49^ The American Naturalist. [June, 
formed, is met by recent experiments which show that the 
capillary action continues in spite of such pressure. 
The presence of volcanic cones and craters on the moon 
would seem to invalidate the " steam engine " theory as well 
as the hydrated lava theory of Professor Judd, unless the 
presence of water in large bodies is admitted, On both the 
earth and moon the expansion of fluid rock in the process of 
cooling would bring to bear an enormous pressure, resulting in 
outwellings of lava, and violent eruptions would be accounted 
for by the development of steam on a large scale. It is gen- 
erally admitted that communication exists not unfrequently 
between reservoirs of molten rock at great distances from each 
other on lines of fissure. Heated rocks, subjected to the hydra- 
tion and aeration of infiltrated water would probably occupy 
more space in a solid than in a pasty or liquid condition, and 
would melt at a lower temperature. Contraction by cooling 
of the solidified part of the globe, works in the opposite direc- 
tion; but while this process is fairly regular and even, solidifi- 
cation may take place unequally, rapidly, and by local causes, 
such as cooling by extensive aqueous percolation. Other 
causes of periodic increases of pressure would be the shrinkage 
of the earth's crust upon the cooling interior, the percolation 
of water through fissures and the closure of these fissures by 
changes of level, so that steam developed at some miles below 
the surface would force the fluid lava through the nearest vol- 
canic vent. As far as the argument from the moon is concerned, 
it can be readily disposed of by admitting the previous exist- 
ence of water on its surface, which has been entirely absorbed 
by the rocky substance. 
Among the attendant phenomena of the eruptions were the 
sea- waves. These caused greater destruction both to property 
and to human life than any other of the attendant phenomena. 
They are treated at great length by Captain Wharton in the 
English Report. Undulations were produced reaching as far 
as Havre, a distance of 10,780 miles from the original source 
of disturbance. The seismic flows and ebbs which thus covered 
a very large part of the globe were composed of long undula- 
tions, with periods of over an hour, and of shorter superposed 
irregular waves at brief intervals. The rate of propagation was 
in all cases less than theory would demand for the supposed 
depth of water. The average speed seems to have been some- 
thing between 330 and 380 miles per hour. The mean depths 
deduced by the usual formula from this speed are less than 
those given by actual soundings. The cause of this discrepancy 
