hayes.] COAL FIELDS OF THE UNITED STATES. 263 
coal thus far discovered is a low-grade fuel, and the field is not now 
producing. The Eagle Pass field is much the larger and more iinpor- 
1 taut of the two, and contains some coal of excellent quality. It 
extends from the northern border of Uvalde County about 75 miles 
southwestward to the Rio Grande, and beyond the international 
boundary expands to a broad area in Mexico. The strata have been 
considerably disturbed, and probably only a small proportion of the 
field will prove to be productive. 
Practically all the available information concerning these Texas 
Cretaceous coal fields is contained in a report by T. WaylandVaughan, 
entitled Reconnaissance in the Rio Grande coal fields of Texas: Bull. 
IT. S. Geol. Survey No. 164, 1900. 
Tertiary coalfields. — The Tertiaiy formations in various parts of 
the United States contain a large amount of vegetable organic matter 
which, in many places, forms beds of lignite. In a few places near 
the Pacific coast conditions have been favorable for the conversion of 
this lignite into true coal. The most important deposits of this Ter- 
tiary coal are in Washington. Here the folding of the inclosing 
strata and the intrusion of igneous rocks have converted the lignite 
into coal of fair quality. Similar conditions have prevailed at a few 
points in the extreme western portion of Oregon and in central and 
southern California. The productive fields are all small and have a 
total area of about 1,000 square miles. How much of this is produc- 
tive has not yet been determined, even approximately, except in a 
few of the most thoroughly developed basins. As in the Rocky Moun- 
tain Cretaceous fields, the coal beds show great variability in thick- 
ness and character, and mining is attended by considerable difficulty, 
owing to the disturbed condition of the strata. 
In addition to the coal-bearing Tertiary areas of the Pacific coast, 
large areas of Tertiary formations occur in the southern portion of 
the United States, which contain extensive beds of lignite. These 
Tertiary lignites contain a large amount of fuel which will doubtless 
some time be utilized. Beginning at the Georgia- Alabama line, a nar- 
row belt of lignite-bearing formations extends westward nearly to the 
Mississippi River. West of the Mississippi the same formations 
occupy a much broader belt, extending from Little Rock southwest- 
ward through Arkansas, Louisiana, and Texas. The boundaries of 
these areas are quite indefinite, owing to the presence of later deposits, 
and probably only a small proportion of the area contains lignite beds 
of sufficient thickness and purity to be utilized. 
CLASSIFICATION OF THE COAL AS FUEL. 
The various fuel requirements call for coals of varying composition, 
and the adaptability of any coal to a particular purpose is determined 
largely by the relative abundance of the several fuel constituents. 
