C. LAPWORTH ON THE MOFFAT SERIES. 341 



foregoing pages, while these fossils are distinct from those in the sur- 

 rounding greywackes. Bearing in mind the rigid vertical restriction 

 of these fossils to definite zones hoth in the Moifat Series and in 

 their foreign equivalents, it is impossible to doubt that each and all 

 of these black bands are due to the repetition of the same deposit of 

 the Moffat Shales. 



(b) Recollecting further the fact of the rapid north-westerly 

 attenuation of the Lower Silurians in Britain and Western Europe, 

 together with that of the gradual disappearance of the highest zones 

 of the Birkhill beds when the latter are followed in a corresponding 

 direction even within the limited area of the Moffat district, we 

 cannot fail to perceive that upon each consecutive repetition the 

 Moffat Series should diminish in collective thickness, and that its 

 fossiliferous zones should disappear one by one from above, as we 

 pass over the Uplands from south-east to north-west. 



I shall show upon a future occasion that this is what does 

 actually take place. In the Dobb's-Linn band, as we have seen, all 

 the zones are present. In the Meggat and Hartfell bands, a few 

 miles to the north-west, the highest zone has disappeared. At 

 Bogrie, yet further in the same direction, a few feet only of the 

 D.-vesiculosus and M.-gregarius bands are all that remain to represent 

 the great Birkhill division of our typical area. On the next reap- 

 pearance of the series to the north of Dairy, the Birkhill division 

 has wholly disappeared, and the PUurograjptus-zo-n.Q of the Lower 

 Hartfell Shale is almost in contact with the greywackes. The same 

 rule holds good to the extreme north-west limit of the Uplands, where 

 scarcely any thing more than a greatly degenerated representative of 

 the Hartfell-Glenkiln division is apparent. 



(c) This furnishes us with a clue to a paradoxical circumstance, 

 hitherto perfectly unaccountable to the palaeontologist, and puzzling 

 alike to those who held the theory of the identity in geological age of 

 the various black bands, and those who preferred rather to consider 

 those to the northward as belonging to a newer formation than 

 the Moffat series — namely, that as we pass from south to north, 

 the fauna of the black bands, instead of remaining comparatively 

 unaltered, or affording evidence of a gradual change into one of a 

 more modern type, undergoes, on the contrary, a rapid and most 

 peculiar impoverishment. In the southern district, though Bala- 

 Llandeilo forms are certainly present, those characteristic of extra- 

 Scottish Llandovery beds are distinctly predominant. In the 

 central areas the Llandovery forms have all disappeared, and the 

 commonest forms are those of Bala age ; while to the extreme north, 

 and therefore in what at first sight appear to be the highest beds, 

 few remain but what are universally admitted to be strictly Llan- 

 deilo species. 



(d) As no fossils of older date than those of the lowest or Glen- 

 kiln division of the Moffat Series have ever been discovered in the 

 Southern Uplands, while the base of that division is nowhere visible *, 

 it may be regarded as almost certain that the strata of the black 



* The Girvan district is»here regarded as a distinct and separate area. 



