6o 
THE ROMAN COMAGMATIC REGION. 
add sufficiently to the femic minerals to carry the rock across the line. This is one 
of several instances of the same kind which occurred in the present investigation. 
This transitional character is also shown by the very close correspondence 
between the analysis of the ciminose and that of the bolsenal vulsinose given in II. 
The difference in position is due partly to the slightly higher FeO and MgO of II, 
as well as to its lower silica and higher alkalis. The two former involved the pres- 
ence of an absolutely greater amount of normative femic minerals, while the two 
latter necessitated the formation of a considerable amount of nephelite from some 
of the soda, and thus reduced notably the relative amount of salic minerals. 
While the effect of such a shifting of the classificatory position through the 
influence of very small amounts of minor chemical constituents may seem to be a 
serious defect in the system of classification, it is in fact not so. For rocks whose 
position can be thus changed are transitional in character, and in the nomenclature 
the change is simply one corresponding to the present case, from ciminose-vulsinose 
to vulsinose-ciminose, and it is a matter of very slight moment in which of the two 
contiguous divisions the rocks strictly belong. 
In the present case it is quite certain that some of the rocks of the region, which 
will be mentioned later, fall in vulsinose rather than in ciminose, so that they belong 
to ciminose-vulsinose, or are even possibly so far from the border as to be well within 
vulsinose itself and not properly transitional. But no analyses of these were made 
and it was not thought advisable or sufficiently important to erect these at present 
into distinct types. 
Mode. The calculation of the mode from the norm is comparatively simple. 
For the augite the composition of that from Ticchiena (cf. p. 134) was assumed, 
for the small amount of biotite, a mixture in equal parts of leucite and olivine. 
After assigning sufficient Na 2 O to form albite in the proportion toward the anorthite 
demanded by the average composition Ab t An 2 , the rest was distributed between 
albite (which enters into the alkali -feldspar) and nephelite, as there was insufficient 
silica to form the polysilicate molecule with all of it. In determining the mode 
by the microscope it was found to be impossible to distinguish accurately between 
the small laths of orthoclase and labradorite, as well as between these and the 
interstitial nephelite, so that the total amount of these is given. 
CALCULATED. 
MEASURED. 
Orthoclase, Or 4 Ab! . 
Labradorite, AbtAn 2 
Nephelite 
Augite 
Biotite 
Magnetite 
Apatite 
65.2 
14-3 
5-4 
8-3 
3-2 
3-3 
Vol. %. Sp. gr. 
85.4 X 2.6 = 222.0 
10.4 X 3.3 = 34-3 
2.1X2.9= 6.1 
2.1 X 5.2 = 10.9 
Wt. %. 
81.3 
12-5 
2.2 
4.0 
273.3 IOO - 
It is seen that the two methods yield very concordant results in this case, the 
error due to overlapping not being serious. Of the two the calculated mode must 
