202 G. L. Elks — Qraptolite Zones in Arenuj Rocks. 



disused, and now much overgrown. For 200 yards north-west 

 of this point a, the beds consist of sandy or slightly micaceous- 

 bluish-black shales with narrow limestone bands, dipping S.E. ait 

 angles of 40°-50'^ ; they are invaded by a few thin dykes of diabase 

 not exceeding one foot in thickness ; between points a-e indicated 

 on the map tliey have yielded numerous specimens of Didymograptus 

 hifidus, Hall, JEglina (Cyclopyge) binodosa, Salt., Placoparia, sp., 

 Caryocaris Wrightii, C. Marri, Hicks, Entomidella Marri, Hicks, sp., 

 Acrotreta ? and Lingula, sp. ; they therefore belong to the zone of 

 D. hijidns. Beds somewhat more slaty in character underlie these 

 to the north-west, and at a point/ due east of the bend in the road 

 above, have yielded a characteristic assemblage of graptolites. 

 Here, however, the fauna has changed its character; the 'tuning- 

 fork ' graptolites are found no more, while the predominant forms 

 are those of the ' extensiform ' or horizontal type, Didymograptus 

 hirundo, Salt., and D. nltidus, Hall, being the characteristic fossils of 

 the beds exposed between the points / and g of the map. At this 

 point g, due south of the Gasworks, the tramway cutting ends, and 

 opposite the Gasworks the section is much obscured by drifts 

 descending to the shore it is, however, soon continued in a north-west 

 direction by a series of tougii calcareous flags with concretions Ii, 

 in which, however, no identifiable fossils were found. These highly 

 calcareous beds are underlain by a series of micaceous sandy flags 

 and shales ^, which extend round the bend of the river; they 

 have yielded numerous specimens of D. hirundo and D. nitidus, 

 a solitary specimen of D. extensiis, Placoparia, sp., and numerous 

 Phyllopods. A little further to the north-west bluish calcareous 

 flags weathering brown are seen to pass under the micaceous beds ; 

 these beds have also yielded D. Idrundo and D. nitidus, but fossils 

 are not abundant, except at k, where in a more shaly band D. hirundo 

 is especially common. From this point to the Boatyard a series 

 of calcareous flags with shaly partings is exposed in continuous 

 downward succession ; they dip slightly more E. than the higher 

 beds, but the inclination is the same in amount. The fauna 

 also changes its character somewhat, for while the beds from k to 

 the old tramway contain D. hirundo as the characteristic fossil, and 

 therefore constitute the zone of D. hirundo, in the beds from k to 

 the Boatyard D. hirundo is rare, and the common graptolite still of 

 the 'extensiform' or horizontal type is D. extensus itself; this fossil 

 is found in abundance at m, and less commonly at / and n ; with 

 it at m are also found D. hirundo (rare), Azygograptus suecicus, Mbg. 

 (one specimen), JEglina [Cyclopyge) binodosa, Salt., and numerous 

 Phyllopods. These flaggy beds must therefore be regarded as 

 belonging to the zone of D. extensus. 



Beyond the Boatyard some calcareous flags occur, which have, 

 however, yielded no organic remains other than Phyllopods; they 

 are seen to be dipping W.N.W. in an opposite direction to those 

 above the Boatyard, and are probably the same beds slightly 

 disturbed by the proximity of the Bangor-Carnarvon fault. There 

 is then a break in the succession, and the next beds, seen at the 



