92 HISTORICAL PALiEONTOLOGY. 



sage is made from one into the other. This method of pro- 

 cedure involves disadvantages 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 pal^ontological 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 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 I^ower 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 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 Gtvup. — 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. 



