrow Inn. 

 They consist of dark-grey shales, with lighter stripes of more sandy 

 material at close intervals. These stripes show the direction of the 

 bedding, which is at an angle with that of the cleavage. The beds 

 contained Didymograptus Murchisoni, Beck, and Siphonotreta micula. 



Confirmatory Sections. 



These beds are so well known elsewhere that we need only 

 mention the chief exposures. Proceeding from the west eastward, 

 we have : — 



(i) The small quarry marked on the 6-inch map as Nant-y-ci 

 sandpit ; the purplish slates contained Didymograptus Murchisoni, 

 Beck, and Climacograptus confertus, Lapw. 



(ii) Pistyll-y-gwion quarry must be specially mentioned, on 

 account of the overlying sandy series. The sandstone was somewhat 

 coarse-grained and very ferruginous, and in it we found Orthis 

 testudinaria, Dalman. The shales yielded Didymograptus Murchi- 

 soni, Beck, Climacograptus confertus, Lapw., Lingula, and Asaplius 

 tyrannus, Murch. 



(iii) In Penlan old quarry and on Penlanffos Hill the same 

 shales are seen. In the old fllled-up quarries on the hill some 

 specimens of Diplograptus foliaceus, Murch., were also found, 

 owing to the fact that we have here a junction with the higher 

 beds. At Cwm-oernant Nurseries specimens of Didymograptus 

 Murchisoni, Beck, were obtained. 



In the Abergwili district to the north-east of the map we have 

 three principal exposures. At Castell-pigyn are dark finely-splitting 

 slates alternating with harder flaggy beds. At Felin-wen the 



