1(38 
EARTHQUAKES. 
is generally not in the ratio of the force of the shocks. 
At Cumana it constantly precedes them, while at Quito, and 
recently at Caracas, and in the West India Islands, a noise 
like the discharge of a battery was heard a long time 
after the shocks had ceased. A third kind of phenomenon, 
the most remarkable of the whole, is the rolling of those 
subterranean thunders, which last several months, without 
being accompanied by the, least oscillatory motion of the 
ground.* 
In every country subject to earthquakes, the point at which, 
probably owing to a particular disposition of the stony strata, 
the effects are most sensibly felt, is considered as tlie cause 
and the focus of the shocks. Thus, at Cumana, the hill of the 
castle of San Antonio, and particularly the eminence on which 
stands the convent of St. Francis, are believed to contain an 
enormous quantity of sulphur and other inflammable mat- 
ter. We forget that the rapidity with which the undula- 
tions are propagated to great distances, even across the 
basin of the ocean, proves that the centre of action is very 
remote from the surface of the globe. From this same 
cause no doubt earthquakes are not confined to certain 
species of rocks, as some naturalists suppose, but all are 
fitted to propagate the movement. Keeping within the 
limits of my own experience I may here cite the granites 
of Lima and Acapulco ; the gneiss of Caracas ; the mica- 
slate of the peninsula of Araya ; the primitive thonschiefer 
of Tepecuacuilco, in Mexico; the secondary limestones of 
the Apennines, Spain, and New Andalusia; and finally, the 
trappean porphyries of the provinces of Quito and Popayan.f 
I n these different places the ground is frequently agitated 
by the most violent shocks ; but sometimes, in the same 
rock, the superior strata form invincible obstacles to the 
* The subterranean thunders (bramidos y truenos subterraneos) of 
Guana* ua to. The phenomenon of a noise without shocks was observed 
by the ancients. — Aristot. Meteor., lib. ii., (ed. Duval, p. 802). Pliny, 
lib. ii., c. 80. 
t I might add to the list of secondary rocks, the gypsum of the newest 
formation, for instance, that of Montmartre, situated on a marine cal- 
careous rock, which is posterior to the chalk. — See the Memoires de 
I'Arndcniie, tone i., p. 31 1 , on the earthquake felt at Paris and its environs 
in 1681. 
