FORMATION OF LEUCITE IN IGNEOUS ROCKS 27% 
TABLE I 
RELATIONS OF HomoLtocous AREAS, REFERRED TO THE PERALKALIC AREA AS UNITY 
Area Peralkalic Domalkalic Alkalicalcic 
I ONN [Oa crate rcorae trie I .000 0.731 0.417 
(OPACIN Niennictissctos eles Seacs I .000 0.724 0.434 
OO ARE eran tenis I .000 0.962 0.855 
EOIN, eee teue nie seer I .000 0.827 0.609 
TABLE 1 
RELATIONS OF HomoLoGous AREAS, IN PERCENTAGES OF THE AREA LON 
Area Peralkalic Domalkalic Alkalicalcic 
EOIN toes ertes pore uci sore oot 21,61 19.11 14.78 
OIAUN Serta ek ig testes 35-91 31.45 25.60 
OOIN Save ovens eine 42.48 49-44 59.62 
TEL OMN eect eterno ry Ca one 100.00 IO00.00 IOO.00 
0.719, along the abscissal axis, this being the molecular amount of silica 
in anorthite. This is also the point of intersection of lines drawn 
through L?, L?, etc., and through O', O?, etc., with the line QN. Con- 
sequently all persalic leucitic norms will lie in the area L*O*An Nt’. 
From the above we may draw the following conclusions as to the 
influence upon the leucitic area of the introduction of anorthite into 
the magma. The possible range in potash for any magma with a 
percentage of silica greater than that of the corresponding mixture of 
leucite and anorthite will be decreased, while for silica. percentages 
below this point there will be but little change. For homologous 
magmas, as regards order and subrang, the absolute amount of potash 
will decrease, while that of silica will also decrease down to a certain 
point, dependent on the magma, after which it will slowly rise.t 
As the amount of anorthite increases, the size of the leucitic 
area decreases, both absolutely and relatively to that of the whole 
area of possible norms, this decrease being increasingly great. 
It is therefore clear that, in general, the introduction of anorthite 
into persalic magmas has little influence on the presence of leucite 
t Compare the figures on p..95 of Professional Paper No. 14, U. S. Geological 
Survey, 1903. Those for the more lenic orders are incomplete, as the compositions of 
the more calcic rangs of these, which cannot exist according to the definition of the 
respective orders, were not calculated. 
