FORMATION OF LEUCITE IN IGNEOUS ROCKS 275 
gous lines. would be parallel to each other. Thus, the lines LN 
would be parallel to those here given, as would be the lines ON 
parallel to the lines ON of the diagram. It follows from this that the 
acute angle LNO of all possible leucitic areas would be the same, and 
also that the angles LN M?! and ONQ would also be constant, so that 
the inclination of the triangular leucitic areas as regards the abscissal 
line QM would always be the same. 
On the assumption that the kaliophilite molecule is excluded, it is 
also evident that all possible rock magmas must fall within the large 
triangle L'QM, since outside of and above it there will be too much 
potash, as explained on a previous page (p. 272), and the points Q and 
M respectively represent pure quartz, or 100 per cent of silica, and 
pure non-siliceous mineral, or o per cent of silica. 
As regards the-sizes of the various leucitic areas, it is clear that, 
while they decrease absolutely with increase in anorthite or femic 
molecules, yet that relatively to the areas OAM, OQA, LON, LNM, 
and LOM, corresponding in the relative amount of anorthite in any 
given case, they remain constant. This follows geometrically from 
the parallelism of the sides of the various homologous areas. Thus 
the ratio of the size of the salfemic and magnetitic leucitic area L*? 
O7?N?? to the whole area L'?Q1?N’? is the same as that of the per- 
salic leucitic area L'O7N! to the whole area L'Q‘N}, since their 
homologous sides are parallel. 
In general, therefore, the influence of the presence of anorthite 
and of standard, non-aluminous pyroxenes is to lower the percentage 
of potash without greatly disturbing the percentage of silica, or, in 
other words, to raise the ratio of silica to potash for the leucitic areas; 
while the presence of olivine molecules tends to lower this ratio to a 
less extent, the change being nil in the case of magnetite, and also to 
diminish to a much greater degree the percentage of silica in the 
magma, this reaching a maximum with magnetite. 
In consequence of these relations, and those discussed in the pre- 
ceding paragraphs, while with increase in femic minerals the leucitic 
areas extend on the whole toward the non-siliceous end of the dia- 
gram, so that eventually they cover the whole area to the right of the 
tM is used to indicate the locus of magnetite where SIOz=o and K,0=o, as 
explained above. 
