Man's physical universe

xanabras

MANY VALUABLE MINERALS 189

Peatlike deposits are then changed into brown Hgnite by moderate

pressures resulting from layers of sedimentary rock formed over them.

In this process water, marsh gas, oxides of carbon, and other gases

escape as the plant compounds are decomposed. Lignite, in turn, loses

gases as it is subjected to greater pressure to form bituminous (soft)

coal. Soft coal is further decomposed as it is subjected to greater

pressure, so that the resulting product is nearly pure, hard, glassy

carbon called "anthracite," or hard coal. Some deposits of anthracite

have been so compressed and heated that they have been changed

into an incombustible form of carbon which is called "graphite."

The large coal beds were formed about 250 to 300 million years ago

from much more luxuriant vegetation than is found in our present

swamps and peat bogs. This vegetation, consisting of huge growths

of ferns, horsetails, mosses, and other plants now extinct, must have

grown in warm swampy regions. It is possible that the carbon dioxide

content of the air was much higher when these plants were growing

than it is today.

Coal deposits have been found widely scattered, but the most

valuable ones are located in North America. No doubt this important

natural resource has greatly contributed to the present advanced position

of the United States.

Coal deposits representing all of the different

stages of their formation are found in the United States. Extensive

coal deposits in the eastern Ohio and western Pennsylvania fields are

bituminous in nature, but farther east the folding of the mountains as

they were lifted up changed these deposits to the finest anthracite

coal in the world. Still farther east in the Adirondack Mountains, so

great were the forces brought to bear that graphite was formed from

similar coal deposits of a different age.

It is not unlikely that coal beds are being formed even now on a small

scale in such places as the Great Dismal Swamp, covering fifteen

hundred miles in the southern part of the United States, and in the

extensive swamp lands near the equator. It is estimated that it would

take about four hundred years or more to produce a layer of coal one

foot thick. Some of the individual coal layers in the Mammoth Bed

of Pennsylvania arc from fifty to sixty feet thick.

Petroleum, Natural Gas, and Asphalt Were Produced from Marine

Organisms.

Petroleum was used by the ancient Babylonians in their buildings,

the Romans used it in their lamps, and the Egyptians embalmed their

dead with it.

Inasmuch as practically all

petroleum deposits have been found in

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