Q2 HISTORICAL PALAEONTOLOGY. 
sage is made from one into the other. This method of pro- 
cedure involves RY oS which appear to outweigh its 
advantages ; and the two groups in question are not only gen- 
erally capable of very distinct stratigraphical separation, but at 
the same time exhibit, together with the alliances above spoken 
of, so many and such important paleontological differences, 
that it is best to consider them separately. We shall there- 
fore follow this course in the present instance ; and pending 
the final solution of the controversy as to Cambrian and Silu- 
rian 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 devel- 
oped in various regions ; and though their gewera/ 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 accept- 
ing the line proposed, for paleontological 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 
piste general succession from below upwards (fig. 34):— 
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” 1 
derived from the town of the same name in Wales, where strata 
of this age were described by Murchison. 
