356 ELEMENTARY GEOLOGY 
shaking, which may last for several minutes, and after 
this a series of lesser shocks, extending over several 
days, or even weeks, before the earth finally settles 
down to quiet. At times the earth has been in a state 
of tremor for months; on other occasions the shock 
is merely one violent shaking, hardly preceded or fol- 
lowed by minor movements. Even then the horizontal 
and vertical motion of the ground is but slight. 
Fortunately the area of great violence is usually small in 
extent, being confined to a tract not far from the epicentrum ; 
but the jar may be perceptible to people hundreds of miles away, 
and by instruments it may be measured at much greater dis- 
tances. The Charleston earthquake of 1886, with the epicentrum | 
near Charleston, South Carolina, was measured at Louisiana, Mis- 
souri, along the Great Lakes, and in Massachusetts. The earth- 
quake which destroyed Lisbon, Portugal, in the year 1755, was 
felt as far as this country on the one side, and Africa on the other. 
The rate of motion of the earthquake wave is variously estimated 
from two or three hundred feet to several miles a second. 
Effects of Earthquakes. — One of the most striking 
effects of earthquake shocks is that upon life. Loose 
and unstable objects are thrown down, and thus much 
destruction is caused. When the epicentrum is near a 
city, the greatest disaster follows from falling buildings 
(Fig. 218), which after bemg thrown down, often take 
fire. Entire cities have been almost completely de- 
stroyed, and with them a large percentage of the inhab- 
itants. In steeply sloping regions, the shaking of the 
