FORMATION OF LEUCITE IN IGNEOUS ROCKS 373 
would seem that the influence of the binding of silica by soda in the 
formation of soda-orthoclase would be felt chiefly in peralkalic rocks, 
or, in more calcic ones, after the formation of labradorite had ceased, 
-at least in great part. In the latter case its influence is probably 
comparatively slight, though in peralkalic rocks the crystallization 
of the alkali-feldspar and leucite is apt to be partially synchronous, 
so that it would be felt more strongly in these and would very materially 
tend to favor the formation of leucite. 
In the preceding explanation it has been assumed to be a case of 
relative affinities, that of potash for silica and of lime and potash for 
soda. Butitis clear that such questions of affinity may be disregarded 
and the result considered merely as dependent on the relative tendency 
toward crystallization of the several minerals involved, especially 
labradorite (or other soda-lime feldspars), orthoclase, and leucite. 
The effect would be the same in either case in influencing the forma- 
tion of leucite. 
Another mineral which may enter into the problem, and which 
would strongly influence the formation of leucite in certain cases, 
is analcite. This contains twice as much silica as does nephelite, 
and its crystallization would abstract silica from the magma, analo- 
gously to the crystallization of albite, though not to the same extent. 
Analcite contains two molecules of water, so that its formation can 
take place only when the molten magma contains water and under 
peculiar physical conditions. But that its presence may explain 
the formation of abnormative leucite in some rocks has been clearly 
shown by Pirsson.? 
The presence of alferric minerals has been disregarded as yet, 
since the amount of these in the exceptional rocks now under con- 
sideration is usually not very large. But a slight consideration of 
their characters and the relations of norm and mode will show that 
the influence of the early crystallization of these is also favorable 
to the development of leucite by abstracting from the magma an abnor- 
mative amount of silica. Considering augite alone, which is the 
alferric mineral most commonly met with in connection with leucite, 
the study of the relations of the norms and modes, as in the Italian 
tL. V. Pirsson, Bulletin No. 237, U. S. Geological Survey (1905), pp. 11, 
170. 
