384 HENRY S. WASHINGTON 
These affinities must be regarded as acting in one direction only, 
that mentioned in the several cases above, since, were they reversed, 
the resulting modes would be quite incompatible in general with 
our observations. 
Formation of leucite—The formation of leucite is an unusual 
occurrence, and the number of leucitic rocks will always remain but 
a very small fraction of all known igneous rocks. 
The formation of leucite is, in general, primarily dependent upon 
the chemical composition of the magma, subject, however, to the 
modifying influence of certain factors, such as physical conditions 
and the mass action of complex mineral molecules, to be mentioned 
later. 
The chief chemical factors involved are the percentages of silica 
and of potash, and the ratio of soda to potash, but this last and the 
amounts of the other constituents are of subordinate importance. 
These statements are subject to the modifications mentioned in the 
preceding paragraph. 
Leucite will not be formed in rocks with an excess of silica—that 
is, more than enough to saturate the bases completely and form the 
most highly silicated mineral molecules possible. It follows from 
this that leucite and quartz are incompatible and will not occur 
together. 
As regards silica, rocks containing leucite may range between the 
silica percentage of orthoclase (64.75) as a maximum, down to the 
absence of silica as a minimum limiting value. The actual possible 
maximum will, however, depend upon the amount of anorthite and of 
femic molecules, all of these decreasing very markedly the possible 
amount of silica. 
As regards potash, the limits lie between the percentage of leucite 
(21.56) as a maximum (though this may be conceivably exceeded 
owing to the existence of the kaliophilite molecule) and the disap- 
pearance of potash asa minimum. The actual maximum will depend 
upon the amounts of anorthite and of femic molecules present, as 
in the preceding case. 
The greatest range in silica consistent with the formation of leucite, 
and consequently the greatest probability of its formation, for any given 
percentage of anorthite and femic molecules, will be found when the 
