as this crust consists very largely of stratified layers
of rock, the enormous forces arising from this contraction are
resisted by the solid rock.

"Notwithstanding the apparent irresistible nature of these layers of
rock, they slowly yield to the enormous lateral pressure of
contraction and gradually huge folds are pushed up in long mountain
ranges. Usually this process goes on so slowly and gradually that the
yielding of the rock masses takes place without noticeable jar, but
occasionally a sudden slip occurs under the gigantic forces, and an
earthquake is the result. This slip is usually only a few inches, but
when two continents fall together for only a few inches enormous
energy is developed.

"Such slips usually occur along the line of an old fissure previously
formed, and the depth below the surface of the earth varies from one
to twelve miles. Thus places situated near these old internal fissures
are more likely to experience earthquakes than those farther away. It
is a well known geological fact that the Pacific coast in California
contains several of these fissures and earthquakes are more common
there. The entire western part of the United States has been slowly
rising for many centuries, and the shifting of soil due to erosion and
transportation doubtless contributes to produce these seismic
disturbances.

"Earthquakes are more common than most persons think. Modern
instruments for detecting slight tremors within the earth's crust show
that there is scarcely an hour in the day free from these shocks. In
mountain regions, and especially in the highest and youngest
mountains, erosion is most rapid, and on the sea bottom, along the
margin of the continents sedimentation is greatest. In these regions,
therefore subterranean temperature and pressure changes are most rapid
and earthquakes most frequent.

"A study of earthquakes develop these general facts. The origin is
seldom more than twelve miles below the surface; the size of the
shaken region bears a certain relation to the depth of the origin or
focus, the smaller shaken region indicating a relatively shallow
origin; the energy of the shock is approximately indicated by the area
of the shaken region; the origin is seldom a point, but generally a
line many miles in length; the subterranean stress is not relieved by
a single movement, but rather by a quick succession of movements
causing a series of jars.

"The transmission of an earthquake shock throu