hayes] COAL FIELDS OF THE UNITED STATES. 259 
Areas of lignite-bearing formations are not included in the above 
table. These areas are extensive and their beds of lignite contain a 
vast reserve of valuable fuel, but they are not strictly comparable 
with the higher-grade fuels of the anthracite and bituminous fields. 
There are approximately 56,500 square miles of lignite-bearing forma- 
tions, chiefly Cretaceous, in Montana, the Dakotas, and Wyoming. 
The Tertiary lignite-bearing formations of Alabama, Mississippi, 
Louisiana, Arkansas, and Texas constitute another area of about 
equal extent. The percentage of the areas of coal-bearing formations 
which is probably productive is fairly well known in a few of the 
thoroughly developed fields. In most of the fields, however, the figure 
given is merely an estimate based on incomplete data, while in a few 
the available data are of such a character that an estimate would 
have little if any value. The estimates given are believed to be con- 
servative in eveiy case. It should further be remembered that large 
areas which under present conditions are, for various reasons, classed 
as unproductive, may in the future, under changed conditions, become 
productive. This is the case with those fields in which the coal lies 
too deep to be mined with profit at the present time. 
The true rank of the several coal fields and States in value of the 
available fuel which they contain is not indicated by the table, since 
area of coal-bearing formations and percentage of productive area are 
only two of the factors which determine that value. Other factors 
are the number and thickness of the workable beds of coal, its quality 
as fuel, and the ease with which it can be mined. The data are not 
at present available for bringing these factors into the problem. 
It will be noted that the rank of the States in production is quite 
different from their rank in area of coal-bearing formations. Thus the 
Northern Appalachian field, which ranks third in area, ranks first in 
tonnage and value of product, while the Western Interior field, which 
ranks first in area, is fourth in production. This result is due to sev- 
eral causes, among the most important of which are (1) proximity to 
markets, (2) suitability of the coal to the fuel requirements, (3) rela- 
tive quantity of workable coal per square mile of productive area. 
GEOLOGIC RELATIONS OF THE COAL FIELDS. 
The coal-bearing formations of the United States range in age from 
Carboniferous to Tertiary. The Carboniferous coals are confined to 
the region east of the one hundredth meridian and the Triassic coals 
to the Atlantic coast. Most of the Cretaceous coal fields lie in the 
Rock}^ Mountain region, between the one hundredth and one hundred 
and fifteenth meridians, and the Tertiary coal fields are between the 
one hundred and twentieth meridian and the Pacific coast. During 
the three great coal- forming periods, therefore, the Carboniferous, 
the Cretaceous, and the Tertiary, there has been a successive west- 
ward shifting of the zone within which conditions favorable for the 
