FORMATION OF LEUCITE IN IGNEOUS ROCKS ela) 
tive leucite is an accessory are quite calcic, belonging to the second 
and third rangs. 
The influence of this factor just discussed is also seen in those rocks 
which carry both normative and modal leucite. In all of these, 
with one exception, where the mode has been adequately estimated, 
as in the Italian lavas, the amount of modal leucite surpasses that 
in the norm, so that a considerable portion of it is really abnormative. 
The exception is the missourite,* in which the amount of modal leucite 
is less than that in the norm, the difference being attributable to the 
formation of some biotite under the intrusive conditions. 
On a previous page (371) it was suggested that the supersession 
of the affinity of lime and potash for soda over that of potash for 
silica could take place ‘“‘under certain conditions.’ An idea of what 
these conditions may be is furnished by the long-recognized fact that 
leucite is characteristically a mineral of effusive igneous rocks. That 
it sometimes occurs in intrusive bodies is now well established by 
occurrences in Montana, Arkansas, and Brazil. It may be noted, 
as bearing on the problem, that there is only one known intrusive 
rock, missourite, in which purely potassic and unaltered leucite is 
found. In the others, as the fergusite and leucite-shonkinite of 
Montana,”? and the leucite-syenites and leucite-tinguaites of Brazil 
and Arkansas, the “‘leucites”’ are really pseudo-leucite, a mixture of 
alkali-feldspars and nephelite.3 
But such occurrences of leucite are quite exceptional, and the 
generally recognized law may still be considered to hold good, that 
conditions which control during the solidification of effusive rocks 
favor the formation of leucite, while those obtaining in intrusive bodies 
tend to prevent it, the chemical composition of the magma admitting 
of the readjustments of silica necessary for the formation of leucite. 
In this connection a few analyses will be worth quoting, which 
show these relations clearly. Others might be added,+ but those 
tL. V. Pirsson, Bulletin No. 237, U. S. Geological Survey (1905), pp. 118, 119. 
2L. V. Pirsson, op. cit., pp. 106, 84. 
3 Cf. Rosenbusch, Mikroskopische Physiographie, Vol. II, first half (1907), pp. 
196, 617. 
4A similar comparison of analyses of leucitic and non-leucitic rocks of almost 
identical chemical composition is given by Lacroix (Comptes Rendus, Vol. CXLI 
[1905], p- 1190). 
