278 HENRY S. WASHINGTON 
The line LN is that of pure lenad, leucite and nephelite, pure 
leucite being found at L and pure nephelite where the curve touches 
its asymptote’ at infinity and where SiO,=o0.704. ‘The line OA is 
that of pure feldspar, orthoclase being found at O, and albite where 
the curves become tangent at infinity with its asymptote at SiO, = 
1.145. The curve LO is that of perpotassic magmas (leucite- 
orthoclase), and is continuous with the curve OQ, that of quartz and 
orthoclase, quartz being found at the point where the curve touches 
its asymptote at infinity and where SiO, =1.667. The curve ON’ 
is that of orthoclase and nephelite, orthoclase being found at O and 
nephelite where the curve touches its asymptote at infinity and where 
SiO, =0.704, N and N’ coinciding here. 
The leucitic area is therefore bounded by the curves LN, LO, and 
LN?:, N and N? rising and approaching each other with decreasing 
potash till they coalesce as just explained. The area N‘OA is that 
of orthoclase-albite-nephelite; and AOQ is that of quartz-orthoclase- 
albite, both of these extending upward to infinity. The whole area 
NLOQ, therefore, embraces all possible magmas, since outside of it 
no norms can exist, as already explained. 
The ordinal curves are shown running down the diagram from 
top to bottom, and with well-marked cusps where they meet the ortho- 
clase-nephelite curve N‘O, the angle of the cusp decreasing as the 
magma becomes more lenic. This change in direction is due to 
causes already explained. ‘The curves representing subrangs run 
across the diagram from left to right. They are all continuous, but 
become successively steeper with increase in soda, for reasons referred 
to in a previous page. 
Introduction of anorthite and of various femic minerals yield 
exactly similar homologous curves and areas, which have all been 
plotted, but which are omitted here. ‘Their influence is quite analo- 
gous to that observed on Plate I]. Thus the introduction of anorthite 
causes the curve LO to rise and shift successively toward the left with 
increasing lime, while the curves NL and N'O are shifted to the right, 
but remain very close to each other. 
But further description is unnecessary, and it may merely be 
« The positions of these fixed asymptotes are indicated by the short straight lines 
at the top of the diagram. 
