247 
Distribution of the Rhabdophora. 
deposit, or to be at least of one and the same geologic age, 
were assigned in Scotland to the middle of the Llandeilo 
period, in England to the Caradoc, and in Bohemia to the 
geological horizon of the Mayhill Sandstone. 
This imperfect knowledge arose in the main from two 
special causes—the one geological, the other palgeontological. 
The geological cause had its origin in the physical peculi¬ 
arities of the ocean-bed upon which the fossiliferous and 
typical Lower Palaeozoic rocks of Wales were laid down. 
The strata composing the formations of the Caradoc and 
Llandovery, which occupy the central portion of Murchison’s 
original Silurian system, are, generally speaking, arenaceous, 
and therefore not of a nature to afford Gfraptolites. Nor did 
they appear to be abundant in the succeeding Ludlow and 
Wenlock formations. For many years only three British 
species were known as occurring in the entire series from the 
base of the Caradoc to the summit of the Silurian. On the 
other hand, they were found to be astonishingly abundant in 
Murchison’s lowest formation—the Llandeilo, which consisted 
in great part of argillaceous and more or less carbonaceous 
schists. When the extra-Salopian areas came to be mapped, 
the same rule was found to hold good everywhere—the Llan¬ 
deilo furnishing Rhabdophora in abundance, while in the 
succeeding formations they were either wholly wanting or, at 
most, were excessively rare. 
In the south of Scotland a corresponding physical accident 
led to the erroneous opinion that the Graptolites were as 
strictly Llandeilo forms as they had proved themselves in 
Wales. The prolific Silurians of Girvan, whose Caradoc 
age was demonstrated by their numerous and well-preserved 
Crustacea and Brachiopoda, appeared to repose at once upon 
those vast thicknesses of non-fossiliferous greywackes and 
schists that floor the Southern Uplands, and which certainly 
bear a striking resemblance to the infra-Caradoc rocks of 
Wales and the west of England. These greywackes were 
therefore naturally paralleled with the Llandeilo rocks of 
Murchison’s * Siluria.’ The black carbonaceous bands that are 
frequently met with amongst them were necessarily regarded 
as of the same general geologic age. These black bands, 
which form the well-known Moffat series, are the most prolific 
Graptolite-bearing strata in Britain ; and their unhesitating 
reference to the Llandeilo formation by Murchison was for 
many years regarded by geologists as one of the best-founded 
generalizations in British geology. 
The published results of the simultaneous investigations 
of foreign geologists appeared to point distinctly in the 
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