THE FORMATION OF LEUCITE. 185 
the diagram, which is drawn from data furnished by a theoretical discussion of 
the subject and which will be referred to later. 
The relations of the potash-soda ratios are not so clear, especially in the critical 
group, where they are quite involved, those of leucitic and non-leucitic rocks being 
both above and below each other. But the tendency to high figures as silica 
decreases and as leucite becomes constant, and to low figures as silica increases 
and as leucite vanishes, is very marked, and seems to admit of no question. 
In regard to the so-called critical magmas, the possibly significant fact may 
be pointed out that its limits of variation in the percentage of silica are practically 
conterminous with those shown by the analyses of natural leucites, which are 54.4 
and 57.2 in the better analyses.* The best analysis of the leucite of Vesuvius,f 
with SiO 2 = 55.4o and =^=4.44, falls just beneath the center of the critical 
S'O 
group, while the theoretical leucite molecule, with SiO 2 = S5.o5 and =7^=4.00, 
lies a trifle to the left of this. The positions of the two are indicated respectively 
in the diagram by a small cross (+) and by a star (*). The position of the average 
magma of the region is indicated by the sign X . 
Conclusions. 
From the facts set forth in the preceding pages the following conclusions as to 
the relation of the magmatic composition to the occurrence of leucite in effusive, 
holocrystalline, igneous rocks can be drawn. They seem to hold good for the 
Roman Region, but whether they are of more general application or not must be 
determined by future investigation. The more special conclusions are first given, 
followed by those of a more general character. 
1. Leucite forms invariably in magmas with SiO 3 less than 52 . 50 per cent, 
the ratio ^--^ being lower than 13 .o, though it may be higher than this. (Analy- 
Jv 2 (J 
tical data are wanting in regard to this last point.) 
2. Leucite is invariably absent, or present only very sporadically, when the 
silica is above 56 . 5 and with the ratio ^-^ at least 10 or higher, the upper limit 
J\. 2 O 
being infinity. (Analytical data are wanting for ratios considerably lower than 10.) 
3. In magmas with silica varying from 54.7 to 56.3, or those in which the 
Q */~\ 
silica percentages are approximately those of the natural leucites, * varying 
from 20 to less than 10, leucite may or may not occur, and these magmas may be 
regarded as critical ones. 
Q" f^\ 
4. In the critical magmas the ratio ^r ^ is uniformly higher, for any given 
Jv 2 L) 
* Cf. Dana, Mineralogy, 1892, p. 342, and Hintze, Mineralogie, II, 1895. P- 13 " 
t G. Steiger, analyst, Bull. U. S. Geol. Surv. No. 220, 1903, p. 3- 
