CLASSIFICATION OF IGNEOUS ROCKS 53 
omission of sections of rangs. Those divisions previously called 
sections of rangs will become rangs with the appropriate change in 
the suffix. That is, the name hitherto used for the section of rang 
is to be retained as the name of the rang of the corresponding 
number, 1, 2, 3, etc., and 7 dropped from the suffix. Thus in 
Class IV, Order 2, Section 2 (paoliare), Rang 1 will be valbonase, 
and Rang 3, paolase. In Section 3, texiare, of the same order, 
Rang 1 will be marquettase; Rang 2 uvaldase. The name texase 
will be dropped unless subsequently applied to either Rang 3, 4, 
or 5 of this Section 3. The subrangs remain as first described 
and named. 
Since in Classes II and III subgrads are formed on a basis of the 
chemical characters of the femic minerals, the process of determining 
them is to be modified in accordance with the method just described 
for determining rangs in Classes [Vand V. That is, subgrads are to 
I vr 
be determined by the ratio Vo ape y sections 
MgO 
HeO>. (Na.O 4K)” 
ceptional cases in which there is a notable amount of femic 
alkalies, subsections of subgrads are to be determined by the 
FeO 
Na,O”-+K,0” * 
Notation of divisions and symbols.—In order to avoid confusion 
in the symbols for various divisions in the system, it has been found 
advisable to number threefold divisions, when used, in conformity 
with fivefold ones; thus, 1-2, 3, 4-5; since the first division com- 
prises 1 and 2 of the fivefold; the second division corresponds 
exactly to 3 of the fivefold divisions; and the third division com- 
prises 4 and 5 of the fivefold divisions. This method has already 
been employed in a number of publications’ since the first appear- 
ance of the system in 1902. It is also advisable to print the num- 
bers of subclasses, suborders, and sections so as to distinguish 
them from the numbers of regular divisions by placing them below 
the line rather than above it, thus: 
and in the ex- 
of subgrads, by the ratio of 
ratio 
«H. S. Washington, ‘‘The Superior Analyses,’ etc., U.S. Geological Survey, 
Prof. Paper 28, 1904; J. P. Iddings, Igneous Rocks, Vol. I, 1909, and others. 
