Under the head of “Silurian Rocks,” Sir Roderick Murchison included all the strata between the summit of the “Longmynd.” beds and the Old Red Sandstone, and he divided these into the two great groups of the Lower Silurian and Upper Silurian.  It is, however, now generally admitted that a considerable portion of the basement beds of Murchison’s Silurian series must be transferred—–­if only upon palaeontological grounds—­to the Upper Cambrian, as has here been done; and much controversy has been carried on as to the proper nomenclature of the Upper Silurian and of the remaining portion of Murchison’s Lower Silurian.  Thus, some would confine the name “Silurian” exclusively to the Upper Silurian, and would apply the name of “Cambro-Silurian” to the Lower Silurian, or would include all beds of the latter age in the “Cambrian” series of Sedgwick.  It is not necessary to enter into the merits of these conflicting views.  For our present purpose, it is sufficient to recognise that there exist two great groups of rocks between the highest Cambrian beds, as here defined, and the base of the Devonian or Old Red Sandstone.  These two great groups are so closely allied to one another, both physically and palaeontologically, that many authorities have established a third or intermediate group (the “Middle Silurian"), by which a passage is made from one into the other.  This method of procedure involves disadvantages which appear to outweigh its advantages; and the two groups in question are not only generally capable of very distinct stratigraphical separation, but at the same time exhibit, together with the alliances above spoken of, 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):—­