258 SCIENCE SKETCHES. 
undisturbed by the fires within, and were clear 
and cold as mountain brooks should be; but the 
rain and melted snow will never all remain on the 
surface. Some of it falls into cracks or joints or 
porous places in the rock, and from this come 
underground streams or springs. But in this 
region a stream could not run long underground 
without coming in contact with the old still-burning 
fires. When a crust is formed over the lava, it 
cools very slowly. When the crust is a rod or 
two deep, the lava within is almost as well pro- 
tected as if it were at the centre of the earth. 
Whenever the water came down into the fire, 
the hot rocks would be furious with indignation, 
and tearing the water to atoms they would throw 
it back to the surface as steam. Then the ex- 
plosive force of the steam would in turn tear up 
the rocks, making still larger the hole through 
which the water came. When the rocks were 
very hot, a little water upon them would make a 
terrible commotion like the shock of an earth- 
quake. When much water came down, it would 
hiss and boil high in the air, as it tried to break 
the cushion of steam which came between it and 
the lava. 
And all this went on in hundreds of places and 
maybe for thousands of years. The hot rocks 
glowed and sweltered in the ground, and the cold 
snow-water crept after them closer and closer, 
while more and more vigorously the rocks re- 
sented the intrusion. Sometimes the water would 
go down in a mass through a cleft, when it would 
be hurled back bodily the very way it came. At 
