EARTHQUAKES AND VOLCANOES. 407 
fact that the emissions consist of ash and lava of slight 
fluidity. In the Hawaiian Islands, where the basaltic lava 
is more fluid than in any other volcanic region, the lava- 
streams often flow for months, and extend fifty or sixty 
miles from the crater, building by successive eruptions a 
cone of great diameter in proportion to their height; 
Mauna Loa having a diameter of ninety miles at the sea- 
level, with a height of less than fourteen thousand feet 
and a slope of about seven degrees. The eruptions of the 
American volcanoes are mainly of masses of rock which 
are piled regularly about the base, in this way increasing 
the height, and great quantities of sand which fills the in¬ 
terstices, and finally of lava in a thick, viscid state which 
clings to the slopes of the growing cone and cements to¬ 
gether the sand and larger fragments. No lava-stream, at 
least of modern times, has been found at any considerable 
distance from its source. 
From the specimens I collected in some of the ravines 
which traverse the older deposits, I saw that in former 
ages the outflow was not only different from that of 
modern times, but of great variety of form in contem¬ 
poraneous streams, although the chemical composition did 
not vary essentially. 
Earthquakes are mainly due to the injection of intensely 
heated lava into strata of cold rock in the process of form¬ 
ing dikes. When a volcano pours its lava out of its sum¬ 
mit-crater, the eruption may be wholly free from earth 
tremors, as is often the case on the Hawaiian Islands; 
and this gives rise to the popular belief that active volca¬ 
noes are in some way a safety-valve for the subterranean 
forces. When, however, the shrinkage of the earth’s crust 
or the explosive force of pent-up vapors Cracks the solid 
