Vol. 65.] HARTEELL-VALENTIAN SUCCESSION AROUND PLYNL1MON. 477 



abundance and in good preservation : — Orthograptus cf. truncatus, 1 

 Olyptograptus cf. sinuatus, Cephalograptus (?) acuminatus, and 

 Climacograptus scalaris var. normalis. 



The succeeding strata are conspicuous by reason of the brilliant 

 colours, ranging from pale yellow to deep rusty-red, which they 

 assume on weathering, whereby they can often be distinguished 

 from a great distance. A few bands of fine-grained grey grit 

 from 4 to 6 inches thick occur at intervals among the flags 

 and shales. The only fossils obtained were graptolites of the 

 species Mesograptus modestus and Climacograptus scalaris var. 

 normalis. The dip is about 40° in the lower part of Nant Nod, 

 but diminishes upwards. The total thickness of strata above tbe 

 gritty bands in the Tarenig, which are provisionally taken as the 

 base of the Pont Erwyd Group, is about 400 feet. Near the ruined 

 buildings of the disused Nant Nod Mine, the beds turn over to an 

 easterly dip ; and in the higher part of the stream the same rocks 

 are exposed in reverse order. 



Near an old level on the west side of the syncline, Cephalo- 

 graptus(l) acuminatus, Climacograptus scalaris var. normalis, 

 CI. meclius, and a form suggesting Mesograptus modestus (F. 3) 

 were obtained in some red-stained cleaved shales with flaggy bands ; 

 and on the tip from an old shaft derived from somewhat lower beds 

 several specimens of a similar form to the last were collected, 

 together with Climacograptus scalaris var. normalis (F. 4). The 

 red-stained strata are underlain by blue flags and mudstones with 

 occasional specimens of Climacograptus sp. ; and these by the 

 unfossiliferous Bryn-glas Mudstones, which are, however, but 

 scantily exposed. About 500 yards to the west is the highest 

 grit-band of the Drosgol Grits, in its natural position below the 

 Mudstone Group. 



Summarily stated, this section proves that a group of flags and 

 shales, with a characteristic mode of weathering and containing a few 

 species of graptolites of the family of Diplograptidse only, overlies the 

 Bryn-glas Mudstones and Drosgol Grits ; the great family of Mono- 

 graptidse is not represented in it. The structure is that of a broad 

 and fairly symmetrical syncline, with an anticline on the east. 



It will be seen from the map (PI. XXIV) that the two limbs of 

 the syncline approach one another towards the north, a fact which 

 indicates that the syncline has a southward pitch. The beds near 

 the nose are exposed by the side of the road, about half-a-mile 

 north of Nant Nod, where they dip at about 18°. 



The geological structure is clearly reflected in the form of the 

 ground. The red-stained flags and shales weather readily into 

 small rectangular prisms, while the underlying mudstones resist 



1 The form referred here, and elsewhere in the Pont Erwyd Stage, to 

 Orthograptus truncatus differs from that species in some important characters ; 

 it is probably a new species. A somewhat similar form was figured as 

 O. truncatus var. abbreviatus by Miss Elles and Miss Wood in Monogr. Pal. 

 Soc. vol. lxi (1907) pi. xxix, fig. 6 e. 



