CRYSTALLIZATION OF IGNEOUS ROCKS. 
107 
ascertained to a certain extent by comparing occurrences 
where the relations of two or more of them are known. 
To judge of the influence of the rate of cooling we must 
compare instances where similar magmas or parts of one 
magma have crystallized under the same or similar pressures, 
and where the mineralizer, if present, has been uniformly 
disseminated through the mass. These conditions are often 
realized, and may be found between neighboring points in 
a chemically homogeneous body, as in a horizontal plane 
across a broad dike. The crystallization often varies very 
decidedly in a rock body in such connection with its sides or 
cooling surfaces that it plainly indicates the great influence 
of the rate of cooling. It is more strongly marked where 
bodies of different sizes have crystallized under essentially 
the same pressure. 
(2.) The influence of the chemical composition of the magma 
upon the extent or degree of crystallization appears to stand 
next in importance. It is very clearly recognized when 
bodies are compared that have crystallized under similar 
geological conditions, which indicate very nearly the same 
rates of cooling and degrees of pressure, the basic magmas 
exhibiting a much greater tendency to crystallize than the 
highly siliceous or alkaline ones. 
(3.) The influence of a mineralizer is very apparent in some 
instances, and can be appreciated by studying a portion of a 
rock body which has solidified under a uniform pressure and 
rate of cooling, but in which the mineralizing agent has been 
irregularly distributed. It is then observed that the crystal¬ 
lization varies conspicuously with the mineralizer. Such 
cases have been noted in spherulitic and laminated obsidian 
and rhyolite and in those with lithophysse. 
The cases in which the influence of a mineralizer can be 
recognized are very much fewer than those in which the 
effect of cooling or of the essential chemical composition is 
observed. Hence it appears to be a much less important 
factor than either of these. If a mineralizer be uniformly 
