474 ME. 0. T. JONES ON THE HARTFELL-YALENTIAN [N~0V. I9O9, 



southwards in the direction of the axis, one passes over the strata 

 which succeed the grits. The best line to take is the one from 

 the anticline at the head of the Peithnant valley, to Craig Tspio 

 about three-quarters of a mile away to the south (fig. 2) : where the 



Pig, 2. — Section along the crest of the anticline, from Drum Peithnant 

 to Craig Tspio, on the scale of 2 inches to the mile. (Section 

 No. II on the Map, PI. XXIV.) 



s.s.w. nr.IT.: 



„ . ,, . _ ... . Drum 



Craig Yspio Peithnant Peithnant: 



[1 = Conglomerate and grit, half a mile east of the cairn. 

 2 = Upper beds of the Drosgol Group (equivalent to the grit-beds on the 



summit of Plynlimon). 3 = Bryn-glas Mudstones (Ac). 



4 = Lower beds of the Eistecldfa Group (Ba).~\ 



ground falls abruptly towards the main road, and the anticline is 

 again seen. On the higher ground there are only scattered 

 exposures of blue-black mudstones, breaking up into phacoidal 

 pieces with shiny surfaces ; beds of grit appear to be absent. 



The rocks exposed in the gullies furrowing the steep slope under 

 Craig Yspio are higher than the mudstones, and consist of blue 

 flags and shales with well-marked parallel cleavage. Their most 

 striking character, however, is the brilliant red-orange colour of 

 their weathered surfaces. This section confirms, therefore, that 

 seen on Drosgol, where, as will be remembered, a great mass of 

 mudstones with peculiar and easily recognized characters intervened 

 between the grit group and the red-weathering shales and flags of 

 Nant-y-Barracks. 



As the pitch of the anticline lies between 12° and 15°, the- 

 thickness of this mudstone group works out at about 900 feet. 



Detailed mapping has shown that the mudstones occupy this 

 position throughout the district. They form the whole of the 

 rather prominent hill of Bryn-glas, which lies between Pont Erwyd 

 and Dyffryn Castell, from which circumstance I have applied to the 

 group the name of Bryn-glas Mudstones. 



The time at my disposal did not allow me to trace, in detail, the 

 higher beds of the Drosgol Grits from Nant Hirnant to Drosgol, so 

 I have somewhat generalized their outcrop from the scattered 

 observations which I was able to make in that region. If traced 

 in detail, it would probably be found to pursue a zigzag course, as 

 it does to the south of Plynlimon. 



