160 GEOLOGY. 
and rain-showers fell over the land, and left there the sandy bays pelted 
with their drops ; and forests of sea-weed waved in the green waters and 
on the rocky reaches; and shells adorned the rocks. Into the seas 
flowed great rivers, whose banks were fringed with reeds and flags ; ferns 
waved on the hill-side, tree-ferns reared aloft their feathery plumes, and 
broad-leaved plants clothed the surface of the landscape; while large 
reptiles roamed through the forests, or crushed the reeds by the river- 
sides. 
ViI.—Carboniferous System. 
Description—Above the Devonian rocks lies a series of strata perhaps 
more generally known than any other, as they afford us what is so neces- 
sary to our comfort, the remarkable combustible stone called coal. They 
receive the name Carboniferous from the fact that they contain coal, although 
they furnish many other important products. These rocks are found in 
most regions of the globe; but in none are they more fully developed, 
compared with the size of the country, than in the British Isles. They 
consist of sandstones, limestones, shales, clays, ironstone, and coal. The 
sandstone is of various qualities and colours, some of it very valuable 
and durable ; the beautiful stone of which the New Town of Edinburgh 
is built being from this system. The limestones are largely developed, 
and are of the greatest service for building and agriculture. The shales 
have of late become very valuable, as from them are distilled oils and 
other substances, including the celebrated paraffin oil and candles. The 
ironstone is of the very greatest value. 
It must not be thought that coal is found only in the Carboniferous 
rocks. Coal being simply compressed vegetable matter, may be found in 
any rock-system in which plants are preserved, and is so found in other 
systems, and often in great abundance. For example, the coal-fields of 
Virginia, some thirty or forty feet thick, belong to another system, the 
Oolitic ; and coal of various kinds can be obtained, more or less, from 
most systems, 
The same is true of other products, such as limestone, sandstone, and 
iron, which last, though found in greatest abundance in this system, yet 
occurs in many others. 
The Coal-strata are divided ‘into three great groups—the Upper and 
Lower Measures, and a thick deposit of limestone, which separates them, 
known as the Mountain or Carboniferous Limestone. The Upper Coal- 
measures are also called the True Coal-measures, as they contain the 
greatest amount of workable coal; the Lower Coal-measures consist chiefly 
of sandstones and shales. The Mountain Limestone is so called because 
where it is most largely developed, as in Yorkshire, it rises into hills 
with great limestone cliffs. 
ee We, eh ‘onigy te 
