
210 |. DYNAMICAL GEOLOGY. — [Boox II. _ 
erust in which at a an earthquake arises. The wave of shock will — 
travel outwards in successive spherical shells. At the pointe it will — 
be felt as a vertical movement, and loose objects, such as paving-stones, 


































































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Fic. 65.—WALL SHATTERED BY AN HARTHQUAKE, OF WHICH THE PATH oF EMERGENCE 
HAS BEEN IN THE DIRECTION SHOWN BY THE ARROW. (AFTER MALLET.) 
may be jerked up into the air, and descend bottom uppermost on 
their previous sites. At d, however, the wave will emerge at a 
lower angle, and will give rise to an undulation of the ground; 
and the oscillation of objects projecting above the surface. In rent 
buildings the fissures will be on the whole perpendicular to the 
path of emergence. By a series of observations made at different 
points, as at g and f, a number of angles are obtained, and the point 
where the various lines cut the vertical (a) will mark the area of 

Fic. 66.—EstTimaTion or Source or Earraquake Movements. 
origin of the shock. By this means Mallet determined that the depth 
at which the impulse of the Calabrian earthquake of 1857 was given 
was about five miles. As the general result of his enquiries he con- 
cludes that, on the whole, the origin of earthquakes must be sought in 
comparatively superficial parts of the erust, probably never exceeding 
a depth of 80 geographical miles. Von Seebach calculated that the 
earthquake which affected Central Europe in 1872 originated ata - 
depth of 9°6 geographical miles; that of Belluno in the same year, 
was estimated by Hofer to have had its source rather more than 4 
