On the Geological Distribution of the Rhabdophora. 423 



XLIX. — On the Geological Distribution of the Rhabdophora. 

 By Charles Lapworth, F.G.S. &c. 



Part II. Data. 



[Continued from p. 341.] 

 Bala Formation. 



In the present state of our knowledge it is impossible to fix 

 upon the physical or palasontological line of demarcation 

 between the Llandeilo and Bala (or Caradoc) formations. 

 The Upper Llandeilo and Lower Bala rocks both consist, in 

 great part, of dark and more or less carbonaceous shales, 

 tolerably prolific in Graptolites. In the south of Scotland, 

 strata of this nature {the Glenkiln Shales), unequivocally 

 superior in systematic position to the generality of the Welsh 

 Llandeilo rocks, afford a Graptolite fauna of a most distinc- 

 tive character, but which, upon the whole, has a facies inter- 

 mediate between that of the typical Llandeilo of Siluria and 

 that discoverable in strata of undoubted Bala age. I have 

 hitherto looked upon this Glenkiln fauna as of Upper Llan- 

 deilo age. Not only, however, have I failed this summer in 

 detecting many of its most characteristic species in the highest 

 Llandeilo rocks of South Wales, but the recent researches of 

 American geologists appear almost to demonstrate that in 

 New York and Canada a similar fauna is characteristic of the 

 shaly strata immediately overlying the Trenton Limestone — 

 in other words, of shales admittedly homotaxeous with the 

 lower beds of the British Bala formation. This also appears 

 to be the systematic place of the same fauna in Ireland and in 

 Scandinavia. Till the lowest boundary of the Bala has been 

 satisfactorily settled by careful research in the typical districts 

 of North Wales, it is perhaps a matter of no great moment 

 whether the fauna in question be considered as of Upper 

 Llandeilo or of Lower Bala age. At present, however, the 

 balance of evidence leans decidedly in the latter direction. 

 It will therefore be more convenient to regard this peculiar 

 fauna, provisionally, as of Lower Bala age. 



Llandeilo- Bala or Lower Bala. 



Scotland. — The Glenkiln or Lower Moffat shales of the 

 south of Scotland, above referred to, yield, both in the typical 

 localities near Moffat and in the district of the Leadhills, the 

 following species : — 



Didymograptus superstes, Lapw. Ocenograptus surcularis, Hall. 

 Oenograptus gracilis, Hall. explanatus, Lapw. 



