THE FORMATION OF LEUCITE. 187 
11. The presence in the norm of such mineral molecules as nephelite or olivine, 
which are capable of being more highly silicated, is a necessary condition for the 
formation of leucite if there is no normative leucite. Of these nephelite seems to 
be the most important, modal leucite being usually formed by a readjustment of 
silica involving the formation of modal albite from normative nephelite. 
12. When leucite is found in the norm of a rock it is also present in the mode. 
13. It is probable that the formation of leucite anticipates and determines the 
distribution of other mineral molecules, as those of Na 2 O in albite (as modal plagio- 
clase rather than nephelite, or as (Fe,Mg)O in hypersthene (as modal augite) 
rather than olivine. 
Returning for a moment to the diagram, and anticipating part of a theoretical 
discussion which will be published elsewhere, it may be pointed out that the line 
A-B represents the limiting value of the ratio ^r-^ for persalic magmas, in which 
Jv 2 U 
leucite would be present in the norm (and hence also in the mode according to 12 
above), on the assumption that the affinity of potash for silica is greater than that 
of soda. For a given silica percentage, if the ratio in question falls below this line 
the rock will be leucitic. If the ratio is above this line for any given silica per- 
centage the rock may be leucitic actually, but is not necessarily so, the tendency 
to the formation of leucite in the mode decreasing rapidly as the silica-potash ratio 
rises above the line. 
It appears from this that the so-called "critical group" is, in reality, one of 
rocks whose silica-potash ratios fall very close to the limiting curve for the forma- 
tion of leucite, and that the apparently sensitive or nearly balanced chemical con- 
dition of their magmas, where very slight changes in the physical or physico-chem- 
ical conditions would either bring about or prevent the formation of leucite, is 
presumably due to this fact. 
The line C-D indicates the limiting values in persalic magmas of the possible 
formation of leucite due to deficient silica. Below it for any given silica percentage 
there is an insufficient amount of silica for even leucite and nephelite, and hence 
such magmas would seem to be incapable of existence, unless such a mineral as 
kaliophilite (K 2 O.Al 2 O 3 .2SiO 2 ), were present. It is interesting to note that the 
only two analyses which fall below it here are Nos. 42 and 44 on page 146, the rocks 
corresponding to which contain notable amounts of akermanite in the norm, and 
of modal melilite in one case at least. 
