part 2] THE VALENTIAN SEJUES. 153 



II. Classification of the Graptolitic Facies 

 OF the Valentiais'. 



The relation of the Tarannon rocks to the Birkhill and Gala 

 having thus been established, it remains to consider the systematic 

 position of the Llandovery rocks. On account of the striking 

 difference between the almost exclusively shelly fauna of the 

 Llandovery and the almost exclusively graptolitic fauna of the 

 Birkhill and Gala, a comparison between them is exceedingly 

 difficult. It is only rendered possible, in fact, by the existence of 

 certain developments or facies which are partly shelly and partly 

 graptolitic, and by the rare occurrence of graptolites in the shelly 

 facies of some districts, or of shelly fossils in the graptolitic facies. 



A standard for comparison is afforded by the succession of the 

 various forms of graptolites, which has been worked out in great 

 detail in many areas, notably in the Moffat district (C. Lapworth),! 

 Lake District (J. E. Marr & H. A. Nicholson),^ Conway (G. L. 

 Elles & E. M. K. Wood),^ lihayader (H. Lapworth),^ Plynlimon 

 and Machynlleth (O. T. Jones & W. J, Pugh).^ There is no reason 

 to believe that the subdivision of these rocks has been pushed too 

 far, and when detailed comparisons are made between the grapto- 

 litic developments in different areas, it is possible to identify even 

 smaller subdivisions in districts as wide apart as Scotland or the 

 Lake District and Central Wales. 



A general classification of the graptolitic facies of the Valentian 

 Series may, therefore, be drawn up as follows (the subdivisions of 

 the older-established main groups being slightly rearranged in 

 accordance with the results of more recent investigations) : — 



f ( Upper Gala sub-stage, comprising' the zones of 



Gala Stage | Moiiograptus creiiulatus and M. griestonensis. 

 or Upper ^ Lower Gala sub-stage, including the zones of 

 Valentian. | Monograptris crispus, M. tiin-iculatiis, and Ras- 

 [ trites maximus. 



( Upper Birkhill sub-stage, formed by the zones of 

 Moiiograptus halli and M. ssdgwichi (=spini-' 

 gems). 

 Middle Birkhill sub-stage, with the zones of Moiio- 

 graptus convolutus (including Cephalograptus- 

 cometa sub-zone), M. leptotheca, Mesograptus 

 tnagnus, and Monograptus triaiigvlatus. 

 Lower Birkhill sub-stage, including the zones of 

 Monograptus cyphus, M. aciiiaces { = ' rheldolen- 

 sis'), M. atavus { = ' tenuis'), Mesograptus mo- 

 destus, Cephalograptus ? acnminatus, and Glypto- 

 i 1^ graptus persculptxis. 



1 Q. J. G. S. vol. xxxiv (1878) p. 240. 



2 Ihid. vol. xliv (1888) p. 654. 



'^ Ibid. vol. lii (1896) p. 273 (G. L. Elles & E. M. R. Wood); & vol. Ixv 

 tl909) p. 169 (G. L. Elles). 



4 Ihid. vol. Ivi (1900) p. 67. 



5 Ibid. vol. Ixv (1909) p. 463 (O. T. Jones); & vol. Ixxi (1915-16) p. 343 

 <0. T. Jones & W. J. Pugh). 





Birkhill Stage 

 or Lower ■{ 

 Valentian. 



