The Age of the Hirnant Beds. 409' 



(10) F. Katzer, " Karst unci Karsthydrographie " : Sarajevo, 1908, pp. 1-95.. 



(11) R. Schubert, " Die Kiistenlander Osterreich-Ungarns " : Handb. d. Reg. 



Geologic, Heidelberg, 1914, Bd. v, jjp. 1-51. 



(12) E. Fleury, " Les Lapics des Calcaires au Nord du Tage (Portugal) " : 



Com'm. Serv. Geol. d. Portugal, Lisbon, vol. xii, 1917, pp. 127-274. 



The Age of the Hirnant Beds. 



By Gertrude L. Elles, D.Sc, Fellow of Newnham College,, 



Cambridge. 



rriHE age of the Hirnant Beds lias for a long time been a matter of 

 -^- uncertainty, occurring as they do at the junction of the 

 Ordovician with the Silurian. They were originally included by 

 Sedgwick in his " Upper Bala 'V and in the Memoir of the Geological 

 Survey on North Wales ^ they are grouped with the Bala, and there- 

 fore in a modern classification would be relegated to the Ordovician. 

 In 1879,^ however, Ruddy stated that the grits representing the 

 gritty facies of these beds in the northern part of the area were the 

 same as those coloured Lower Llandovery oii the Geological Survey 

 map, and he then held this view of their age, though later, in 1885,* 

 he placed them at the top of the " Caradoc or Bala Series ". Hughes ^ 

 also regarded the Hirnant Beds as representing the base of the 

 Silurian, though he regarded the Ashgill Shales of Fairy Gill and 

 elsewhere as Ordovician. Marr ^ has also always regarded the Ashgill 

 Shales of Fairy Gill as " Upper Bala ", and has included them in his 

 Ashgillian division at the top of the Ordovician. It is perhaps 

 significant that Salter in his Catalogue '^ includes the fossils from both 

 sets of beds in the " Upper Bala ", but divides this into two : — ■ 



(a) The Hirnant Limestone and Llanfyllin Beds. 



(b) The Llandovery (Lower Llandovery of the Survey). 



Thus, whilst opinion regarding the age of the Hirnant Beds has 

 clearly been divided, there has been a general consensus that the 

 equivalent beds in the Lake District area were of Ordovician age. 



The fauna that these beds contain appears to be a very definite 

 one, though some forms are difficult of identification, in fact, much 

 of the difficulty has centred round the identity of the two species, 

 Orthis hirnantensis M'Coy and Strophomena siluriana Dav. ; some 

 geologists have considered these synonymous, whilst others have 

 regarded them as distinct fossils characteristic of different horizons. 



1 have recently been able to show ^ that the two fossils, whilst 

 possessing certain undoubted resemblances, are nevertheless quite 



^ Quart. Journ. Geol. Soc, vol. i, 1845, p. 6. 



2 Mem. Geol. Surv., vol. iii, 1866 and 1881 (2nd edition). 



3 Quart. Journ. Geol. Soc, vol. xxxv, 1879, p. 200. 



* Proc. Chester Soc. Nat. Hist., No. 3, 1885, p. 113. 

 ^ Proc. Chester Soc. Nat. Hist, pt. iv, 1893, p. 141. 



" Quart. Journ. Geol. Soc, vol. Ixix, 1913, p. 1 ; and Quart. Journ. Geol. Soc, 

 vol. Ixxi, 1916, p. 189. 



'' Cat. of Cambrian and Silurian Fossils, 1873, p. 72. 



* Quart. Journ. Geol. Soc, vol. Ixxviii, 1922, p. 132. 



