We
shall therefore follow this course in the present instance; and
pending the final solution of the controversy as to Cambrian and
Silurian nomenclature, we shall distinguish these two groups
of strata as the "Lower Silurian" and the "Upper Silurian."
The _Lower Silurian Rocks_ are known already to be developed
in various regions; and though their _general_ succession in
these areas is approximately the same, each area exhibits
peculiarities of its own, whilst the subdivisions of each are
known by special names. All, therefore, that can be attempted
here, is to select two typical areas--such as Wales and North
America and to briefly consider the grouping and divisions of
the Lower Silurian in each.
In Wales, the line between the Cambrian and Lower Silurian is
somewhat ill-defined, and is certainly not marked by any strong
unconformity. There are, however; grounds for accepting the line
proposed, for palaeontological reasons, by Dr Hicks, in accordance
with which the Tremadoc Slates ("Lower Tremadoc" of Salter) become
the highest of the Cambrian deposits of Britain. If we take this
view, the Lower Silurian rocks of Wales and adjoining districts
are found to have the following _general_ succession from below
upwards (fig. 34):--
1. The _Arenig Group_.--This group derives its name from the
Arenig mountains, where it is extensively developed.
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