THE LOWER SILURIAN PERIOD. QI 
parts of North America, similar strata, now consolidated into 
shales, sandstones, and limestones, may be found resting with 
a very slight inclination on still older sediments. 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 char- 
acteristic 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 de- 
lighting 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 con- 
tained in the rocks. On the other hand, when the rocks are 
tolerably soft and uniform in their nature, or when few disturb- 
ances 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 Murchi- 
son included all the strata between the summit of the ‘‘ Long- 
mynd” beds and the Old Red Sandstone, and he divided these 
into the two great groups of the Zower Silurian and Upper Silu- 
rian. 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 palzeontological grounds—to 
the Upper Cambrian, as has here been done; and much contro- 
versy 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 “ Silu- 
rian” 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 pale- 
ontologically, that many authorities have established a third 
or intermediate group (the “ Middle Silurian”), by which a pas- 
