FORMATION OF LEUCITE IN IGNEOUS ROCKS 277 
acters as regards anorthite and femic minerals, it should have leucite 
in the mode; while, if it falls outside of this area, it should be modally 
free from this mineral. 
5. From the relative sizes of the leucitic and non-leucitic areas we 
should expect leucitic rocks to be much less abundant than non- 
leucitic ones. 
6. We should not expect the presence of femic molecules in the 
magma to exert much influence on the presence of leucite, while the 
presence of anorthite should tend to lessen the probability of its 
presence, this influence being increasingly felt as the magma becomes 
more calcic. | 
Consequently we could expect to find leucitic rocks with very 
large amounts of femic molecules, but should not expect to find them 
with large amounts of salic lime, that is, when the amount of this 
exceeds that of the alkalies. 
7. Other things being equal, there should be more probability of 
a rock being leucitic the higher the potash, while with increase in 
soda as regards potash the tendency toward the formation of leucite 
should diminish rapidly, so that we should not expect to find many 
leucitic rocks with soda in excess of potash. 
There are given on Plate III the results of the data for persalic 
and peralkalic magmas plotted with the abscissas representing per- 
centages of silica (but with 100 per cent to the right), and with the 
SiO, 
K,0: 
the diagram on Plate I. It is introduced because it is explanatory of 
my previous study of the subject, where this method of plotting was 
adopted, and because it is illustrative of the complexity introduced by 
the adoption of ratios rather than percentages as a basis for plotting, 
while the general relationships are still manifest. A brief descrip- 
tion must suffice. 
The lettering, and the signification of the lines and areas, on 
Plate III are the same as on Plate I; only it must be borne in mind 
that the directions, both up and down and right and left, are reversed, 
and that non-potassic and purely sodic magmas and normative 
minerals lie above the diagram at infinity, a serious distortion being 
thus introduced. 
ordinates representing the ratios Otherwise it is analogous to 
