Vol. 56.] WENLOCK SHALES OF THE WELSH BORDERLAND. 383 



to me to show that there has been some overlap on the part of the 

 beds belonging to the zone of Cyrtograptus Lundgreni, and that 

 this overlap conceals the highest beds of the O.-rigidus zone. This 

 view is certainly borne out by the palaeontological evidence, for the 

 fauna of the beds near Park Farm suggests the beds of Dulas 

 Brook rather than those of Nant Prophwyd. 



In the normal state of the river these lowest beds of the C -Lund- 

 greni zone must be only exposed in vertical section along the bank ; 

 this section reaches a maximum height of 5 feet just below the 

 point at which a small stream comes in on the opposite bank. 



The section is continued again on the left bank, the lowest beds 

 observed being the same as those last seen on the right bank, and 

 consisting of hard calcareous flagstones. These beds are succeeded 

 by a series of soft black shales, which are exposed in vertical section 

 in the bank, and are fully 4 feet thick. They are deeply stained 

 with iron, and much weathered ; the underlying flags are exposed 

 for a considerable distance at the water-level, and the softer beds 

 pass beneath the water near an old oak-tree, about 50 yards below 

 Irfon Bridge. The graptolites of these soft beds are enumerated 

 in Table II, cols. D & E (p. 384). 



The next good exposure is about 20 yards below the bridge. 

 Here the beds consist of hard calcareous shales and flagstones with 

 limestone-concretions ; the strike is the same as before, but the 

 dip has increased to 15°. Graptolites, as before, are abundant in the 

 softer bands, and some of the specimens of Monograptus irfonensis, 

 sp. nov., reach a length of 8^ inches. This fossil is particularly 

 abundant in a band about 15 inches thick, which is marked on the 

 map (fig. 1, p. 381) as 13 x ; other fossils found here are enumerated 

 in Table II, col. F (p. 384) This band is succeeded by 12 feet 

 of hard flags, which are overlain by another graptolite-bearing 

 band, in which M. Ftemingii var. y is the predominant form. 

 This is succeeded by another series of flags, and then immediately 

 below the bridge by still another fossiliferous band, the fossils of 

 which are enumerated in Table II, col. H. This band is followed 

 by another series of flagstones, and the exposure comes to an 

 end. 



There is then a considerable gap in the section, there being no 

 exposures on either bank for some little distance above the bridge. 

 Below the big bend of the river excellent exposures, however, 

 are seen on both banks, and the beds form a high cliff- section on 

 the right bank. They consist of hard calcareous shales with large 

 limestone-concretions ; the strike is still E. 10° N., but the dip 

 has yet further increased, and is now 35°. 



These beds have yielded only two graptolites (see Table II, col. I, 

 p. 384) : Monograptus Flemingii var. I and M. dubius, Orthoceras 

 striato-punctatum var. originate, 0. subundulatum, and Cardiola sp. 

 are also abundant. They are succeeded by a well-marked band 

 of flagstones, in which, after much searching, I discovered two 

 specimens of M. dubius and a head-shield and pygidium of Phacops 

 Musheni. 



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