FORMATION OF LEUCITE IN IGNEOUS ROCKS 379 
are entirely too great to be caused only by the slight variations in 
silica, soda, or potash shown by the analyses given above. In such 
magmas, therefore, whether leucite is formed or not must be held 
to depend upon some factor or factors extraneous to the magma itself; 
and these are to be looked for in the physical conditions which obtain 
during the solidification. 
These conditions have been discussed by many petrographers, 
and it seems unnecessary to enlarge on them here, in view of the fact 
that the attitude of this paper is dominantly chemical, and because of 
the inadequacy of our knowledge of the physico-chemical proper- 
ties of leucite, and other minerals, mentioned at the beginning of this 
paper (p. 259). It must suffice to say that the chief physico-chemical 
factors which under effusive conditions induce the formation of leucite 
in magmas chemically capable of permitting it seem to the writer to be: 
a) The great tendency toward crystallization of the more calcic 
soda-lime feldspars and of leucite, that of nephelite being also con- 
siderable, but that of orthoclase being very slight under such condi- 
tions. 
b) The relative stability of orthoclase, soda-lime feldspars, leucite, 
and nephelite, at different temperatures and pressures, the stability 
of the second and third of these being apparently greater under effusive 
conditions than those of the first and fourth. This is a matter which 
demands thorough experimental investigation. 
c) The absence of mineralizers under effusive, and their presence 
under intrusive, conditions; the former permitting the magma to 
solidify and crystallize at a higher temperature, and so favor the for- 
mation of leucite, while the latter diminishes the viscosity, and thus 
permits the crystallization to proceed at a lower temperature and in a 
more fluid medium, which would favor the formation of orthoclase. 
d) It is possible that, whether a molten magma is at rest or in 
motion, or whether it is subject or not to shocks, as of explosions of 
steam in a volcanic vent, may be of importance in inducing the crys- 
tallization of some minerals such as leucite; since it is known that 
tapping or agitation will frequently bring about the crystallization 
of an undercooled liquid. 
The influence of other factors might be adduced, but the above 
must suffice. In any case, an appeal to them in explanation of the 
