THE FORMATION OF LEUCITE. 
Although it is now well recognized that a magma of a certain composition 
may crystallize as very diverse aggregates, the variations depending largely if not 
entirely upon the conditions obtaining during solidification, it is nevertheless true 
that the chemical composition of the magma controls the mineral composition of 
the rock within limits. Without going into a general discussion of this interesting 
topic, the special case of the relation between the occurrence of leucite in igneous 
rocks and the composition of the magma may be investigated. 
As to the occurrence of this mineral the only general "law" which has yet been 
recognized is that leucite, as well as nephelite and melilite, are never found in asso- 
ciation with quartz,* that is, in rocks with an excess of silica. It is also generally 
understood that it occurs most frequently in rocks which are rich in potash, but 
the limitations of this term are very vague, and it is known to occur in rocks whose 
potash content can not be called high. 
For the study of this relation from the side of actual occurrences the Roman 
Region is especially favorable. The number and variety of its leucitic rocks are 
great and they have a wide range in chemical composition; both leucitic and non- 
leucitic rocks occur here of similar as well as of different chemical compositions; 
and the disturbing factor of differences in physical conditions during solidification 
is almost entirely eliminated, or at least minimized, by the uniform occurrence 
of the rocks as effusive lava flows. We may, then, examine the rocks belonging to 
magmas of the various kinds as to the presence of leucite, drawing such conclusions 
as we may from these data. The full theoretical discusssion of the problem will 
be published elsewhere, probably in the Journal of Geology. 
The Roman Region. 
Among the persalane rocks of the Roman Region leucite may be said to be 
non-existent in those which belong to the sodipotassic subrang phlegrose (I. 5. i. 3), 
though it occurs very sporadically in the blocks of cumal phlegrose at the Vico 
Volcano. The mineral begins to appear in notable amount in the dopotassic 
vulsinose rocks (I. 5. 2. 2), many of which are highly leucitic, with the viterboid 
habit, though the majority of the types of this subrang are non-leucitic. The same 
is true of the rarer sodipotassic pulaskose (I. 5. 2. 3) rocks, while in the types 
belonging to beemerose (I. 6. i. 3), procenose (I. 6. 2. 3), and appianose (I. 7. 
i. 3) leucite is a constant and important constituent. 
In the dosalanes it is, of course, absent from the sorianal harzose (II. 4. 3. 3), 
as there is an excess of silica in this magma. The ciminose (II. 5. 2. 2) rocks 
* Cf. F. Zirkel, Lehrbuch der Petrographie, Leipzig, 1893, Ii P- 646. 
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