FORMATION OF LEUCITE IN IGNEOUS ROCKS 259 
In a paper on the rocks of the central Italian volcanoes? the rela- 
tionships of the leucitic and non-leucitic rocks, and the chemical 
characters of their respective magmas, were discussed with the view 
of throwing light on the chemical characters which controlled the 
formation of leucite in this region. Some general conclusions were 
reached, which will be referred to subsequently, and some of which 
seem to be of wider applicability. The present paper, as was men- 
tioned above, is an extension of this study, the field being enlarged 
to include all known leucitic rocks. The theoretical limitations of 
leucitic norms are first determined mathematically on the basis of 
certain assumptions, and these are afterward compared with the 
actual facts as revealed by a study of the rocks themselves. 
A great deal of work has been done in recent years by Morozewicz, 
Vogt, Doelter and his pupils, Day and Allen, and others, in deter- 
mining the physico-chemical conditions involved in the formation of 
many of the rock minerals. While this work is of great value and 
importance, and highly suggestive in many ways, little reference will 
be made to it in the present paper, in which the more essentially 
chemical factors will be the main subject of investigation. This is 
partly because such an extension of the scope of our inquiry would 
unduly lengthen the paper, and partly because it is felt that such 
physico-chemical investigations, are not yet so complete and so de- 
tailed, especially as regards leucite, as to make their introduction, 
and the conclusions to be derived from them, satisfactory. They 
may and will be invoked in the future for the explanation of many 
of the facts set forth in the subsequent pages, but such an application 
here seems to be unjustified and rendered merely tentative by the 
paucity of our knowledge at the present time. 
THEORETICAL DISCUSSION 
While it is now well recognized that a given magma solidifying 
under different physical conditions may form very diverse mineral 
combinations, within the limits imposed by the chemical composi- 
tion, yet for any given set of physical conditions it would seem that 
the relative affinities of the chemical constituents of the magma for 
1H. S. Washington, ‘‘The Roman Comagmatic Region,” Carnegie Publication 
No. 57 (1906), p. 181. 
