250 ECONOMIC GEOLOGY disturbance, which is not always the epicentre as, the vibra- tions 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 earth- quake 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 un- reliable 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 consider- able damage; (8) overthrows chimneys and cracks walls; (9) characterized as *‘ violent,” destroys buildings; (10) a ** catastrophic earthquake,” complete devastation through- out 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