Man's physical universe

xanabras

THE GRADATION OF THE EARTH 143

Slichter, consulting engineer of the United States Geological Survey,

has estimated that the amount of ground water is nearly one-third as

great as that in the oceans.

Generally, the ground waters settle on an impervious layer of rock

and reach a level called the water table, sometimes near the surface

and at other times quite deep. When streams cut below these water

tables, springs appear along the banks. Water seeps into wells when

they are driven below the water table. Sometimes portions of land lie

below the water table, and a swamp or lake results.

Ground waters may move through the soil or rocks very slowly,

sometimes only a few hundred feet per year. These waters frequently

Fig. 38. Thousand Springs, Snake River Canyon, Idaho. Outlet for groundwater

flowing through porous lava. (From Reeds, Chester A.. The Earth,

published by the University Society.)

become charged with carbon dioxide, which gives them the solvent

action necessary to dissolve limestone and form such huge series

caves as the Carlsbad Caverns in New Mexico and the Mammoth

Cave in Kentucky, which extend scores of miles underground.

Sometimes ground waters collect in fissures between rocks and pour

out as veritable rivers.

sunk into ground water that is under pressure. Occasionally the

pressure of the steam formed in

Artesian wells are produced when pipes are

the water to shoot upwards suddenly as a geyser.

of

the lower portions of fissures causes

This action of geysers

may occur regularly, as in the case of Old Faithful in Yellowstone

National Park, or irregularly, as in the case of Eaimangu in New

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