34 WILLIAM H. PICKERING 
Connected intimately with the origin of the continents is the 
problem as to the cause of volcanoes, and why they are at present 
always situated near the sea. A point that is of the utmost conse- 
quence in its bearing on this question is the fact, noted by Charles 
Darwin, that active volcanoes are found only where the coast-line 
is rising. Clearly the same cause produces both effects. 
A rising region, as pointed out by Dutton, must evidently be 
increasing its volume. This increase may occur either with or 
without an increase of mass. In the latter case the increase must be 
due toarise of temperature. It has been shown that, if a part of the 
Earth’s crust fifty miles in thickness were to have its temperature 
raised 200° F., its surface would be raised to the extent of 1,000 to 
1,500 feet.‘ The Bolivian plateau has an elevation of two and a 
half miles. That of the Himalayas is about a mile higher. It is 
improbable that these elevations are due to this cause. 
The alternative is that in the rising regions we have an increase 
of mass. If the mass were increased materially, it has been shown 
by Gilbert? that the hot subterranean region should yield to the 
added pressure, thus neutralizing the elevation. An added column 
of rock two miles in height could not possibly be supported. Appar- 
ently our last resort is to introduce some lighter material, such as 
water or steam. ‘The pressure on the steam, if its temperature were 
above the critical point, would be so great that its density would be 
but little less than the equivalent extrapolated value for water. It 
might have one-fourth of the weight of an equal column of rock. 
Liquid lava is full of water, and as the lava cools the water is 
expelled from it. The lava at Hilo, Hawaii, contains innumerable 
bubbles, indicating the presence of steam, which had been retained 
by it within its structure for many days, ever since it had left the 
crater of Mauna Loa, fifty miles distant. 
Since volcanoes are intermittent in action, the charging process 
must still be going on at the present time; otherwise there would have 
been one long discharge in the distant past, which would have ren- 
dered all our present volcanoes extinct. 
Since volcanoes are active only near the oceans, it has been sug- 
t Judd, Volcanoes, p. 347. 
2 Continental Problems of Geology, Smithsonian Report, 1892, p. 165. 
