PLACE OF ORIGIN OF THE MOON 35 
gested that the eruption is due to sea water that has entered by 
cracks in the Earth’s crust and is subsequently discharged from the 
volcano. Volcanoes do discharge salt water, but the solid ingre- 
dients of the water do not occur in the same proportions that they 
do in the sea. Some of the sea salts are often found to be absent, 
while other salts are often found that do not occur at all in sea water. 
This fact, together with the inherent improbability that sea water 
should be sucked in at a low level and pumped out at a high one, 
renders this explanation improbable. 
Another explanation of the universal presence of water in vol- 
canic products is that it is derived from rain water, which has per- 
colated down through the soil. This theory, however, does not 
account for the fact that volcanoes are always found near the sea. 
Neither of these theories account for the gradual elevation of the 
land in volcanic regions. 
Since the process of charging volcanoes with steam is still going 
on, and since it appears that the necessary water is not derived from 
either the sea or the atmosphere, the only alternative seems to be that 
it comes from the heavy stony material forming the ocean beds, and 
does not come in appreciable quantities, at present, from the lighter 
material forming the continents. It is evident, however, that this 
lighter material is sometimes cracked, permitting the discharge to 
take place through it. This was the case with the extinct volcanoes 
in central Europe, and those near the Yellowstone Park and Arizona 
in this country. The volcanoes at present active in North and South 
America seem to rise from what was probably formerly the edge 
of the continental plateau. | 
The next question that arises is: From what depth does the 
lava come? Judged by its temperature at the vent, unless it becomes 
heated by friction, by compression, or by radio-activity, on its way 
to the surface, which seems improbable, it must have come from a 
considerable distance. The rate of increase of temperature with 
the depth varies in different parts of the world from 20 to too feet 
per degree Fahrenheit. It may fairly be taken near the surface at 
too° per mile of depth. From its surface temperature, Bonney 
estimates? that ‘‘the lava is generally supplied from a zone situated 
t Volcanoes, p. 284. : 
