"Vol. 56.] SILURIAN SEQUENCE OP RHAYADER. 81 



nodular bluish-grey and greenish-grey mudstones and flags, charac- 

 terized by the occurrence of nodules of carbonate of lime. All soft 

 shales have disappeared. 



The calcareous nodules reach a maximum diameter of about 

 12 inches in the basement-beds, but throughout the remainder of 

 the division they average only from 3 to 4 inches in diameter. As 

 a rule the carbonate of lime has been dissolved, leaving a small 

 nodule or ball of dark brown or black, powdery, ferro-manganese 

 oxide. In the river-bed itself the nodules have been washed away 

 by the river-waters, but a few may be seen standing out above 

 the average river-level in marked contrast against the pale grey 

 mudstones. Another peculiarity of the group is the presence of 

 curious blue and black lenticular markings on the fractured surface 

 of the mudstones and flags. These occur in nearly all the beds 

 of the river- exposure, up to the point where the Birmingham 

 Aqueduct crosses the Wye. As we approach this crossing the flags 

 become somewhat sandy, and assume a curious speckled appearance. 

 Thin hard blue bands are not uncommon in the group. Fossils 

 are rare ; but immediately above the overflow -outlet occur Diplo- 

 graptus magnus, Qlimacograptus rectangularis, 01. undulatus, and 

 Orihoceras sp. 



A very rich graptolitic fauna was detected in a few black flags,, 

 which were exposed in the river-trench for the Birmingham 

 water-pipes. These black beds cannot be seen in the river-course 

 itself, being covered t up by alluvium. The graptolites extracted 

 were in exquisite preservation, and consisted of the following 

 species : — 



Monograptus attenuatus. 



leptotheca. 



gregarius. 



lobiferus, M'Coy. 



Nicoli, Harkn. 



Monograptus crenularis. 

 Rastrites peregrinus, Barr. 

 Petalograptus palmeus, Barr. 

 Biplograptus tamariscus (?). 

 Climacograptus undulatus. 



Monograptus leptotheca is by far the commonest fossil in this 

 band ; the examples collected are perhaps the best preserved fossils 

 in the district. • 



The exposure extends for a short distance above the Aqueduct- 

 crossing, and brings the total thickness of the group to at least 

 140 feet. The dip is about 26° north-westward. 



Zone of Monograptus convolutus (Ad 2 ). — Eor the next 

 100 yards or so (equivalent to a vertical thickness of about 120 feet) 

 there is a break in the continuity of the river-section, and only a 

 part (a few feet) of this zone is exhibited. This portion lies in the 

 floor of the river-course, and the rock can be seen only when the 

 river is low. Prom such hand-samples I have extracted a single 

 specimen of Monograptus convolutus, occurring in a slightly nodular 

 pale bluish-grey mudstone with green ferruginous bandings. 



Q. J. G. S. No. 221. g 



