The Age of the Hirnant Beds. 413 



ataviis, the lowest of the three subzones into which the Orthograptus 

 vesiculosiis zone can now be divided. All these beds are well seen 

 at the top of tlie Pass ; they are then succeeded on the southern 

 slope by definitely banded G-raptolitic Shales, which are not very 

 fossiliferous at first, but contain more graptolites to the south, in 

 the neighbourhood of Blaen y Pennant, where all the main Birkhill 

 horizons can be recognized ; these pass up in turn into the still 

 more conspicuously banded shales containing the Gala-Tarannon 

 (Upper Valentian) fauna Monogmptus discus Tqt., M. crispus 

 (Barr.), and M. niarri Perner. 



There would therefore seem to be little room for doubt that 

 the Hirnant Beds must be regarded as being of Lower Valentian 

 age, their peculiar character in the typical locality and the com- 

 parative rarity of this fauna in general being possibly accounted for 

 by the fact that the beds are essentially of a transitional nature 

 between the typical shallow- water sediments of the north and the 

 deeper-water deposits of the south. 



These facts also agree well with those noted at Conway,^ where the 

 Conway Castle Grit, which seems to contain this Hirnant fauna, 

 rests upon beds with Phacops imicronatus, and is itself overlain by 

 Graptolite Shales, with a Lower Birkhill fauna. 



If, as seems to be the case, the Ashgill Shales of the Lake District 

 also contain this " Hirnant " fauna, it would appear as if the upper 

 limit of the Ashgill ian should be drawn beneath them, and in this 

 connexion it is worthy of notice that the '' Dimorphograptus " 

 beds, referred to as constituting part of the basal series of the Silurian 

 in the areas where the Ashgill Shales are described contained a fauna 

 characteristic of an horizon still higher within the Valentian than that 

 of the beds seen at Bwlch y groes. 



If the zone of Orthogmjitus vesiculosiis be subdivided into three, 

 as has been done by Jones, ^ viz. :— 



j Sub-zone, Monog. cijphus = 

 '■' Dimorphograplus Zone ", of 

 Lake District. 

 Sub-zone, Monog. acinaces. 

 Sub-zone, Monog. atavus = 

 Hirnant Beds. 



the difference in horizon is clear. The assemblage of graptolites 

 in the Dimorphograptus zone of the Lake District is that characteristic 

 of the sub-zone of Monog. ci/phus, so that in a shallow-water facies 

 there must be a considerable thickness of beds below to be included 

 in the Valentian as at present understood. If the base of the 

 Silurian be taken as suggested above, at the base of the Hirnant 

 Beds or their equivalents, the horizon is easily recognizable from a 

 palaeontological standpoint either by the advent of the '' Hirnant " 



1 Elles, Quart. .Journ. Geol. Soc, vol. Ixv, 1909, p. 169. 



- Jones, " Gothlandian or Silurian (Great Britain) " : Handbuch der 

 Regionalen Geologic, Bd. iii, i, 1917, p. 85. 



Zone 9. Orthograptus vesiculosus 



