82 MR. H. LAP WORTH ON THE [Feb. I9OO, 



Pale Grey Mudstones (Ad 3 ). — From the termination of the 

 last zone tip to the Rhyd-hir Brook the bed of the Wye is floored 

 by pale bluish-grey mudstones and flags. Some of these are banded 

 with pale green or blue seams, but the whole group is extremely 

 monotonous in its lithological characters, and is practically barren 

 of fossils. ■ Near the summit, however, the flags yield Climacograptus 

 normalis, Leptama, and Orthoceras sp. The total thickness of this 

 division measures rather less than 250 feet. 



Immediately succeeding the above is a great group of pale 

 shales, the Rhayader Pale Shales (C), which are not exposed 

 in the river itself at this point, but may be seen about 50 yards 

 or so up the brook at the. water-wheel of the Tannery. At this 

 exposure graptolites are abundant in certain beds, from which 

 the following fossils have been obtained : — 



Monograptus Sedgwickii, Portl. 



var. distans, Portl. 



involutus, Lapw. 



lobiferus. 



Becki, Barr. 



Monograptus pandus, Lapw. 



crassus, Lapw. 



jacuhm (?) Lapw. 



Climacograptus extremus sp. nov. 

 Peltocaris aptychoides (?) Salt. 



It will be seen that this assemblage, considered as a whole, is 

 very different from the collective fauna of the zones which we have 

 traced up to this point. Only one species, Monograptus lobiferus, 

 in the fauna of these Pale Shales occurs in the beds hitherto 

 described. It is evident, therefore, even at first sight, that we are 

 here dealing with a new rock-group, separated from the Gwastaden 

 Group by an enormous extent of geological time. At this point, 

 therefore, may be drawn the line marking the top of the Gwastaden 

 Group. 



Confining our attention to the typical section described in the 

 preceding paragraphs, we see that here is a collective series of 

 rocks over 1800 feet thick, apparently conformable from base 

 to summit. This group — the Gwastaden Group — is underlain 

 with apparent conformity by a group of Blue-black Shales, and 

 overlain with apparent conformity by a group of Pale Shales. 

 The Gwastaden Group itself includes grits, flags, and shales, 

 the grits being thickest at the base. Prom the base the grits 

 gradually thin out, and are replaced by hard blue-and-grey flags. 

 These pass up in their turn into soft shales, and these again into a 

 series of pale mudstones. 



It is evident, judging from the included fossils, that the various 

 divisions of the Gwastaden Group are closely bound together, for 

 there is no break in the life-succession, but merely a gradual 

 evolution from the base to the summit of the group. In the lowest 

 rocks occur only the genera Climacograptus and Biplograptus. The 

 Monograptidse make their first appearance at about one-third the 

 thickness of the group, measured from the base, and, in company 

 with the Climacograptidse and the Diplograptidae, reach the topmost 



