528 ME. O. T. JONES ON THE HARTEELL-VALENTIAN [Nov. I909, 



their physical characters, and does not necessarily indicate that two 

 sets of movements were required to produce it. 



In the Rhayader district the existence of lower beds of the 

 Plynlimon Stage is -perhaps indicated by the dark shales 

 ' often intermixed with grits and sandstones, and occasionally with beds of 

 •conglomerate ' x 

 which extend for several miles south and east of Rhayader. 



The conspicuous Cerig Gwynion Grits, which form the base of 

 the Gwastaden Group near Rhayader, are unrepresented in the 

 Ront Erwyd district, where the greatest difficulty was experienced 

 in finding a satisfactory base-line. There is reason to believe that 

 they are of local occurrence even in the neighbourhood of Rhayader, 

 for at the extreme eastern limit of that district 



' where the grits have dwindled down to extreme tenuity, there seems evidence 

 of a passage, upward from these Blue-black Shales to the Gwastaden Group: 

 for .... it is almost an impossibility, in the field, to say where the one group 

 ends and the other begins' (op. cit. p. 96). 



That the absence of the grits in the Pont Erwyd district is not 

 due to overlap within the Pont Erwyd Stage is sufficiently proved 

 by the fact that the lowest zone in that district indicates, if any- 

 thing, a lower horizon than any fossiliferous beds near Rhayader. 



I was unable to separate the zones of Cephalogrctptus (?) acumi- 

 natus and Mesograptus modestus, as has been done in other districts ; 

 the latter occurs throughout the Eisteddfa Group, and is accom- 

 panied everywhere in the upper part by C. (?) acuminatum. It 

 is possible that the modestus zone may be represented in part by 

 the lower portion of my zone of Monograptus atavus, since the 

 characteristic form was only obtained near the top of the zone. 

 Or again, it may be equivalent to the upper portion of my acumi- 

 natus zone, that fossil having perhaps a greater range than in the 

 Rhayader district. 



There is also a slight difference in meaning attached to the zone 

 of Monograptus cyphus in the two districts. At Rhayader it is 

 probably equivalent to the zones of M. rheidolensis and M. cyphus 

 in the Pont Erwyd area. 



The characteristic leptotheca band was by Dr. H. Lapworth 

 ascribed to the M. convolutus zone, whereas in the area here de- 

 scribed I have given my reasons for including it with the M. com- 

 munis zone. However, the band is so clearly a link between the 

 two zones, that it is of little importance which view is adopted. 



The remarkable fact has been established that the great Caban- 

 Conglomerate Group of Rhayader is entirely absent in the Pont- 

 Erwyd region, although the localities in the two districts where 

 the contemporaneous deposits occur are only about 15 miles apart. 

 Eurther, there is a complete and gradual passage, both lithological 

 and palseontological, throughout the Castell Group, the upper part 

 of which is palseontologically equivalent to the Caban Group. The 

 marked unconformity at the base of that group has, therefore, 

 completely disappeared in that distance. The graptolites obtained 

 1 See Quart. Journ. Geol. Soc. vol. lvi (1900) p. 96. 



