Yol. 65.] SUCCESSION AROUND PLYNLtMON AND PONT EKWID. 485 



and range in a direction south 10° west; the pitch is 12°. As the 

 rocks are laid bare for a considerable distance in the bed of tbe 

 river, tbese pitching folds are beautifully displayed in plan. Ou 

 the axis of the anticline close to the bank of the river a shaly 

 band (F. 11) yielded the following graptolites : — Monograptus 

 ■atavus (v.c), Dimorphograptus erectus, and CUmacograptus scalaris 

 var. normalis ; wbile a few yards away, at about the same horizon, 

 CI. tdrnquisti occurred in abundance. As the river swings round 

 to the east, tbe same rocks are passed over in reverse order, 

 until, about 200 yards below the bend, a band of grit crosses 

 the river, which lies exactly in the line of strike of the highest band 

 mentioned above. Immediately below it I found Cephalograptus (!) 

 ■aeuminatus and a CUmacograptus. Some 18 yards away another 

 band of grit emerges from the northern bank of tbe river, but only 

 reaches mid-stream, being there truncated by a strike-fault of con- 

 siderable throw. The fracture is so clean-cut that it escaped my 

 ■attention for a long time, and the abrupt ending of the grit band 

 puzzled me considerably. A shaly film within an inch or two of 

 tbe fault on the west side afforded many specimens of Mesograpius 

 modestus, together with Cephalograptus (?) aeuminatus. The occur- 

 rence of these species proves that the strata up to the highest 

 grit band belong to the acuminatus-zone of the Eisteddfa Group. 



The first species of Monograptus appears some distance up in 

 the succeeding beds, and is there accompanied by that peculiar 

 genus Dimorphograptus, wliich forms, as it were, a connecting link 

 between the Diplograptidae and the Monograptidse. 



On the east side of the fault are some shales and thin flags which 

 resemble the beds with Monograptus atavus, but the characteristic 

 fossil was not obtained. In a few yards they roll over to the east 

 in an anticline with the usual southerly pitch ; this fold is one of the 

 leading structures of the gorge. 



Immediately east of the crest is a line of small calcareous nodules, 

 which seems to mark a slight change in the physical conditions : for 

 the succeeding strata are somewhat harder and more sandy with 

 a smaller proportion of shales ; while they weather a dark rusty 

 brown, in contrast with the pale orange colours of the underlying 

 beds. This change is also accompanied by a change in the fauna, 

 for the characteristic graptolite of the higher beds is Monograptus 

 rheidolensis, sp. nov. This form can be obtained in considerable 

 numbers at the point numbered F. 12 in the gorge. It is accom- 

 panied by Monograptus attenuatus, Ortliograptus muiabilis (c), 

 Glyptograptus tamariscus, Dimorphograptus confertus cf. var. swan- 

 stoni, CUmacograptus rectangidaris, CI. scalaris var. normalis (c), 

 and CI. tdrnquisti (v.c). 



I propose to include the rocks above the line of small nodules in 

 the zone of M. rheidolensis, which, although less abundant than 

 CI. tdrnquisti, has a smaller vertical range. The part of the gorge 

 where they crop out runs nearly due north and south, but soon swings 

 round to the east again. At the elbow a band of large calcareous 



