230 MESSRS. T. C. CANTKILL AND H. H. THOMAS ON THE [May I906, 



several small exposures, and have yielded graptolites of the D.-bifidus 

 type ; while, south-east of the grit, shales may be seen in the road 

 bordering the western side of the Lambstone porphyry, against 

 which they are faulted : these have yielded similar graptolites. 



South-west of Lambstone Farm these shales extend towards 

 Llandeilo-Abercowin ; the grits form the northern slopes of the hill 

 (crowned by an old camp named ' Castle ' on the 6-inch map) 

 west of Gardd-erfin ; and shales in a cart-road 200 yards west of 

 that farm have yielded Dichograptid stipes, a small Diplograptus or 

 Olimacograptus, and a form of the Didymograptus-bifidus type. 

 South of this section they are exposed in a brook, in which, at a 

 point 350 yards north-west of Gelli, they have yielded specimens 

 of Didymograptus of the bifidus-type, Diplograptus dentatus (?) 

 Brongn., and Didymograptus of. stabilis, Elles & Wood, together 

 with lYinucleus, sp. nov. (?), and Ampyx nudus (?) Murch. 



They are faulted against the igneous rocks of the Coomb complex, 

 from the Coomb dingle on the north-east, by Gelli, to Pentre- 

 newydd on the south-west. At Gelli they contain grits, and in a 

 lane 150 j'ards south-west of the farm they have afforded Didy- 

 mograptus bifidus, Hall, and D. artus, Elles & Wood, within a yard 

 or two of the andesites. At Pentre-newydd, some included grits are 

 exposed in the yard by the side of the house. These and the grits 

 at Gelli are probably on the horizon of the chief band, which is to 

 be found some 250 feet above the base of the subdivision in other 

 parts of the district. 



South-west of Pentre-newydd the shales are well-exposed, 

 although somewhat weathered and contorted, in a road-cutting west 

 of the Cross Roads, and have yielded graptolites of the Didy- 

 mograptus-bifidus type and D. Murchisoni (?), together with 

 JEglina sp., Ampyx nudus, Murch., Ampyx sp., Barrandia Cordai, 

 M'Coy, and Barrandia sp. 



Immediately south of this section they are unconformably overlain 

 by the basement-beds of the Old "Red Sandstone, and faulted on the 

 south-west against the Bala Limestone and Dicranograptus-Shales 

 of Llandeilo-Abercowin. 



(3) The Old Red Sandstone. 



The lowest beds of the Old Red Sandstone are a group of sage- 

 green and buff, rocky, calcareous marls and cornstones, with some 

 interbedded conglomerates and micaceous green sandstones. They 

 form a well-marked basement- group to the red marls of the Lower 

 Old Red Sandstone of the district. 



(a) The Main Outcrop. 



At the eastern end of the area they are poorly exposed in Pern- 

 hill Brook at Rhyd-lydan, where they strike towards the Tetra- 

 c/raptus-Beds of Glog, and are presumably separated therefrom by a 

 fault running along the brook. This fault bends westward along 



