258 
THE DEVONIAN VOLCANOES 
BOOK V 
reiiKiiiis, emerge from under the escarpments of Carboniferous Limestone, 
and stretch into In-oad uplands until tliey are lost at the top of the Silurian 
system. 
But when the geologist crosses the Bristol Channel to the opposite 
shores of Nortli Devon, he encounters a remarkably different assemblage of 
rocks. It is true that he has not yet been able to detect there any equivalents 
of the uppermost Silurian strata of Glamorganshire, nor does he find any 
comspicuous band of Carboniferous lamestone, sucli as that which encircles 
the Welsh Coal-field. He is thus unable to start from a known definite 
horizon in the attempt to work out the order of succession, either in an 
up^vard or downward direction. Lithological characters likewise afford him 
no means of establishing any satisfactory parallelism. As he follows the 
f levoman strata, however, he finds them to disappear conformably under the 
Culm-measures, wliich, though strangely unlike the Carboniferous strata on 
the opposite coast, are yet proved by their fossils to hehjng to the Carbon- 
iferous system. Hence the Devonian type, like the Old Bed Sandstone, is 
pro\ ed to be immediately anterior to, and to graduiite into, the Carboniferous 
rocks. 
Theie is no stratigraphical change in Britain so rapid and complete as 
that from the Old Bed Sandstone on the one side of the Bristol Channel to 
the Devonian series on the other. Ho satisfactory explanation has yet been 
found for this sudden transformation, whicli still remains one of the 
unsolved jjroblems in British geology. 
As tlie ol)server follows the Devonian assemblage across tlie land to the 
southern coast-line, he is conscious that its general characters, both litho- 
logical and paheontological, depai-t more and more from the type of the Old 
lied Sandstone, and apiproach more closely to the common Devonian facies of 
the Continent. He is forced to admit that the Old Bed Sandstone, not- 
withstanding its extensive development in Britain, must be regarded as an 
exceptional type of sedimentation, while tlie Devonian facies represents 
that which is most widely prevalent, not only in Europe, but generally 
over the world. ^ 
Ihe broad estuary of the Bristol Channel unfortunately conceals from 
view the tract which lies between the typical Old Bed Sandstone of 
Glamorganshire and the typical Devonian formations of Devonshire 
Whether this intervening space of some fifteen miles was occupied by 
a physical barrier, which separated the respective areas of deposit of 
these two types, or the circumstances of sedimentation in the one recdon 
merged insensibly into those of the other, must remain matter for 
speculation. 
The geographical conditions betokened by the Old Bed Sandstone will 
be considered in the next chapter. There can be no doubt that those 
indicated by the Devonian system were marine. The organic remains so 
plentifully distributed through the argillaceous and arenaceous sediments of 
that system, and so crowded together in its limestones, were obviously 
denizens of the open sea. Yet the ordy tract of Britain over which this 
