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.
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