SILURIAN SYSTEM. 93 



sight into the physical and vital aspects of this primary 

 period. 



The last of the primary systems which form the subjects 

 of our Sketch is the SILURIAN, so named by Sir Roderick 

 Murchison, because well displayed and first examined by 

 him in that border country between England and Wales 

 which in ancient times was inhabited by the Silures. 

 Eocks of Silurian age have been found in almost every 

 region of the globe in Central and Northern Europe, largely 

 in both Americas, and in Australia and though they differ 

 much in their mineral composition, some being more crys- 

 talline and slaty than others, still on the whole thera is a 

 wonderful similarity among them, both in their lithological 

 and fossil aspects. Occasionally they are so metamorphosed 

 as to be undistinguishable from the crystalline schists of the 

 Cambrian and Laurentian; but, generally speaking, their 

 sedimentary character is abundantly apparent in the numer- 

 ous alternations of sandstones, slaty shales, and limestones, 

 and we see in these strata, with their fossil corals, shells, 

 and Crustacea, the clearest evidence of deep and wide- 

 spreading seas. Altogether the geological record becomes 

 more legible, and we can form some notion of the earth's 

 terraqueous conditions during the long and gradual deposi- 

 tion of the Silurian sediments. We say long and gradual 

 deposition, for in our own islands these strata are from 

 20,000 to 30,000 feet in thickness, embracing numerous 

 alternations of rock-material, and repeated removals and 

 renewals of genera and species of animals. 



In these Silurian strata we perceive limestones formed of 

 coral-reefs and calcareous debris, slates, and slaty shales 

 arising from the deeper sea-muds, and sandstones, grits, and 

 conglomerates composed of the sands and pebbly shingle of 

 the shallower waters. Here and there through the mass we 



