f, so many and such important palaeontological
differences, that it is best to consider them separately. 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. It consists
of about 4000 feet of slates, shales, and flags, and is divisible
into a lower, middle, and upper division, of which the former
is often regarded as Cambrian under the name of "Upper Tremadoc
Slates."

2. The _Llandeilo Group_.--The thickness of this group varies
from about 4000 to as much as 10,000 feet; but in this latter
case a great amount of the thickness is made up of volcanic ashes
and interbedded traps. The sedimentary beds of this group are
principally slates and flags, the latter occasionally with calcareous
bands; and the whole series can be divided into a lower, middle,
and upper Llandeilo division, of which the last is the most
important. The name of "Llandeilo" is derived from the town of
the same name in Wales, where strata of this age were described
by Murchison.

3. The _Caradoc_ or _Bala Group_.--The alternative names of this
group are also of local origin, and are derived, the one from
Caer Caradoc in Shropshire, the other from Bala in Wal