74 Geological Society. 



emerge one by one in its centre, till finally, in the widest exposures, 

 we recognize the deepest strata of the Glenkiln shales. 



It was shown, by plans, sections, and descriptions of every expo- 

 sure of consequence within the Moffat district, that precisely similar 

 results are arrived at with respect to the remaining black shale - 

 bands. To the south of Moffatdale, the Moffat beds agree essentially 

 with those of Dobb's Linn ; but to the north the whole formation 

 diminishes in collective thickness, and the highest division gradually 

 loses its fossiliferous black shales. 



These facts place it beyond question that all the carbonaceous and 

 Graptolitiferous shales of the Moffat area are portions of one and the 

 same originally continuous deposit — the Moffat Series, which is now 

 the oldest visible rock-group in the district, being everywhere in- 

 ferior to the prevailing greywackes, through which it invariably 

 rises from below in greatly elongated anticlinal forms. 



In the rigid restriction of distinct groups of fossils to a few feet 

 of the succession, the rocks of the Moffat series resemble the thin- 

 bedded Silurians of Scandinavia and North-eastern America. Prom 

 analogy it may be suspected that they similarly represent an enor- 

 mous period of time. The correctness of this inference is demon- 

 strated by the evidence afforded by the known geological range of 

 their organic remains. The Graptolithina of the Lower or Glenkiln 

 division are those of the highest Llandeilo Flags of Wales, the 

 corresponding Middle Bicranograptus -schists of Sweden, and the 

 Norman's-Kiln shales that underlie the Trenton (Bala) Limestone of 

 New York. The Hartfell species occur in the Bala beds of Conway, 

 &c, the higher Dicranograptus-schiats of Sweden, and the Utica 

 and Lorraine shales that overlie the Trenton Limestone. Those of 

 the Birkhill shales agree almost species for species with the fossils 

 of the Coniston Mudstone of Cumberland, the Kiesel-Schiefer of 

 Thuringia, and the Lobiferous beds of Sweden, which lie at the 

 summit of the Lower Silurians of their respective countries. Hence 

 it may be considered certain that the Glenkiln shales are of highest 

 Llandeilo age, that the Hartfell shales stand in the place of the Bala 

 or Caradoc of Siluria, and that the Birkhill shales correspond to the 

 Lower Llandovery. 



The insignificant thickness of these three formations in the Moffat 

 district is in strict agreement with the well-known north-westerly 

 attenuation of the Lower Silurian rocks in Wales, England, and in 

 Western Europe generally. 



It was pointed out that these results, when carried to their 

 legitimate conclusion, harmonize all the apparently conflicting facts 

 hitherto collected among the Lower Silurians of the south of Scot- 

 land. We have a complete explanation of such difficulties as the 

 remarkable lithological uniformity of the predominating strata, the 

 absence of associated igneous rocks, the peculiar localization of 

 the fossils, their identity along certain lines, and their rapid and 

 peculiar impoverishment along others. We reduce, at a single 

 stroke, the apparently gigantic thickness of the South Scottish 

 Silurians to reasonable limits, and at the same time bring them into 

 perfect harmony with those of Western Europe and America. 



