SUBJERIAL PYROPLASTIC ROCKS. 
115 
earth’s surface in the form of waves or undulations, which 
travel with great rapidity in all directions from the focus of 
disturbance. The intensity of this movement, or the force of 
the shock communicated to the strata, is supposed to be in 
some way dependent upon the diameter of the vent through 
which gases and melted matter have escaped. The openings 
of Vesuvius, Etna, and the South American volcanoes, are nar¬ 
row and constricted, and at the same time their coverings over 
and above the seat of activity are thick, and strengthened by 
repeated accessions of layers of rock from beneath, and braced 
by numerous intersecting dykes. Under these circumstances 
volcanic forces are confined by strong walls and narrow fun¬ 
nels ; and hence, when the forces have acquired strength suffi¬ 
cient to rend asunder these walls, or force the safety-valve, it 
will be attended with tremendous earthquake shocks. On the 
other hand, where there is an opportunity for a free escape of 
gas and melted matter, as in the Sandwich islands, Mouna Loa 
for example, where the craters are rather deep and wide exca¬ 
vations, volcanic action of great intensity begins without 
endangering the surrounding country by earthquakes. The 
seat of activity seems to be more superficial, and the resistance 
to be overcome far less, than those of South America and 
Europe. An eruption of the volcanoes of Europe and South 
America, therefore, is attended with violent movements or 
undulations of the crust, which are propagated from the center 
of action in all directions. It is to be recollected, that as the 
crust is not homogeneous, and the material through which the 
impulse is propagated is not equally dense, the effects of that 
impulse must be modified in its transit. The impulse here spo¬ 
ken of generates a wave in the crust which usually moves 
onward, as has been already stated, with great velocity in all 
directions from the center which receives the first shock of the 
explosion. The surface rises and falls like waves of the sea; 
or, in other words, the undulations travel onward with great 
speed in obedience to the ordinary law of a force propagated 
through a resisting medium. The undulation is modified, how- 
