112 
IDDINGS. 
law holds for solutions at high temperatures and great press¬ 
ures. 
The results of the experiments of Pelouze and others upon 
the saturation of artificial glasses are applied to the satura¬ 
tion of natural glasses, and the conclusions drawn— 
That the degree of saturation of rock magmas depends 
primarily on the temperature and pressure. 
That the character of the saturation depends primarily on 
their chemical composition. 
That in cooling magmas the order of crystallization will 
depend primarily on their chemical composition. 
That the nature of the crystallization should vary with 
the physical conditions attending solidification. 
The consideration of magmas during their eruption takes 
account of— 
The uniform decrease of pressure accompanying the ascen¬ 
sion of the magma in its conduit. 
The accelerated decrease of temperature as it approaches 
the surface of the earth, and its resultant influence on the 
saturation of the magma. 
The changes in the temperature of the walls of the con¬ 
duit, which may become highly heated and permit the magma 
to reach the surface of the earth before crystallization sets in. 
The effects of large conduits in producing different rates 
of cooling at the sides and in the center of large bodies of 
magma. 
The variability in the velocity of eruption and the retarda¬ 
tion of crystallization, or the resorption of crystals. 
It appears that the order of resorption should be according 
to the solubility of the minerals, which is inversely as their 
power to saturate the solution. Consequently the last min¬ 
eral crystallized should exhibit the greatest amount of resorp¬ 
tion. 
The rate of cooling affects the grain of the rock—slow 
cooling producing fewer and larger crystals; rapid cooling, 
