260 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
accumulation of coal prevailed. Exceptions to this westward pro- 
gression of the coal-forming zone were the deposition of coal east of 
the Carboniferous fields in Triassic time and south of the Carbonifer- 
ous fields during Tertiary time. 
Carboniferous coal fields. — There are five main subdivisions of the 
Carboniferous coal fields. They may be briefly characterized as 
follows : 
The anthracite field is confined to eastern Pennsylvania and con- 
tains 484 square miles of productive area. It consists of several long, 
narrow, synclinal basins, whose axes are approximately parallel, 
extending in a northeast-southwest direction. They do not differ 
materially from the ordinary synclines of the sharply folded Appa- 
lachian belt, except that they are sufficiently deep to have preserved 
the Coal Measures, which have elsewhere throughout this folded belt 
been generally removed by erosion in the synclines as well as upon 
the anticlines. This field has been thoroughly developed, and a larger 
proportion of its coal has been mined than of any of the other fields. 
The Appalachian field, which has been subdivided into northern 
and southern fields, extends from the northern border of Pennsylvania 
southwestward 850 miles to central Alabama. It embraces portions 
of nine States, and contains, approximately, 70,800 square miles, of 
which about 75 per cent contains workable coal. The eastern margin 
of this field forms the western border of the sharply folded Appala- 
chian belt, and along this margin the strata have suffered some fold- 
ing, a few outlying synclines being nearly or quite separated from the 
main field by steep eroded anticlines. In general, however, the strata 
in this field are either gently undulating or essentially horizontal. 
The formations which make up the Coal Measures are generally 
thickest along the eastern margin of the field, thinning rapidly west- 
ward. In the same direction there is a corresponding decrease in 
number and thickness of the coal beds. These formations consist of 
overlapping lenses of conglomerate, sandstone, shale, coal, and occa- 
sionally limestone, none of which can be traced throughout the entire 
field. Some coal beds, as the Pittsburg and Sewanee, may be identi- 
fied over several thousand square miles, but more generally the work- 
able coal is in local thickenings of beds that are elsewhere worthless. 
For this reason correlations of individual beds in distant parts of the 
field are of doubtful value, although particular horizons may often be 
closely correlated by means of the fossil plants they contain. Some 
portions of the field have been carefully prospected, chiefly those hi 
which development has been most active, but large areas, particularly 
in West Virginia and Kentucky, remote from lines of transportation, 
remain practically unknown. 
The Northern Interior field lies wholly within the State of Michigan 
and has an area of approximately 11,000 square miles. It forms an 
oval area Avhose outlines are imperfectly known, since the region is 
