Vol. 56. J SILURIAN SEQUENCE OF RHAYADER. 79 



The chief fossil of this group is Monograptus tenuis, Portl. which 

 has been met with in one band onlj*, but in great abundance. Its 

 associates are Climacograptus normalis and Cl.rectangularis. Other 

 exposures of the shales of this group are to be seen up the little 

 brook previously mentioned, and also in the sides of the lane that 

 runs from Cae-newydd to the .Rhayader Workhouse. The thickness 

 of this division may be estimated at 250 feet. 



Zone of Monograptus cyphus (Ac^). — Succeeding the space 

 allotted to the foregoing division on the line of the typical section, 

 from about 100 yards south to about 50 yards north of Ddol 

 Farm, is an almost continuous section of bright orange-weathered, 

 thickly-bedded, calcareous, hard, grey sandy flags and grey 

 shales, with thin, ferruginous, sandy bands. From the southern 

 extremity of this exposure to the southern end of Ddol Farm the 

 individual flags reach a thickness of about 6 inches, and are 

 intercalated with dark-grey and light-blue flaggy shales. They 

 are distinctly nodular in structure, often micaceous in composition, 

 and, like the shales, are peculiarly striped at frequent intervals 

 with ferruginous sandy and green gritty bands. All the beds 

 are more or less calcareous. Iu places the shales have become 

 thoroughly rotted and disintegrated by the weather, but a compact 

 band of limestone, 2 feet thick, is to be seen in the river-bed in 

 line with the hedge at the south side of Ddol Farm. It is accom- 

 panied by large concretions of carbonate of lime embedded within 

 the shales. 



A strike-fault at the southern end of the exposure cuts out a 

 portion of this group, but the total thickness cannot be less than 

 200 feet. The dip is less than that of the underlying beds, varying 

 from 15° to 20° north-westward. 



Passing up towards the top of this zone occasional carbonaceous 

 bands are seen to occur in the flags. From a few bands in the shales 

 near the summit of the group I have extracted Monograptus cyphus, 

 Lapw., Climacograptus normalis, and CI. rectangular is. By far the 

 commonest fossil is the last-named, which is found throughout the 

 division ; but as Monograptus cyplius is exclusively confined to it, 

 it is preferable to name the zone after this fossil, especially as 

 CI. rectangularis has a considerable range both above and below. 



Zone of Monograptus fimbriatus (Ac 3 ). — A set of lime- 

 stones, 3 or 4 inches thick, may be taken as the basement-band of this 

 zone. A few beds with limestone-concretions overlie this band, and 

 are succeeded by a series of soft, blue-banded, blue-hearted shales, 

 with occasional grey flags. The grey beds of the underlying group 

 are here entirely supplanted by a series of more or less carbon- 

 aceous blue shales, yielding so rich a graptolitic fauna that this 

 zone is marked out as one of especial palseontological interest. 

 These graptolite-shales are generally shivery, and a persistent 

 cleavage cuts everywhere through their bedding-planes at a very 

 acute angle ; the shales consequently take the form of thin sheets 

 with sharp, knife-like edges. The graptolites — which are usually 



