FORMATION OF LEUCITE IN IGNEOUS ROCKS 375 
of potash and small amounts of alferric minerals and often of nephe- 
lite, so that the mass action of these (which would also certainly 
tend to prevent the formation of leucite) may be neglected. Further- 
more, some of these abnormatively leucitic rocks are vitreous, so that 
the leucite has been formed in spite of their incomplete crystallization. 
The explanation of the abnormality in the behavior of potash 
toward silica in these cases may be explained by two sets of factors, 
or rather by a combination of the two. 
The tendency of the CaAl,Si,Og molecule to crystallize with that 
of NaAlSi,Og as the mixed crystals of the soda-lime feldspars is well 
known. Pure anorthite is a rather rare feldspar, being met with, 
for the most part, only in rocks belonging to docalcic and percalcic 
rangs, gabbros and some basalts, though it occurs in a few rocks, 
andesites, vulsinites,and ciminites, which are decidedly alkalic. But, 
speaking generally, the affmity of lime (in anorthite) for soda (in 
albite) seems to be very strong. Likewise purely potassic orthoclase 
and microcline are rarely met with, these minerals nearly always 
carrying very notable amounts of soda, as may be seen in the analyses 
given in Hintze’s Mineralogie. On the other hand, both albite and 
anorthite, as well as their mixed crystals, the soda-lime feldspars, 
contain very little potash, the amount of this constituent in the analysis 
of the soda-lime feldspars being usually very small, and generally 
far less than the amounts of soda in orthoclase and microcline. 
It is reasonable, therefore, to suppose that, under certain con- 
ditions, the affinity of lime and potash for soda to form feldspars 
may supersede the usually superior affinity of potash for silica, with 
the result that the CaAl,Si,Og molecules in the magma, and to a 
less extent those of KAISi,Og, will capture a certain amount of the 
soda present, forming soda-lime feldspar and soda-orthoclase. This 
soda will therefore bind three times as much silica as it would do 
if it entered into nephelite, leaving a correspondingly less quantity 
of this available for the potash, which would necessarily form leucite, 
if the amount of silica left available for it were less than enough to 
form only orthoclase. 
Confining our attention for the moment to the soda-lime feldspars, 
it is evident that, on this hypothesis of the superior affinity of lime 
for soda, the abnormative formation of leucite would be aided by 
