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IDDINGS. 
another would be the absorption of water under great 
pressure and at high temperature.” 
He considers that King’s theory that the relief of pressure 
has resulted from the erosion of superficial material is not 
in accordance with the geological conditions observed in 
many regions of volcanic activity. In many such regions 
there has not only been little or no erosion, but in some 
cases even there has been recent deposition. 
“ From a dynamical standpoint,” he adds, “ the problem 
to be explained is the passage of lava-forming materials 
from a dormant to an energetic condition,” and that this 
means the passage of solid materials into the liquid condi¬ 
tion with a decrease of density, and this involves, he thinks, 
an increase of temperature. 
As to the mechanics of eruptions he remarks that “ a care¬ 
ful examination of the details of volcanic eruptions leaves 
the impression that they are pressed up by the weight of 
rock which overlie their reservoirs, and that their extrava¬ 
sation is merely a hydrostatic problem of the simplest 
order.” 
“ Lava will rise to the surface or not, according to its 
density. If it be lighter than the mean density of the rock 
above the reservoir, it will reach the surface; if it be heavier 
than the overlying rock, it will never reach the surface.” 
Upon this conclusion is based his explanation of the order 
of succession of volcanic rocks, which is the same sequence 
as that of Von Richthofen. The two essential conditions 
for their eruption are: First, the}^ must be fused; second, 
they must be lighter than the overlying rocks. He con¬ 
siders the most basic rocks to be the most fusible, but that 
they require a higher temperature to be sufficiently light 
to reach the surface of the earth, while the acid rocks fuse 
at the highest temperatures, but are the lightest. From this 
he argues that the intermediate varieties may be fused and 
attain sufficient lightness first, and that the others follow in 
the order observed. He thinks that the basic rocks have 
reached the surface in a superfused condition, which gives 
