FORMATION OF LEUCITE IN IGNEOUS ROCKS 261 
silica, this order is: potash, soda, lime, magnesia, and ferrous oxide. 
A second order of affinity of almost equal importance is that of the 
bases for alumina, usually the next most abundant constituent after 
silica. This order is the same as that for silica. Upon these are . 
based very largely the calculation of the norm in the quantitative 
system of classification, that is, the simplest expression of the mineral 
molecules which experience has shown are most likely to be formed 
in igneous rocks. 
On these assumptions the most important mineral molecules 
formed in the norm by each of the above five oxides may be referred 
to two pairs for each one, diopside being considered as split up into 
simpler molecules, and kaliophilite being disregarded on account of 
its excessive rarity. These pairs, one member of which has the 
highest and the other the lowest amount of silica, are as follows: 
Base to Base to 
Molecule Silica Molecule Silica 
@rthoclase; KAISi203 W412 6 Ieucites KAIST, Oy ne eee 
Salic Albite, NaA1Si;08 ets 0 Nephelite, NaAlSiO, «2. ..- -r 3-2 
Anorthite, CaAl,Si,0g . Tees Wanting 
(ener ene CaSiO; niet Akermanite, Ca,Siz0;15  . 42 3 
emie Enstatite, MgSiO; Te Jor Forsterite, Mg2SiO, al assem 
Ferrosilite,t FeSiO; . Ten Havalite, Hes sl@yihn. a.) que. 2a 
Magnetite, Fe;O, it 40) 
Considering holocrystalline rocks alone, in accordance with these 
assumed orders of affinity of the bases for silica and for alumina, 
quartz would be formed only when there is an excess of silica over 
that needed to permit the existence of the higher silicated pair for 
each base present, when the lower silicated member would be absent. 
If there is insufficient silica for this, the potash will form orthoclase 
rather than leucite, the magnesia and ferrous oxide forming olivine 
rather than hypersthene, and soda forming nephelite rather than 
albite if necessary. On the basis of the assumed order of affinity for 
silica, melilite should be formed rather than nephelite; but it is found 
that this is not the case, melilite being notoriously much more rare 
than nephelite. This undeniable exception may be attributed (in 
great part at least) to the mass-action and the high stability of the 
complex pyroxene molecules, into which lime enters in preference to 
melilite. After the potash is satisfied, the soda would form albite 
t Cf. Washington, Professional Paper, U. S. Geological Survey, No. 14 (1903), 
Pp. 9o. 
