Cee ee 
“ 
‘ 
268 DYNAMICAL GEOLOGY. — [Boox III. _ 
1872 was estimated to have been 2433 feet in a second, that of an earth- 
quake at Travancore in Southern Hindostan 656 feet in a second. 
Duration.— The number of shocks in an earthquake varies 
indefinitely, as well as the length of the intervals between them. 
Sometimes the whole earthquake only lasts a few seconds; thus the 
city of Caracas, with its fine churches and 10,000 of its inhabitants, 
was destroyed in about half-a-minute; Lisbon was overthrown in 
five minutes. But a succession of shocks of varying intensity may 
continue for days, weeks, or months. ‘The Calabrian earthquake 
which began in February, 1783, was continued by repeated shocks for 
nearly four years until the end of 1786. 
Modifying influence of geological structure.—In its 
passage through the solid terrestrial crust from the focus of origin 


the earth-wave must be liable to continual deflections and delays, — 
from the varying geological structure of the rocks. To this cause, 
no doubt, must be ascribed the marked differences in the rate of 
propagation of the same earthquake in different directions. The 
wave of disturbance, as it passes from one kind of rock to another and 
encounters materials of very different elasticity, or, as it meets with 
joints, dislocations, and curvatures in the same rock, must be liable 
to manifold changes alike in rate and in direction of movement. 
Even at the surface one effect of differences of material may be seen 
in the apparently capricious demolition of certain quarters of a city, 
while others are left comparatively scatheless. In such cases it is 
usually found that buildings erected on loose inelastic foundations, 
such as sand and clay, are more liable to destruction than those 
placed upon solid rock. In illustration of this statement the accom- 
panying plan (Fig. 64) of Port Royal, Jamaica, was given by De la 

Fic. 64.—PLAwn or Port Roya, JAMAIOA, SHOWING THE EFFECTS OF THE 
IWARTHQUAKE OF 1692 (B.). 
P ©, Portions of the Town built on Limestone and left standing after the Earthquake; 
a a, 1, the Boundary of the Town prior to the Earthquake; N N, Ground gained 
by the drifting of sand up to the end of last Century ; I LH, Additions from the 
tame cause during the first quarter of the present Century. 
Beche * to show that the portions of the town which did not disappear 
* “Geological Observer,” p, 426. 
