482 M.B. 0. I. JONES ON THE HARTFELL-V ALEUTIAN [Xov. I909, 



Graptolites are fairly plentiful, preserved either in full relief in 

 pyrite or in low relief in a greenish-white substance ; the follow- 

 ing were identified (F. 9) : — Ortliograptus ef. truncatus (v.c.), 

 Mesograptus modestus? (r.), Ceplialograptus (?) acuminatus (v.c), 

 Climacog rectus scalaris vars. normalis & miserahilis (?), and CI. 

 medius. 



These beds are followed by hard blue flags and blue and olive- 

 green gritty shales, weathering deeply to a buff colour with brown 

 and yellow stripes. They contain three bands of tough olive-green 

 gritty mudstones, which give rise to small waterfalls. Towards the 

 upper part of the section the surfaces of the rocks are so deeply 

 stained to a bright orange colour that it comes off on the fingers. 

 The fresh rocks were probably calcareous, and some of the beds 

 remind one of rottenstones in their mode of weathering. Grapto- 

 lites are not plentiful, and are for the most part poorly preserved. 

 They are of the same species as those just enumerated ; with, in 

 addition, Mesograptus modestus var. diminutus (c), and a well- 

 preserved Dictyonema sp. : also a species of Orthoceras, which was 

 rather common (F. 10). The section ends about 300 yards south 

 of the gate mentioned above, the stream being crossed hereabouts 

 by an important fault with a downthrow to the south. 



This section illustrates the difficulty of finding a definite base- 

 line to the Eisteddfa Beds. There seems to be a gradual change in 

 the lithological characters of the strata from the crushed-looking 

 sheeny mudstones of the Bryn-glas Group, devoid of fossils, to the 

 smooth, flaggy, red-weathering strata with Dipiograptidae. I have 

 adopted, for want of better, the thin quartzitic band exposed in 

 the stream about 150 yards north of Fuches-gau as the line of 

 separation in this section ; it may correspond to the gritty horizon 

 which was adopted as the base-line in the Afon Tarenig section 

 (see p. 475). 



The group thus limited is characterized by the prevalence 

 of forms of Mesograptus modestus, which appear earlier than 

 Ceplialograptus (?) acuminatus. The flaggy mudstones with Meso- 

 graptus modestus var. pqrvulus and Glyptograptus persculptus, 

 which occur near the base of the group, may well be separated 

 off as a distinct zone, and this may be called for short the 

 persculptus zone. The zone of Cepludograptus (? ) acuminatus 

 (or acuminatus zone) then commences with the red-weathering flags 

 .and shales. 



From Nant Fuches-gau the strata swing round to a westerly 

 strike, and range towards Pont Erwyd. The base runs for some 

 distance parallel with, and north of, the road to Devil's Bridge. 

 South of the road are several exposures of rusty flags and shales, 

 which weather down into small rectangular fragments; in them 

 are some hard thickly-bedded grits and gritty mudstones, which 



