46 Mr. C. Lapworth on the Geological 



scheme of classification of the Lower Palaeozoic rocks not one 

 is perhaps more important, or more certain eventually to be 

 adopted by geologists in general, than the transference of the 

 theoretical line of demarcation between his Lower and Upper 

 Silurian from the middle of the Lower Llandovery to the 

 base of that formation. The immediate adoption of this 

 new divisional line by those who rely more especially upon 

 marked physical evidence is hardly to be expected. In the 

 typical district of Llandovery Murchison's plan of classifica- 

 tion appears at first sight the only one possible, as there seems 

 to be an uninterrupted sequence from the Llandeilo into the 

 Lower Llandovery. Not only so, but the magnitude of the 

 stratigraphical discordance below the Pentamerus-Qxits and 

 Limestones of the Upper Llandovery, where Murchison draws 

 his line of demarcation, is clear and unequivocal from Wen- 

 lock to Llangadock. 



On the other hand, however, the Lower Llandovery grits, 

 which, in Central Wales, follow immediately upon the dark 

 shales of the Upper Bala, afford unmistakable evidence of 

 important and widespread changes in the physical condition 

 of the sea-bottom at the advent of the Llandovery epoch. 

 Even in the typical district of Llandovery itself this change, 

 according to Messrs. Salter and Aveline, is, probably, marked 

 by an unconform ability. Throughout the basin of the Dee 

 the Lower Llandovery beds, according to the most recent 

 researches of Professor Hughes and others, retain their coarse 

 arenaceous character. The relations of the Bala shales to 

 similar grits at Conway appear to me impossible of interpre- 

 tation except on the hypothesis of an unconform ability or 

 overlap at the base of the latter. The most convincing argu- 

 ment, however, in favour of the proposed change is found in the 

 fact that, if we except the typical district of the higher portion 

 of the valley of the Towey, the most distinct physical and 

 palaeontological break in the strata that lie between the 

 Arenig and the Ludlow is that at the summit of the Bala 

 formation and its extra-British equivalents. In Scotland, for 

 example, the only palaeontological break of any magnitude is 

 that at the base of the equivalent of the Lower Llandovery 

 — the representatives of the Lower Llandovery, Upper 

 Llandovery, and Tarannon graduating imperceptibly the one 

 into the other. In Scandinavia the same rule holds good 

 generally, though there are beds of passage where the Ordo- 

 vician and Silurian forms are for a time commingled. I 

 believe that the same rule obtains in Bohemia and Thuringia; 

 but our present evidence is too defective to enable us to 

 bring forward decisive proofs. In America (Anticosti ex- 



