1J:U DE. G. L. ELLES OX THE EALA COrXTET : [vol. IxXYlii, 



SO that the bedding is often quite obliterated on the weathered 

 surface, in marked contrast to the prevalent clear stratification of 

 the beds below the ash. The cleavage throughout the district 

 is commonly north 10° east, and dips a little east of south. In the 

 higher parts of this series some veiy fine-grained harder bands 

 make their appearance : these, when weathered, exhibit a peculiar 

 smoothed surface, and when glaciated may take on a very high 

 polish ; their bedding-planes are commonly about 6 inches apart, 

 and thev are verv easily recoo-nized in the field. They are not of 

 themselves very f ossiliferous ; but, since they resist cleavage to 

 an extent comparable with that of the hard sandy mudstones of 

 the Grlyn-Grower Series, the fossiliferous surfaces of the softer 

 beds in Avhich they are intercalated are often well preserved in a 

 series of dip-slopes and scarps, forming a conspicuous and dis- 

 tinctive feature in the landscape. 



An especially good series of the scarps and slopes is seen all 

 along Allt-Ddu, from which these beds derive their name, though 

 they are also well displayed at Bryn-bedwog, Bryn Porfa, Bryn- 

 yr-aber, and on Cefn-ddwj'-graig. 



In these Allt-Ddu Beds the fauna is both rich and varied, the 

 bedding-surfaces being often croAvded with numerous brachiopods. 

 of which Seterorthis alternaia and the variety reirorsistria,^ 

 PJafi/st7'opliia hiforata, and Dinortliis flahelluhim are the most 

 conspicuous; while among the trilobites a big AscqyJnis of the 

 jjowisi type, Triniicleus gihhifrons, and Ccdymene caractaci are 

 characteristic, though less abundant. Seterorfliis cdfernata and 

 its variety refro'rsist'ria are by far the most numerous fossils 

 found, so that the beds may appropriately be termed the Heter- 

 oo'this alt em at a Beds, a name long ago given to them by 

 Buddy. 



Pont-y-Ceuuant Ash. 



The Pont-y-Ceunant Ash succeeds the Allt-Ddu Beds, but its 

 junction with these mudstones is characteristically very irregular, 

 as may be well seen in the quarry at \ Grarnedd, where the ash 

 has been extensively quarried : it has a thickness of about 25 feet 

 m the northern part of the district, the only part of the area where 

 it is at all really well-developed ; southwards it becomes more 

 completely merged in the Calcareous Ash, until in the neighbour- 

 hood of Bryn-Pig only the lowest part (1 to 2 feet) can be 



^ Thomas Davidson first regarded this fossil as a variety of ff. altevnata, 

 but subseqnently, upon the evidence of specimens from Cerrig-y-druidion now 

 in the Sedgwick Museum, he raised it to the rank of a distinct species 

 (Appendix to ' Silurian Brachiopoda '). These specimens, however, in common 

 with all from the same locality, have suffered severely from the deformation 

 of the rocks in which they lie, and cannot be regarded as normal. Specimens 

 of the same fossil from many other Welsh localities appear to be so very 

 close in general structural details to H. alternata, apart from the more 

 conspicuous bending-back of the surface-ribbing, that I regard Davidson's 

 original diagnosis as the more satisfactory. 



