i8 THE ROMAN COMAGMATIC REGION. 
DETERMINATION OF MINOR CONSTITUENTS. 
The remarks above lead to the consideration of another matter of some 
importance which is clearly brought out in the following pages, namely, the impor- 
tance in the quantitative system of determining the chemical constituents which 
are usually regarded as minor, or at least some of them. It will be observed in a 
number of cases that the classificatory position of a rock is changed by the deter- 
mination of and introduction into the calculation of some constituent, as TiO 2 or 
P 2 O S , which is present only in very small amount, sometimes less than one-half of 
i per cent. Thus the arsal vulsinose-ciminose [vulsinite] of Vetralla was formerly 
placed in vulsinose, but the recent determination of 0.61 TiO 2 and 0.17 P 2 O 5 shows 
that it is properly in ciminose; similarly, the determination of 0.77 P 2 O S has shifted 
the " leucite-tephrite " of La Scala from vesuvose into braccianose. Such cases are 
to be expected, and while they may be considered by some to be a practical defect 
in the quantitative system in demanding what may seem to be ultra-refinements in 
the chemical analysis, they can not justly be regarded in this light. This has been 
briefly discussed elsewhere,* and it was pointed out that it is "not an objection, but 
really a very strong point in favor of the new system. For it postulates as funda- 
mental to the classification the absolute necessity for only the best class of analyti- 
cal work." And it may be said in addition, though it can not be held as an excuse 
for, or as justifying, incomplete analyses, that changes thus brought about by the 
determination of these minor constituents will usually happen only when the rock 
is transitional as to magmatic position. 
In the present case, however, some of the rocks involved are of such a char- 
acter that the determination of small amounts of TiO 2 , P 2 O S , etc., or slight inac- 
curacies in the determination of the main constituents, as silica or the alkalis, will 
bring about very serious changes in the magmatic position. These rocks are those 
which belong to the more lenic orders of dosalane, braccianose and vesuvose, and 
to the salfemanes. In the present region these rocks are leucitic without exception; 
that is, are dopotassic and with a deficiency of silica. On account of the silica 
relations of orthoclase and leucite with 64.75 an d 55-5 P er cen t of silica respectively, 
a very slight difference in the amount of silica or the alkalis, or in other compo- 
nents, may bring about very decided changes in the norm, owing to the readjust- 
ments of potash between the polysilicate orthoclase, KAlSi 3 Os, and the metasili- 
cate molecule, KAlSi 2 O6- 
In other words, the limits of the dopotassic subrangs of these lenic orders are 
very narrow. This is very clearly shown by Iddingsf in his diagram of the limits 
of orders and rangs for theoretically pure potash-salic rocks. In this the uniformly 
smaller area of the potassic subrangs, as compared with the purely sodic ones, and 
especially the extremely small dimensions attained by the potassic divisions as the 
silica decreases and the amount of the lenad leucite rises, are very clearly brought 
* Washington, Prof. Paper U. S. Geol. Surv. No. 14, 1903, p. 44. 
t Iddings, Prof. Paper U. S. Geol. Surv. No. 18, 1903, p. 72 and Plate II. 
