408 
GUATEMALA. 
rock, thus giving passage to the molten mass which must 
be supposed to underlie this volcanic region, the sudden 
contact of two bodies of very different temperatures (per¬ 
haps two thousand degrees) must cause vibrations entirely 
sufficient to account for the worst earthquake recorded. 
That the supply of molten rock is ample beneath the crust 
of this region, we have proof in the constant activity of 
Izalco, which for more than a century has poured out lava 
with the other ejections. 
This theory of earthquake action is so simple that it 
must commend itself to any one who has observed the 
powerful vibrations excited by placing a cold kettle upon 
a hot stove, or by admitting with force a stream of hot 
water into a bath-tub partly filled with cold water. It 
may be stated also that lava is a remarkably poor con¬ 
ductor of heat (I have been able to walk over a crust that 
bent beneath my weight, and again where I left footprints 
in the half-hardened lava), and solid lava might retain 
a temperature of less than two hundred within a few 
feet of a molten mass ranging among the thousands of 
degrees. The secular refrigeration of the subterranean 
molten masses due to the slight conductivity of solid lava 
is well illustrated in the temperature of hot-springs, that 
remains unchanged for centuries. 
Eruptions are usually of an explosive nature in the 
Central American region (as described in the outbreak of 
Coseguina), and the ejected ash is scattered often to a 
great distance to form by its decomposition layers of soil 
especially fitted for the cultivation of coffee, sugar, and the 
vine. Sulphur is not so abundantly deposited as at JUtna, 
Hekla, or even the Mexican volcanoes. 
