HAYES.] COAL FIELDS OF THE UNITED STATES. 261 
deeply covered by glacial drift. Prospecting is done entirely by 
means of the drill, and on account of the expense involved the pro- 
portion of the field underlain by workable coal can not at present be 
estimated. The strata appear to dip from the margins of the field 
toward its center, the formations thickening in the same direction. 
It is probable that this field was formed in an isolated basin and that 
its strata have never been continuous with those of the fields to the 
southeast and southwest, in Ohio and Indiana. 
The Eastern Interior field embraces portions of Indiana, Illinois, 
and Kentucky, having an area of 58,000 square miles. It forms an 
oval basin whose longer axis extends northwest and southeast, nearly 
at right angles to the axis of the Appalachian field. The strata about 
the margins of the basin have gentle dips toward its center, while in 
the interior of the basin they are practically horizontal. The work- 
able coal beds are confined to the lower portion of the Coal Measures, 
and hence reach the surface in a broad belt about the margins of the 
basin. The development of the field has been confined to this belt, 
although the coal beds are supposed to extend beneath the unproduc- 
tive formations which occupy the surface in the central portion of the 
field. It is estimated that about 55 per cent of the area is productive 
under present conditions, and that a considerable proportion of the 
remainder will become productive when conditions render mining at 
greater depths profitable. The Eastern Interior field is separated 
from the fields on either side by broad, gentle anticlines, from which 
the Coal Measures, which may originally have been continuous, have 
been removed by erosion. 
The Western Interior and Southwestern fields form a practically 
continuous belt of Coal Measure rocks extending from northern Iowa 
southwestward 880 miles to central Texas, and embrace an area of 
94,000 square miles in Iowa, Missouri, Nebraska, Kansas, Indian 
Territory, Arkansas, and Texas. At the eastern margin of these 
fields the underlying older formations reach the surface, while along 
their western margin the Coal Measures pass beneath the Permian 
and other formations of the plains region. 
In the Western Interior field and the northern portion of the South- 
western in Indian Territory, as well as in the portion lying in Texas, 
the strata are nearly horizontal, having a uniform gentle dip to the 
west. In that portion of the field which lies in Arkansas and extends 
westward through the central part of Indian Territory the strata are 
somewhat sharply folded. This belt forms the northern border of the 
intensely folded and faulted Ouachita Mountain zone, whose structure 
corresponds closely with that of the Appalachians. 
Triassic coal fields. — Several small basins of Triassic rocks in the 
Piedmont region of Virginia and North Carolina are coal bearing. 
They contain an aggregate area of about 1,000 square miles. The 
most important of these, and the only ones at present productive, are 
