656 MISS E. M. R. WOOD ON THE [~NoV. I906, 



band above they contain many flaggy layers. They pass downward 

 gradually into 6 feet of purple shaly mudstones; and these again 

 are underlain by 6 feet of greenish mudstones, having a prominent 

 but thin flaggy bed at the base. All these purple and green strata 

 are well exhibited in the high and steep banks on the east side of 

 the river. 



Below this thin band of green occurs 130 feet of soft purple 

 mudstones and shales which form the base of this sub-group (Bd 2 ). 

 As these lowest strata contain but few beds of flags, they are not 

 conspicuous in either bank, although they may be seen in the stream 

 when the water is low. They end at a point 20 to 30 yards below 

 the footbridge opposite the farm of Dolgau. The total thickness 

 of this Middle Sub-group, which, as we have seen, consists of three 

 bands of purple and two of green, must therefore be about 225 feet; 

 it appears to be quite barren of fossils. 



Lower Green Mudstone Sub-group (Bd^. — Below the 

 lowest bed of purple mudstone a third sub-group of strata comes 

 on, consisting of green shales and mudstones intercalated with 

 numerous thin flags, and containing a few black carbonaceous bands 

 at the base. This group has a total thickness of about 95 feet. 

 The five black bands are shown below the footbridge, and occur 

 within a distance of 18 yards. They all yield graptolites, but these 

 are, as a rule, indifferently preserved. Collectively they contain the 

 following species : — 



Monograptus crenulatus (C). I Monograptus griestonensis, Nicol ? 



Monograptus priodon (C). Monograptus nuclns, Lapw. ? 



Monograptus subconicus, Tornq. ? (f. C). Betiolites Geinitzianus (f. C). 

 Monograptus discus, Tornq. (R). 



As these dark graptolite-bearing shales, which have a thickness of 

 about 30 feet, are readily recognized and can be identified else- 

 where, I regard them as forming the base of this Lower Green 

 Mudstone Group. 



We see, therefore, that immediately below the Nant-ysgollon 

 Shales comes a group of strata some 420 feet thick, showing great 

 mineralogical similarity throughout, but varying in colour from 

 purple to green. It is made up of three divisions : — 



Feet. 



Bd 3 . Upper Green Mudstones, unfossiliferous 100 



Bd 2 . Middle Purple Mudstones, with some flaggy beds, unfossili- 

 ferous 225 



Bd r Lower Green Mudstones, with numerous flaggy beds and 



containing graptolitic bands 95 



Hitherto fossils have only been obtained in the lowest beds ; 

 four of the species are common to the Nant-ysgollon Shales 

 above ; but Qyrtograptus Murchisoni, the characteristic fossil of 

 the Wenlock Shales, is absent. The remaining three species, Mono- 

 graptus griestonensis, M. discus, M. subconicus, which are absent 

 from the Nant-ysgollon Shales, are rare forms at this horizon, 

 and are far more characteristic of the beds below. 



