AMERICA THE OLD WORLD. 
17 
sible, and when the atmospheric conditions neces- 
sary to their maintenance were already estab- 
lished. Many of the names given to tlieso 
periods are by no means significant of their char- 
acter, but are merely the result of accident : as, 
for instance, that of Silurian, given by Sir Rod- 
erick Murchison to this set of beds, because he 
first studied them in that part of Wales occu- 
pied by the ancient tribe of the Silures. The 
next period, the Devonian, was for a similar rea- 
son named after the county of Devonshire, in 
England, where it was first investigated. Upon 
this follows the Carboniferous period, with the 
immense deposits of coal from which it derives 
its name. Then comes the Permian period, 
named, again, from local circumstances, the first 
investigation of its deposits haring taken place in 
the province of Permia in Russia. Next in suc- 
cession we have the Triassic period, so called 
from the trio of rocks, the red sandstone, Mus- 
cliel Kalk (shell-limestone), and Keuper (clay), 
most frequently combined in its formations ; the 
Jurassic, so amply illustrated in the chain of the 
Jura, where geologists first found the clue to its 
history ; and the Cretaceous period, to which the 
chalk cliffs of England and all the extensivo 
chalk deposits belong. Upon these follow tho 
so-called Tertiary formations, divided into three 
periods, all of which have received most char- 
