220 PHYSIOGRAPHY, GEOLOGY AND MINERAL RESOURCES

series of folds which increase in their sharpness toward the east.
Three principal types of coal are distinguished as a result of the fold-
ing. Where the rocks have been but little disturbed the coal con-
tains a high proportion of volatile gases, or less than 70 per cent of
carbon. This coal, known as bituminous coal, is especially valuable
for the manufacture of coke and gas. Where the rocks have been
somewhat more disturbed, as along the northern and eastern edge of
the Alleghany field in Pennsylvania, Maryland and West Virginia,
the coal contains from 70 to 84 per cent of carbon. This is known as
semi-bituminous coal and is superior to any other for generating
steam. Where the coal-bearing slates are much more disturbed, as
in eastern Pennsylvania, the amount of carbon exceeds 84 per cent
and the coal is of a hard, glistening variety, called anthracite. This,
on account of the cleanliness-, is especially fitted for domestic use.

The Maryland coals belong entirely to the first two classes, and the
only kind being worked to any extent is the latter or the semi-bitu-
minous type. This coal basin is of comparatively small size, but con-
stitutes by far the most important of the state's mineral resources.
It is commonly called the " Cumberland basin, " and sometimes also
the "Frostburg" or "George's Creek" basin. It is situated in an
elevated trough to the west of the city of Cumberland between two
parallel ridges, known as Dan's and Savage Mountains, which are less
than 4 miles apart. The most important coal seam in the Cumber-
land basin is known as the " Big Vein " or the " 14-foot Vein, " and
belongs to the Elkgarden formation, which is the uppermost member
of the Carboniferous series. This bed is very limited in extent, but
once covered the entire basin, the continuity of the formation having
been greatly reduced by the natural processes of erosion. Below the
14-foot vein are several other workable seams, viz., the 18-inch vein
of the Fairfax formation, the 4-foot vein and the 3-foot vein of the
Bayard formation, and the 6-foot vein of the Savage formation.
These lower veins have not been as yet fully exploited, although sev-
eral companies are to-day removing more or less coal from them.
These seams become very important farther south in West Virginia.
The Maryland coal is unsurpassed in quality and has been proved by