458 N. L. BOWEN 
plane of the page. S, indicates the direction of another section, 
cut as before, showing magnetite centrally placed in a plate of 
plagioclase, part of which is without doubt younger than the 
magnetite but part of which may be older. These sections are 
shown in plan in the figure. In the great variety of sections 
such as a rock slide commonly contains, some few would show 
a peripheral placing of the magnetites, but this would be regarded 
as mere accident in the face of the great number that showed a 
random placing. The conclusion as to order of crystallization 
would be that magnetite had crystallized before plagioclase, when 
as a matter of fact the greater proportion of the plagioclase might 
have crystallized before magnetite. Obviously the only safe con- 
clusion that can be drawn is that some of the plagioclase continued 
to crystallize after magnetite had ceased to crystallize. 
The conclusions that may be drawn from the fact that one 
mineral is idiomorphic against another or that one mineral indents 
another are qualified in a similar manner, as will be apparent 
from an examination of Figs. 2 and 3. 
These figures represent sections of an augite crystal with a 
plagioclase crystal indenting it (Fig. 2) and idiomorphic against 
it (Fig. 3). The two cases are 
strictly analogous and do not re- 
quire separate consideration. The 
relation of the minerals makes it 
apparent that only the outer rim 
A of the augite is necessarily of 
later crystallization than the pla- 
gioclase. The inner portion B may 
have crystallized later than, or simultaneous with, or earlier than, 
the plagioclase. 
The alternatives mentioned would, however, not be apparent 
in most other sections. S represents the direction of such a section, 
cut as before, and shown in plan in the figure. A natural conclu- 
sion from such sections (and a rock slide would show every variety) 
would be that augite crystallized after plagioclase when really 
the greater proportion of augite might have crystallized before. 
The relation of the two minerals indicates with certainty only 
Fic. 2 
