182 THE ROMAN COMAGMATIC REGION. 
resemble those of vulsinose in that some of the types are non-leucitic and some 
highly leucitic, this mineral assuming the form either of large phenocrysts or of 
small individuals in the groundmass. In these, as well as in the preceding cases 
of persalane rocks, it is to be noted that the modal leucite is formed at the expense 
of normative nephelite, leucite not being found in the norms, except in the tavolatal 
appianose. In the closely similar but sodipotassic monzonose (II. 5. 2. 3) rocks 
leucite is not found, except very sporadically in that from L'Arso, while in the few 
rocks which belong to auruncose (II. 5. 3. 2) leucite is uniformly present in abun- 
dance. In the only rocks of this region so far known which belong to shoshonose 
(II. 5. 3. 3) leucite is wanting. 
With the rocks belonging to vicose (II. 6. 2. 2) we meet leucite both in the 
norm and in the mode, and from here on to the end of the series leucite is a constant 
and important constituent, no non-leucitic types of vicose (II. 6. 2. 2), braccianose 
(II. 7. 2. 2), vesuvose (II. 8. 2. 2), or of the salfemane subrangs jugose (III. 7. 
2. 2), fiasconose (III. 7. 3. 2), or albanose (III. 8. 2. 2) being known in the Roman 
Region. 
From these facts we may draw the following conclusions as to the formation 
of leucite in effusive rocks, in the Roman Region at least : It will not be formed 
in rocks belonging to quaric orders, or those with an excess of silica (norm- 
ative quartz). It is not liable to be formed in sodipotassic magmas, especially 
when of the dosalane class and in persalanes when the norm is perfelic. But this 
prohibitive influence of the abundance of soda is counteracted by paucity in silica, 
as in beemerose, procenose, and appianose rocks, whose norms all show notable 
amounts of lenads. 
In magmas which belong to perfelic orders and to dopotassic subrangs, whether 
in persalane or dosalane, as vulsinose (I. 5. 2. 2) and ciminose (II. 5. 2. 2), leucite 
may or may not be formed, and these magmas seem to be in a somewhat critical 
condition as regards the occurrence of this mineral. The occurrences are all 
extrusive lava flows and no constant differences in the conditions obtaining during 
solidification can be made out for the leucitic and the non-leucitic flows, nor can 
any definite order of succession be established. From the chemical side the analy- 
ses at hand would lead us to suppose that leucite will not be formed if the silica 
is above 56, assuming that 8 or 9 per cent of potash is present, though it can form 
with this amount of silica if the potash percentage runs above 10 (with about 3 or 
4 per cent of soda). Stated in other terms, if no nephelite, or only i or 2 per cent, 
is found in the norm no leucite or only sporadic crystals will occur in the rock. 
In the subrangs of dosalane which show notable amounts of nephelite in the 
norm and in the similar ones of salfemane, all of these being dopotassic, leucite 
invariably occurs in the mode. 
Although the number of chemical factors possibly involved is large, and the 
possibilities of mutual readjustments between normative mineral molecules, as 
between hypersthene and olivine, are therefore complex, it would seem that the 
