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.
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