50 The American Geologist. •'" Iy - 1: "' :: - 
to which few will take exception. In carrying out this plan 
the principal rock-making minerals are divided into two 
groups consisting, on the one hand, mainly of the more highly 
siliceous, alkali- and calci-alnminous ones and, on the other, 
of the ferro-magncsian ones, the first group being known as 
salic and the second as femic. For the purpose of completely 
classifying a rock by this system, it is, therefore, necessary to 
know its composition, as determined by chemical analyses, or, 
if holocrystalline, by optical methods. 
Recognizing the fact that certain magmas may crystallize 
into quite different mineral combinations, according to attend- 
ant conditions, the authors have found it necessary to select 
a certain set of these salic and femic minerals as uniform 
standards of comparison, the standard mineral composition of 
the rock being called its norm, while its actual mineral com- 
position, which may or may not be different from the norm, is 
called its mode. 
Starting with these groups, the rocks are divided into five 
classes, according as one or the other of the groups of stand- 
ard minerals constitute the norm or is extremely abundant, 
whether one or the other is dominant, or whether the two are 
in about equal proportions. The classes thus formed are di- 
vided into orders, which are based on the proportion of quartz 
to feldspar and feldspar to feldspathoids, or that of silicate to 
non-silicate minerals, accordingly as salic or femic minerals 
preponderate. Orders are again divided into fangs, which are 
founded on the chemical nature of the bases in the preponder- 
ant salic or femic group, and rangs into grads, which in their 
turn are based on the general acidic proportions of the sub- 
ordinate minerals. Each of these divisions may be further di- 
vided, giving rise to sub-orders, sug-rangs, and sub-grads, if 
occasion demand. 
In searching for a nomenclature the authors have adopted 
one which is necessarily entirely new and it must be confessed, 
not at all times quite pleasing at first glance. As proposed, 
this is polynomial and based upon the very sensible plan of 
conveying an idea of the character of the mass to which any 
name may be applied. To this end names are given which 
convey a definite cenception of magmatic and consequent 
norm or standard mineral composition ; these are then qualified 
