FORMATION OF LEUCITE IN IGNEOUS ROCKS 369 
physical conditions during solidification may induce or prevent the 
formation of leucite. 
It has been noted above that the replacement of orthoclase by 
leucite may take place to varying degrees, and when the positions 
of these colored squares are compared with their respective leucitic 
areas, it is found that, as a general thing, the nearer they lie to these, 
the richer they are apt to be in leucite, and the more completely the 
potash has entered this mineral; while the farther away they are, 
the poorer the rock is in leucite. So that it may be said that the 
tendency to form leucite increases with the nearness of the locus of 
the rock to its leucitic area, or as silica and soda decrease and as 
potash increases, other things being the same. 
Indeed, an examination of the published descriptions of the rocks 
which fall well to the left of and far from their leucitic areas reveals 
the fact that the amount of leucite in most of them is very small, 
and that this mineral is made prominent in the name chiefly because 
of its rarity, and of the desire to distinguish varieties containing it, 
even in small amounts, from similar rocks which are free from it. 
These cases would include such rocks as the leucite-banakites and 
shoshonites of the Yellowstone National Park, the leucite-kulaite of 
Phrygia, leucite-tephrites of Bohemia, and leucite-rhomben-porphy- 
ries of Kilimanjaro, and many others, into the details of which it is 
unnecessary to enter here. 
But, even granted the accessory character of the leucite in these 
rocks, they still do not conform to our assumed order of affinity. 
There are also those rocks in which the abnormative leucite forms 
an essential and abundant component; and, furthermore, we have to 
take into consideration the fact that in the rocks with both normative 
and modal leucite the amount of the latter almost always exceeds that 
of the former, so that actually a greater amount of leucite has been 
formed than is demanded by theory. 
In this connection it will be of interest to give a few illustrations 
of such relations of norm and mode, so as to show the extreme varia- 
tions which are possible, and the actual divergencies in some cases. 
In the adjoining table are given the norms of some Italian leucitic 
rocks, in which the amount of modal leucite has been estimated. 
The columns headed ‘‘a” are calculated on the regular assump- 
