FORMATION OF LEUCITE IN IGNEOUS ROCKS 267 
leucite; while to the right of it they contain orthoclase, leucite, and 
nephelite, but no albite. 
This orthoclase-nephelite line ON is therefore a highly important 
one in our discussion, marking, as it does, one limit of leucitic norms, 
the others being the leucite-nephelite and leucite-orthoclase lines LN 
and LO, beyond which no norms can exist, as we have seen above. 
Within the leucite-orthoclase-nephelite area LON, therefore, must 
fall all leucitic peralkalic and persalic norms, and this may be called 
the leucitic area. Several features of this leucitic area deserve remark 
as bearing on the subsequent discussion. ‘The first is the small size 
as compared with the feldspar-nephelite area OAN and the feldspar- 
quartz area OQA. Calling the area of the leucitic area of LON 
unity, that of OAN is 1.662, and that of OQA is 1.966. Or expressed 
in percentages of the whole area LON they are as follows: LON= 
21.61, OAN =35-o01, and OQN =42..48. 
The second feature is the shape and the position of the leucitic 
area—an acute-angled, almost isosceles, triangle, with the apex down 
and inclined sharply to the right. In consequence of this shape and 
position, the greatest variation in potash occurs at the silica percen- 
tage of leucite, S=o.9194, and the greatest variation in silica at the 
potash percentage of orthoclase, K=o.180. ‘Therefore, as the per- 
centage of silica of the magma falls below that of leucite, the possible 
range in potash rapidly diminishes till the silica percentage of nephe- 
lite is reached at S=o.704, while as the percentage of silica of the 
magma rises above that of leucite the range in potash diminishes at 
a more rapid rate until the silica percentage of orthoclase is reached, 
above which no leucite can exist. 
Crossing the whole area LQN are seen two sets of lines, one repre- 
senting the loci of the center-points and border-points of the orders, 
and the others the center-points and border-points of the subrangs. 
ube lines a*b*c?, a2b7c?, etc., are the loci of the different ratios’ of 
lenad to feldspar and of feldspar to quartz, corresponding to the 
center-points and border-points of the orders. The ordinal lines 
atbtc! to a7b7c7 are broken at the intersections b‘, b?, etc., with the 
orthoclase-nephelite line ON, the lower limiting value of the leucitic 
area, while a°c° to a?®c?® are straight and continuous. This change 
in direction or refraction in passing out of the leucitic area follows 
