376 HENRY S. WASHINGTON 
orthoclase diminish the silica in the magma relatively to potash, 
and hence favor the crystallization of leucite. This was found to be 
true of soda-lime feldspar alone, as the portion of the magma remain- 
ing after the abstraction of the constituents of the minerals mentioned 
above, and neglecting the presence of albite in the alkali-feldspar, 
always yielded a greater amount of normative nephelite than that 
shown by the rock, thus proving that silica had been abstracted. 
And, naturally, the effect was still more marked if soda was assumed 
to go into the alkali-feldspar prior to the crystallization of leucite, 
the amount of this calculated from the composition of the last portion 
of the magma being the same as that determined in the mode, even 
if none is present in the norm, of the rock. 
It will be observed that this favorable influence of the crystalliza- 
tion of soda-lime feldspars on the formation of leucite, consequent 
on the presence of considerable salic lime in the magma, is apparently 
at variance with the theoretical conclusion reached previously (p. 277), 
and based on the relative sizes of the leucitic areas, that the presence 
of anorthite should tend to lessen the probability of the presence of 
leucite. But it must be remembered that this was based only on a 
consideration of ideal norms, and that the influence of anorthite 
in forcing the soda to take up extra silica to form the modal albite 
molecule was disregarded. 
It may also be pointed out that this action of the anorthite molecule 
offers a satisfactory explanation of the fact, noted by Zirkel and 
mentioned above (p. 258), that leucite is more often found in combina- 
tion with plagioclase than with orthoclase. 
On this assumption, the tendency to the formation of abnormative 
leucite will increase with the potash content of the magma, since 
the ratio of potash to silica will increase in the last portions to crystal- 
lize, and also, especially when potash is not very high, with its rich- 
ness in salic lime and in soda, since the chemical composition 
will then be favorable to the formation of soda-lime feldspars. In 
conformity with these conclusions we have already noted the fact that 
the loci of these abnormative rocks are apt to fall rather near their 
respective leucitic areas, and that they are apt to contain the more 
leucite the nearer they approach these—that is, the higher they are 
in potash. It will also be noted that those rocks in which abnorma- 
