GENERAL PRINCIPLES. 
115 
in its lowest term, the primordial, which crosses the northwest border 
from Tennessee, in a few points along the Smoky Mountains. 
DEVONIAN SYSTEM. 
In the Devonian system, (so called from Devonshire, Eng.), the fossils 
belong largely to higher types ; land plant and fishes being abundant and 
the latter the predominant type of organic life, so that it is known as the 
Ichthyan or Dish Age. Rocks of this series are found in many parts of 
Canada, of New England, New York and Pennsylvania, West Virginia, 
and in various parts ©f the Mississippi Valley, and west as far as Utah 
The series occupies also an important place in the geology of other con- 
tinents. The sandstones of this horizon are the chief source of petroleum. 
The former name of the group is “ Old Red Sandstone.” 
CARBONIFEROUS SYSTEM. 
The distinctive feature of this system is the enormous development of 
its fossil vegetation preserved in the form of coal. The most valuable 
beds of this mineral in Europe and America occur in the formations of 
this age; in Asia and the western side of North America, however, the 
coalbeds are generally of a later date. Limestones abound in these for- 
mations, and the lower terms are largely made up of conglomerates, grits 
and coarse sandstones. The coal measures, which are found mostly in 
the Carboniferous proper, are underlaid very generally by extensive beds 
of these latter rocks. 
The seams of coal, which are sometimes very numerous and of various 
thicknesss, from that of paper to 40 feet and more, are commonly inter- 
stratified with shales, slates and sandstones. 
The coal is simply the result of a chemical transformation of vegetable 
matter, accumulated very much as beds of peat are now seen to form in 
various parts of the world. The extent of these beds, which are fre- 
quently called basins , from the basin or trough-like form into which they 
have been thrown by the folding or bending up of the strata, is often 
very great. The largest in the w'orld is the Alleghany Coal Field, which 
extends from the southern edge of New York, nearly 700 miles, to the 
middle of Alabama, and is estimated to contain an area of more than 
60,000 square miles. The great coal basin of Illinois and Indiana is 
nearly as large, and there are many others of great extent in different 
parts of the continent, amounting in all to about 200,000 square miles, 
while the whole of Europe hardly contains one-tenth of that amount. 
