CRYSTALLIZATION OP IGNEOUS ROCKS. 
105 
may be well to consider how the re-solution of minerals 
would take place when several kinds existed in the same 
magma. 
If the magma, upon cooling under decreasing pressure, 
reaches a point where the molecules of one kind of mineral 
can no longer remain in solution, they crystallize out; and 
at a subsequent point the molecules of a second kind of 
mineral can no longer remain in solution, and they crystal¬ 
lize out; and so on until three or four kinds of minerals have 
crystallized out of the magma. If, now, the ratio between 
the temperature and pressure diminishes, so that the result 
is a greater capacity of the magma to contain matter in solu¬ 
tion, then the mineral molecules should be resorbed in the 
reversed order to that in which they were excluded from solu¬ 
tion ; for the tendency of a substance to saturate a solution 
is in a measure inversely proportioned to its solubility in the 
solvent. Hence the relative solubility of minerals in the solu¬ 
tion out of which they have crystallized should be inversely 
as their power to saturate it. Thus the mineral which crys¬ 
tallized last would be the first to be resorbed. Consequently 
the youngest mineral should exhibit the greatest amount of 
resorption. 
Rate of Cooling and Rock Structures .—It appears to be un¬ 
questionably the rule that the crystallization due to slow 
cooling proceeds from a comparatively few centers, while that 
due to rapid cooling proceeds from a much greater number 
of centers closer together. Hence the slower rate of cooling 
which a magma may experience in the earlier stages of its 
eruption will lead to the crystallization of a comparatively 
few individuals of any mineral. These may grow to consid¬ 
erable size before the rate of cooling is sufficiently increased 
to set up crystallization from a greater number of centers. 
Such a change would start other individuals of the same 
mineral that would not reach the size of those started earlier, 
which may continue growing. In this manner there may be 
produced a porphyritic structure in which there is a grada- 
12—Bull. Phil. Soc., Wash., Vol. 11. 
