FORMATION OF LEUCITE IN IGNEOUS ROCKS 265 
quently obtains curves instead of straight lines. Furthermore, 
Iddings’ first diagrams were constructed before the elaboration of the 
magmatic divisions of the quantitative system, so that the center- 
points and border-points of these are not considered, while in his 
later diagram the limits are given for the orders and rangs of potash- 
lime and soda-lime rocks, the alkalies being considered separately. 
It may be pointed out that all such diagrams represent loci of 
chemical equilibrium, the lines or curves being those of definite com- 
binations of the chemical or mineral constituents. ‘They are analo- 
gous to the pressure-temperature, concentration-temperature, and 
other diagrams employed to represent physico-chemical equilibria. 
It may also be noted that, apart from considerations of simplicity, 
convenience, or intelligibility, it makes no difference whether ratios 
or percentages are chosen as bases for plotting. The relations will be 
evident in either case. 
Turning to Plate I, the line LN is the locus of norms composed 
wholly of leucite and nephelite, passing through the center-points of 
the various subrangs of order 9 in a persalic and peralkalic magma. 
Pure leucite is found at L and pure nephelite at N. Between these 
extremes are varying mixtures of the two, the points for definite com- 
binations, which correspond to the center-points and border-points 
of the subrangs, being indicated by the intersections with the solid 
and broken lines which slope down toward the left. To the right 
of or above this line no peralkalic and persalic norms can exist, since 
there will be insufficient silica to satisfy the alkalies unless kaliophilite 
is formed, and this molecule has been excluded from consideration. 
To the left of or below it there will be an excess of silica, so that either 
orthoclase or albite or both will be present. 
The line OA is the locus of purely feldspathic norms, that is of the 
center-points of order 5, composed wholly of orthoclase and albite in 
varying proportions, pure orthoclase being found at O and pure albite 
at A, and the definite combinations which correspond to the center- 
points and border-points of the subrangs being indicated by the 
intersections with the solid and broken lines which extend upward 
toward the right. To the right of this line the norms must contain 
nephelite or leucite, or both of these, the maximum amounts being 
reached at the leucite-nephelite line LN, along which lenads alone 
