106 
IDDINGS. 
tion in the size of the crystals of one or more of the essential 
minerals, from the largest porphyritical individual to the 
smallest, which occur in far greater numbers in the ground- 
mass. Such structures are common among the andesites. 
It is evident that a sudden increase in the rate of cooling 
would produce a fine-grained groundmass with distinct por¬ 
phyritical crystals ( phenocrysts ). 
Where the magma retained sufficient heat during its erup¬ 
tion to prevent the separation of crystals early in its history 
it would reach its stopping place without porphyritical crys¬ 
tals {phenocrysts). The structure would then result from a 
nearly uniform rate of cooling, and would become compara¬ 
tively even-grained if the magma was homogeneous. The 
rate of cooling would affect the size of the grain, which might 
be extremely fine or very coarse. 
Extremely rapid cooling would prevent all crystallization 
and would solidify the mass as glass. 
Relative Influence of the Various Causes of Crystallization in 
Rock Magmas. —(1.) When we consider the relative impor¬ 
tance of the causes leading to the crystallization of rock 
magmas we conclude that of the various factors treated of in 
the foregoing discussion the most evident and essential is the 
loss of heat, or cooling. All igneous rocks have solidified as 
cooling bodies from a fluid state, and so long as their origi¬ 
nal temperature existed they would not have crystallized 
under the conditions existing near the surface of the earth. 
It is also evident that the element of time is almost as essen¬ 
tial as that of cooling; for it is known that a certain amount 
of time is necessary for the development of crystals, and that 
very rapid cooling may solidify any rock magma in the form 
of glass. The rate of cooling , therefore, becomes the compound 
factor whose influence is to be compared with that of the 
chemical composition, mineralizers, and pressure. 
The relative efficiency of these factors, which have taken 
part in the crystallization of most if not of all rocks, can be 
