FORMATION OF LEUCITE IN IGNEOUS ROCKS 385 
percentage of potash is that of a mixture of orthoclase with the given 
amount of the other constituents mentioned. From this most favorable 
maximum the range in silica, and consequently the probability of the 
formation of leucite, will diminish rather slowly with decreasing potash, 
while, at the same time, the probability of the rock being non-leucitic, 
as well as the possible number of rocks, will increase. On the other 
hand, only leucitic rocks can be expected with higher potash than this 
amount, but their range in silica and the probability of their occurrence 
will diminish rapidly with increasing potash. 
Similarly, the greatest range in potash, and hence the greatest 
probability of the formation of leucite, will be found when the silica 
percentage is that of a mixture of leucite with the given amount of 
anorthite and femic molecules. From this most favorable maximum, 
the range in potash and the probability of the formation of leucite will 
diminish slowly as silica decreases, but more rapidly as silica increases. 
While the maximum ranges of silica and of potash, expressed in 
percentages of the whole rock are about the same (for example, 
12.54 and 12.00 respectively in peralkalic persalanes), yet relatively 
to the amounts of each present the possible range of potash is much 
greater than that of silica. So that we may expect to find the potash 
range of leucitic rocks very great, while that of silica will be compara- 
tively small. 
If leucite is present in the norm of a rock, it should also be present 
in the mode, provided that the crystallization is complete and the 
magma has solidified under effusive conditions. The formation of 
leucite in such normatively leucitic magmas may, however, be, and 
often is, wholly prevented by the presence of femic constituents in 
large amounts, giving rise to the modal presence of alferric minerals 
with highly complex constituents, such as biotite, hornblende, augite, 
or melilite, the potash being incorporated in these by their mass action. 
The presence of abundant soda may also prevent the formation of 
leucite by the incorporation of the potash in the complex nephelite 
molecule. Both of these effects are most apt to occur if the rock has 
solidified under intrusive conditions. Leucite should therefore be 
rarely met with in highly femic rocks, as the dofemanes and per- 
femanes, as well as in those which are highly sodic, both of these 
characters implying, of course, that the magma is not very potassic.. 
