CLASSIFICATION OF IGNEOUS ROCKS 557 
which is in Order 4, near 3, and in Rang 3, near 4; the name for 
the subrang is tonalose, but as it is near Rang 4 it should be called 
bandose-tonalose; and the transition toward Order 3 should be 
expressed by describing it as extremely quaric bandose-tonalose. 
If it were transitional toward Class I, it would be extremely salic; 
if toward Class III, extremely femic—the term extremely signifying 
that it is near the limit of the division with respect to the factor 
named, the actual amount being small in some cases. A scale of 
ratios to be used in determining the limits of transitional divisions 
is shown in Fig. 2. 
In order to express in the symbol that a rock has a transitional 
position in the quantitative classification, the number of the divi- 
sion toward which it is transitional is to be placed in curves, thus 
AIG )e27(3)3, or HES. 3.4(5).- 
Experience has already shown that it is eminently undesirable 
to base subrang or other names on occurrences of transitional rocks. 
Changes in the norm.—It has been found advisable to omit the 
calculation of normative sodalite and noselite on a basis of the 
Cl and SO, in rock analyses, because these substances are such 
small components of sodalite and noselite molecules that any error 
in the assignment of Cl or SO, to these molecules may make a 
large error in the amount or normative noselite and sodalite. 
Moreover, there are difficulties in the analytical determination of 
Cl and SO, on the one hand, and they may occur in quite different 
mineral compounds on the other. The method of calculating the 
norm has been modified, therefore, by allotting to Cland SO, sufficient 
Na,O to form NaCl and Na,SO,, and including these compounds 
among the salic components. This modification has been incor- 
porated in the statement of the Quantitative System in Vol. I of 
Igneous Rocks by one of the authors. 
The rule with regard to the allotment of CaO and TiO, to 
form titanite or perovskite is to be modified to suit those cases in 
which there is no excess of CaO after calculating anorthite; that is, 
when there is no femic CaO. In the ordinary case in which titanite, 
or perovskite, is formed there is femic CaO available, but there are 
magmas, not commonly met with, in which anorthite is associated 
with abundant rutile, as in certain rutile-bearing pegmatites. 
