250 ECONOMIC GEOLOGY

disturbance, which is not always the epicentre as, the vibrations
 there being vertical, may have little overthrowing
effect. Away from the centre, as the angle of emergence
of the waves become less, the horizontal component in the
vibrations becomes larger and more destructive, until it
gradually loses strength. The meizoseismic area or zone is
where the combination of angle of emergence and strength
of shock has the greatest destructive power.
The determination of the source and cause of an earthquake
 requires a map of its range. Homoseists are easily
drawn where there are many daily rated clocks, as along
railways and telegraph lines. Where time records are unreliable
 isoseists are used, as they are based upon facts and
experiences that can be observed and verified afterwards.
Earthquakes are classified according to intensity by the
ten grades of the Rossi-Forel scale: (1) light tremors recorded
only by seismographs; (2) noticed only by a few people at
rest ; (3) felt by most people who are awake but lying down ;
(4) felt by people in motion, light objects moved, and plaster
ceilings cracked ; (5) felt universally, furniture moved, and
light mechanically swung bells rung; (6) awakens sleepers,
rings most bells, stops clocks, swings chandeliers, and shakes
trees; (7) a * strong earthquake,” overthrows objects, rings
heavy church bells, causes general alarm, and does considerable
 damage; (8) overthrows chimneys and cracks walls;
(9) characterized as *‘ violent,” destroys buildings; (10) a
** catastrophic earthquake,” complete devastation throughout
 a large area.
Depth or Oricin—Hypotheses as to the structure of the
earth’s crust led to the view that earthquakes are of shallow
origin, and arise at a depth of generally not more than
5 or 6 miles, and probably never more than 10 miles. This
conclusion is improbable as a shallow movement would not
be likely to devastate an area hundreds of miles in diameter.
The Charleston Earthquake of 1886 was of extreme severity
throughout a region 1500 miles across, where any widespread
horizontal earth-movement is improbable ; its range appears
inexplicable if its origin was at the depth assigned of only
10 miles. In 1917 the late G. W. Walker, from the angles
of emergence of world-shaking earthquakes at Pulkova,
concluded that they start from depths down to 800 miles