FORMATION OF. LEUCITE IN IGNEOUS ROCKS 301 
or more definite points of the leucitic area desired, when its size and 
position on the diagram can be readily laid down, the parallelism 
of the various lines here coming into use, and its relations to the 
locus of the rock can be studied. For this purpose any two of the 
points L, O, or N, may be used, but for practical purposes the simplest 
determination is that of the point NV, the locus of a mixture of nephelite 
with the amounts of normative anorthite and femic minerals corre- 
sponding to the rock. From this the directions of the points Z and O 
may be readily determined. 
The actual process is as follows: The norm of the rock having 
been calculated in the usual way, the total percentage of the normative 
anorthite and combined femic molecules, and the total amount of 
silica belonging to them, are estimated. The difference between 
their combined percentage Ioo per cent. is then assumed to be nephe- 
lite, and the amount of silica belonging to this is calculated. The 
sum of these percentages of silica (those of normative anorthite, 
femic minerals, and of nephelite) is then reduced to molecular ratio 
by dividing by 60, and the result is the locus N of the leucitic area 
corresponding to the rocks on the abscissal line QANM. The 
leucite-nephelite and orthoclase-nephelite boundaries of this area are 
determined by drawing lines from NV parallel to the other LN and 
ON lines of the diagram, the points L and O being readily determined 
by calculating the amount of potash or silica corresponding to a 
mixture of anorthite +femic molecules and either leucite or orthoclase, 
respectively, in the same proportions that were used in the case of 
nephelite.* 
In actual practice, however, it was found that the determination 
of the points Z and O was seldom necessary, and the comparison 
between the leucitic area and the locus of the rock was accomplished 
without defacing the plotted diagram and very quickly, after the 
point V had been determined, by means of a square sheet of stiff 
paper from which a triangle had been cut, whose lower angle LNO 
and inclination as regards the lower edge of the paper corresponded 
to those of the plotted leucitic areas in the diagram. By placing the 
sharp lower apex of this triangular opening at the determined point NV, 
1 The locus of N for every rock, as thus calculated, is given in the list of analyses 
in the Appendix. 
