1 86 
THE ROMAN COMAGMATIC REGION. 
silica percentage, in the non-leucitic than in the leucitic rocks, the division line as 
expressed on the diagram falling slowly as silica increases and rising as silica 
decreases. It is probable that this division line extends an indefinite distance in 
both directions beyond the limits of the critical group, but analytical data are 
wanting to determine this. 
K O 
5. Leucite is most apt to occur in the mode when the ratio 2 is greater 
J\a 2 \_) 
than unity (perpotassic and dopotassic subrangs), the tendency to the formation 
of leucite being proportional to the ratio; leucite is not apt to be formed if the ratio 
approximates to unity (sodipotassic subrangs); and leucite does not form when 
the ratio is less than unity (dosodic and persodic subrangs). 
6. The prohibitive tendency of high soda may be overcome by a low ratio of 
silica to potash. 
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4 5i'O 2 46 48 SO 52 54 56 58 6O 6t 
= Non leucitic rocks 6- Leucitic rocAs 
FIG. 3. 
The more general conclusions which may be drawn are: 
7. The formation of leucite, or its modal occurrence, is directly dependent 
upon the chemical composition of the magma. 
8. The chief chemical factors involved are the percentage of silica, the ratio 
of silica to potash, and the ratio of potash to soda. The relations of the other 
constituents are apparently of subordinate importance, if not quite negligible in 
some cases. 
9. The ratio of potash to soda is of subordinate importance to that of silica 
to potash. 
10. Leucite does not occur in rocks with an excess of silica, or more than 
sufficient to saturate the bases completely and form the most highly silicated mineral 
molecules possible, that is, in magmas where quartz would occur in holocrystal- 
line types. 
