480 Prof. Sedgwick on the May Hill Sandstone, 



From the above statements the following conclusions seem 

 naturally to follow : — 



1st. The Pentamerus beds (or Norbury limestone, No. 4) must 

 be arranged with the shales (No. 5. fig. 2), as their fossils belong 

 to a group which is entirely distinct from that of the Builth flag. 

 The case is exactly like that at Norbury and in the neighbour- 

 hood of Wenlock, where the same beds were cut off from the 

 Caradoc sandstone, and arranged by Professor M'Coy and myself 

 as the base of the upper or true Silurian series. Here (just as 

 in Bohemia) there is, after the intervention of a bed of trap, 

 an immediate change in the fossil fauna — the upper being Silu- 

 nan, while the lower is Cambrian : and a similar remark applies, 

 we believe, to one of Professor Phillips's Malvern sections, which 

 is based on the HoUybush shales. 



2ndly. Spite of the almost mathematical parallelism in the 

 bedding of the preceding groups, there can be little doubt but a 

 great geological interval exists between the true Builth flags 

 (No. 3) and the overlying beds (Nos. 4 and 5). The section is 

 not perfect and continuous, and cannot (on that account) give 

 us a guidance to a true nomenclature of the groups. The case 

 is analogous to that already quoted from the section south of 

 Llangollen {supra, fig. 3. p. 306). 



Ill, Conglomerates, slates and sandstones of Dol Fan, ^c. 



Under this title is included a large and ill-defined group of 

 slates and sandstones, sometimes passing into a coarse conglo- 

 merate. This group, commencing near the southern end of 

 Radnor Forest, ranges through Dol Fan towards the south-west ; 

 generally contorted and forming saddles with sides of high incli- 

 nation ; the conglomerates not continuous, but breaking off" and 

 reappearing ; first forming a part of the mountain barrier to the 

 comparatively low country of Wenlock shale which runs to the SW. 

 of Builth ; then thrown further back among the western moun- 

 tains ; and at length forming a kind of high rugged plateau near 

 the water-shed of the Cothi and the Towy. That this singular 

 and ill-defined group has a high geological place among the un- 

 dulating masses between Teifi and the Towy can admit of no 

 doubt; and I think it certain (as I have done since 1832) that 

 these undulating masses (as well as many others of a very diff"er- 

 ent type, fm-ther west towards Cardigan Bay) are superior to the 

 Bala limestone. 



Of what date then are the Dol Fan conglomerates and the 

 associated beds of grit and slate ? The answer can only be sup- 

 plied by the fossils. In my old collection, made in 1846, 1 have 

 but one species from the neighbourhood of Dol Fan, viz. Euom- 

 phalus tricinctus. 



During the year 1846 I made, however, another traverse 



