e rock. The effect of this action is immensely to
increase the amount of detritus which the streams convey to the sea.
After the great Jamaica shock, above noted, the rivers for a while
ceased to flow, their waters being stored in the masses of loose
material. Then for weeks they poured forth torrents of mud and the
_debris_ of vegetation--materials which had to be swept away as the
streams formed new channels.

In all regions where earthquake movements are frequent, and the shock
of considerable violence, the trained observer notes that the surfaces
of bare rock are singularly extensive, the fact being that many of
these areas, where the slope lies at angles of from ten to thirty
degrees, which in an unshaken region would be thickly soil-covered,
are deprived of the coating by the downward movement of the waste
which the disturbances bring about. A familiar example of this action
may be had by watching the workmen engaged in sifting sand, by casting
the material on a sloping grating. The work could not be done but for
an occasional blow applied to the sifter. An arrangement for such a
jarring motion is commonly found in various ore-dressing machines,
where the object is to move fragments of matter over a sloping
surface.

Even where the earth is so level that an earthquake shock does not
cause a sliding motion of the materials, such as above described,
other consequences of the shaking may readily be noted. As the motion
runs through the mass, provided the movement be one of considerable
violence, crevices several feet in width, and sometimes having the
length of miles, are often formed. In most cases these fissures,
opened by one pulsation of the shock, are likely to be closed by the
return movement, which occurs the instant thereafter. The consequences
of this action are often singular, and in cases constitute the most
frightful elements of a shock which the sufferer beholds. In the great
earthquake of 1811, which ravaged the section of the Mississippi
Valley between the mouth of the Ohio and Vicksburg, these crevices
were so numerously formed that the pioneers protected themselves from
the danger of being caught in their jaws by felling trees so that they
lay at right angles to the direction in which the rents extended,
building on these timbers platforms to support their temporary
dwelling places. The records of earthquakes supply many instances in
which people have been caught in these earth fissures, and in a s