e in
the aggregate from 70 to 207 feet. Some of the lower seams individually
attain an exceptional thickness, that at Lehigh Summit mine containing a
seam, or rather a bed, of 30 feet of good coal.

A remarkable seam of coal has given the town of Pittsburg its name. This
is 8 feet thick at its outcrop near the town, and although its thickness
varies considerably, Professor Rogers estimates that the sheet of coal
measures superficially about 14,000 square miles. What a forest there
must have existed to produce so widespread a bed! Even as it is, it has
at a former epoch suffered great denudation, if certain detached basins
should be considered as indicating its former extent.

The principal seam in the anthracite district of central Pennsylvania,
which extends for about 650 miles along the left bank of the Susquehanna,
is known as the "Mammoth" vein, and is 29-1/2 feet thick at Wilkesbarre,
whilst at other places it attains to, and even exceeds, 60 feet.

On the west of the chain of mountains the foldings become gentler, and
the coal assumes an almost horizontal position. In passing through Ohio
we find a saddle-back ridge or anticline of more ancient strata than the
coal, and in consequence of this, we have a physical boundary placed upon
the coal-fields on each side.

Passing across this older ridge of denuded Silurian and other rocks, we
reach the famous Illinois and Indiana coal-field, whose
coal-measures lie in a broad trough, bounded on the west by the uprising
of the carboniferous limestone of the upper Mississippi. This limestone
formation appears here for the first time, having been absent on the
eastern side of the Ohio anticline. The area of the coal-field is
estimated at 51,000 square miles.

In connection with the coal-fields of the United States, it is
interesting to notice that a wide area in Texas, estimated at 3000 square
miles, produces a large amount of coal annually from strata of the
Liassic age. Another important area of production in eastern Virginia
contains coal referable to the Jurassic age, and is similar in fossil
contents to the Jurassic of Whitby and Brora. The main seam in eastern
Virginia boasts a thickness of from 30 to 40 feet of good coal.

Very serviceable lignites of Cretaceous age are found on the Pacific
slope, to which age those of Vancouver's Island and Saskatchewan River
are referable.

Other coal-fields of less importance are found between Lakes Huron and
Erie, where the