182 
IDDINGS. 
plex solution. It is a ch emico-physical problem involving 
differences of temperature, pressure, and all of the intricate 
properties of mixed solutions. 
A comparison of the phenomena connected with a definite 
volcano or group of volcanoes and those of a region of fis¬ 
sure eruptions shows that at a volcanic center differentiation 
takes place in a smaller body of magma and proceeds to 
greater extremes within a given time, and is independent of 
the differentiation in other centers; but the resulting pro¬ 
ducts are similar when the general magmas at these centers 
were similar. It is also observed that the eruptions of a 
volcano are comparatively frequent or continuous, and that 
the volume of the magma erupted at any one time is com¬ 
paratively small, while in a region of fissure eruptions the 
outbursts are few and are often separated by long lapses of 
time, and the volume of lava at each outburst is compara¬ 
tively large. The composition of the magma erupted at 
successive phases of the activity is more uniform, and the 
differentiation has evidently taken place in a larger body of 
magma than in the case of a volcano. The localization of 
the magma, therefore, was not so restricted, and we may well 
imagine that the differentiation took place farther beneath 
the surface of the earth than in a volcano, the resulting pro¬ 
ducts being the same in kind, but not always in degree. 
It is perfectly evident that the course of dynamical events 
must have great influence on the character of the magma 
erupted from different centers or in different regions. The 
volume of magma erupted at one time and the constancy of 
the supply will affect the uniformity of its composition ; con¬ 
sequently the variations in the character of the rocks at dif¬ 
ferent centers of eruption and the order of their eruption will 
depend on the course of dynamical events in various parts 
of the region. 
The objection has been raised to any theory of separation 
which would derive such extreme kinds of rock as rhyolite 
and basalt from one original magma, that we should expect 
to find transitional forms of rock associated with them. But 
