he
above, Silurian districts usually present the bold, rugged, and
picturesque outlines which are characteristic of the older
"Primitive" rocks of the earth's crust in general. In many instances,
we find Silurian strata rising into mountain-chains of great
grandeur and sublimity, exhibiting the utmost diversity of which
rock-scenery is capable, and delighting the artist with endless
changes of valley, lake, and cliff. Such districts are little
suitable for agriculture, though this is often compensated for
by the valuable mineral products contained in the rocks. On the
other hand, when the rocks are tolerably soft and uniform in
their nature, or when few disturbances of the crust of the earth
have taken place, we may find Silurian areas to be covered with
an abundant pasturage or to be heavily timbered.

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 o