92 HISTORICAL PALAEONTOLOGY. 



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 

 contained 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 Lower 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 palseontological 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 

 recognize 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 palae- 

 ontologically, that many authorities have established a third 

 or intermediate group (the "Middle Silurian"), by which a pas- 

 sage is made from one into the other. This method of pro- 

 cedure involves disadvantages which appear to outweigh its 

 advantages; and the two groups in question are not only gen- 

 erally capable of very distinct stratigraphical separation, but at 

 the same time exhibit, together with the alliances above spoken 



