154 THE ROMAN COMAGMATIC REGION. 
it is the only sodic mineral. Of the other lenic minerals, halite and thenardite 
(existing modally in sodalite and noselite or haiiyne) are found only in the persa- 
lanes, and of these only in the sodipotassic subrangs, phlegrose, beemerose, and 
appianose. In the dosalanes and salfemanes they seem to be entirely lacking. 
The femic constituents need less discussion, as they are present in much smaller 
amount than the salic. This is especially true of the persalanes, in which the 
amounts of femic minerals are negligible. Of these it may only be said that acmite 
occurs but once, and that rocks falling in this class are about the only ones which 
show normative wollastonite, this being present generally when the subrang is 
sodipotassic. 
Diopside is constantly present, being absent only in two cases, while norma- 
tive hypersthene is rare, but this last can not, of course, be present in the norm 
along with the lenads. Olivine is almost as constantly present as diopside, and 
is absent only when there is normative quartz or wollastonite. The amounts of 
both of these vary considerably, but increase gradually though irregularly as the 
magmas become more femic. Magnetite, ilmenite, and apatite are always present, 
but in small amounts, and each varies within a very narrow range. 
Relations of Norm and Mode. 
The readjustments of the molecules of the norm to form the modes of the several 
types have been spoken of in each case in the preceding pages, but it will be 
instructive to summarize the main features. 
In leucite-free types the readjustments are almost always of minor impor- 
tance and the modes are essentially normative. In these the augite is the chief 
disturber of the normative relations, this alferric mineral taking some of the norma- 
tive anorthite, and still less of the magnetite, ilmenite, and nephelite, to combine 
with the normative diopside. But as these non-leucitic types are only found in 
the persalanes and dosalanes, where the femic minerals are either negligible or 
subordinate, and as the amount of anorthite involved is only about one-fifth of the 
diopside (and that of the other minerals involved much less), the disturbance is 
seldom serious. It is noteworthy that wollastonite is more apt to be present in 
the norms of the persalane rocks than hypersthene or olivine, and this is probably 
connected with the presence of this mineral in the norm of the augite of the region 
when it is calculated into standard minerals. The high wollastonite in the norm 
of the tavolatal appianose [haiiynitic leucite-tephrite, tavolatite] is of interest in 
connection with the occurrence of garnet in this rock. 
In a few types characterized by the presence of biotite readjustments are needed 
for this mineral, involving normative leucite, olivine, and also anorthite to furnish 
the extra alumina in exact calculations. But here also the divergences from the 
norm are notable only in one or two cases. The reciprocal exclusion of biotite 
and leucite, so commonly observed elsewhere, is also evident throughout the region. 
Though there are a few cases where the two minerals do occur together, as in the 
