In a great
many regions, however, the Silurian deposits are found to have
undergone more or less folding, crumpling, and dislocation,
accompanied by induration and "cleavage" of the finer and softer
sediments; whilst in some regions, as in the Highlands of Scotland,
actual "metamorphism" has taken place. In consequence of the
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.
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