490 ME. 0. T. JONES ON THE HARTFELL-VAEENTIAN [NOV. I909,. 



wherever the band was examined. As will be shown in the sequel, 

 there is reason to believe that this peculiar rock has a wide dis- 

 tribution outside the district here described. The beds between the 

 calcareous nodules at the base and at the top of the dark band (8) are 

 characterized by forms which may be grouped around Monograptus 

 communis, and may therefore be referred to the zone of that graptolite. 

 The rocks of this part of the Bheidol section may, then, be summarized 

 as in the following table : — 



Thickness in feet inches*. 

 ("Cast ell Group. Pale mudstones. 



f f Leptotheca band. ~") 



Zone of 



R lie idol Group. < 



Magnus band. 



Monograptus ■{ f- 63 



I Trittiit/ulatits band. 

 communis. a 



{JTriangulatus-v&x. band.J 



I Zone of Monograptus cyphus 40 



Zone of Monograptus rheidolensis 160 



l^Zone of Monograptus atavus 150 



v;„t-^AAt-. r',.^„„ (Zone of Cephaloaraptvs (?) acumi- 

 1 Jiii stud uri Group. < . r a . r -, ) y en a 



*- I natus; seen to about 50 



It will be seen from the table that in this section there is a 

 thickness of over 400 feet of beds between the Eisteddfa Group 

 and the base of the pale mudstones which are assigned to a higher- 

 group. They are characterized by a large number of species of" 

 Monograptus and Iiastrites, and by some well-marked forms of 

 Diplograptidas. I propose to denote the group as the Eheidol 

 Group. 



In accordance with the anticlinal arrangement revealed by the 

 lower beds in the gorge, the black shales of the M.-communis zone 

 swing up the hillside along the base of the precipitous crags over- 

 looking the river opposite Bryn-chwith Farm, where the strong 

 jointing of the overlying M.-convolutus beds is remarkably con- 

 spicuous. The highest beds just reach the top of the hill, and 

 then immediately meeting the axis of the anticline roll over to the 

 west. Along the hilltop the course of the anticline which brings 

 these soft strata to the surface is indicated by a narrow depression, 

 bounded by rocky ridges of pale mudstones. In about a quarter of 

 a mile the outcrop of the black shales intersects another of the 

 great meanders of the Eheidol, 300 yards north of the Barson's. 

 Bridge. Eor nearly half a mile above this point the direction 

 of the river is determined by an important fault which locally 

 carries lead-ore ; the fracture can be seen in several places in the 

 bed and banks of the river, and also in a gully on the west which 

 follows the line of weakness. The black shales on the north of 

 the fault are thrown against pale gritty mudstones on the south,. 



