110 
IDDINGS. 
which have not been discussed in this paper. Some of them 
may have a direct bearing on the points here raised. Many 
of them are more closely connected with certain phenomena 
which belong to another phase of the question which has 
been intentionally left for another paper, the purpose of the 
present paper being to present only the essential facts which 
seem to contribute to an understanding of the act of crystal¬ 
lization and of the nature of rock magmas. The facts con¬ 
cerning the phenomena of crystallization have been arranged 
so as to show: — 
The general structure of different igneous rocks and the 
manner in which the mineral crystals occur in them. 
The various kinds of minerals composing the rocks, their 
chemical nature and association in different rocks. 
The character of these minerals as crystals and the means 
of distinguishing their relative age in glassy and in holo- 
crystalline rocks. 
The nature of the inclusions in these minerals, their kinds, 
mode of occurrence, associations, and their significance. 
The zonal structure of certain minerals and its significance. 
The tendency of certain minerals to include certain other 
substances and minerals; to crystallize together as synchro¬ 
nous intergrowths or as successive parallel growths. 
The crystallization of the constituent minerals in one con¬ 
tinuous period or in two or more interrupted periods. 
The order in which these minerals crystallize and its varia¬ 
bility for different kinds of rocks. 
The essential difference between minerals of different gen¬ 
erations in some cases. 
The essential identity of minerals of different generations 
in other cases and its important bearing on the apparent in¬ 
fluence or lack of influence of the pressure under which 
these generations took place upon the nature of the minerals 
crystallized. 
The extraneous differences between minerals of different 
generations in one rock combined with identity of mineral 
substance. 
