enings; but when
they have reached the height mentioned, the passages up through
their centres, corresponding in each case to the throat of a real
volcano, become clogged and new holes are formed in the mud at
the base.

[Illustration: FIG. 32.--AN EXTINCT MUD VOLCANO

With small active one at its side]

Many of these mud volcanoes closely resemble true volcanoes in form
and structure. The mud which pours out at the top forms streams
down the slopes very like those of molten lava. New cones are built
upon the sides or at the bases of the old ones in much the same
way as are those in the volcanic regions.

There are no signs of volcanic action in the vicinity of these mud
springs, and it is likely that the water is forced to the surface
by large quantities of gas produced by chemical changes taking
place deep within the clay beds of the old lake. Similar springs
occur farther south, nearer the mouth of the Colorado River, in
the Yellowstone Park, and near Lassen Peak, but nowhere in America
except in the Colorado desert have they formed such large and
interesting mounds.

THE HISTORY OF A COAST LINE

The story of our Pacific coast reads more like a tale from the
"Arabian Nights" than like a plain statement of events which have
actually happened.

The meeting place of the land and ocean is not really so permanent
a line as it appears. The shore has been continually moving back
and forth throughout the long history of the earth. That which was
dry land at one time was at another time deeply buried beneath the
ocean. The Pacific border seems never to have been at rest. It has
risen and sunk again repeatedly. It has been squeezed, folded, and
broken, shaken by earthquakes, and disturbed by volcanic eruptions.

One might be led to think from this statement that it would not
be safe to live on the Pacific coast, and that both animals and
men would shun the region. The fact is, however, that these changes
usually come to pass so very slowly that we are not aware of them.
Severe earthquakes and volcanic disturbances take place so rarely
in comparison with the length of a man's life, that we may pass our
whole lives without experiencing any of these violent disturbances.
The Pacific coast region, with its forest-covered mountains, fertile
valleys, and beautiful homes, presents so quiet and peaceful an
appearance that it is difficult to believe that parts of its history
have been so tumultuous.

Perhaps you wil