CLASSIFICATION OF IGNEOUS ROCKS 555 
In order to simplify the expression of these divisions they are 
printed with prime marks as follows: 1, 1’, ’2, 2, 2’, ’3, 3, 3’, etc. 
A rock may belong to I, 2’, ’4, 3, or I’, 2, ’2, 3’. For convenience 
in determining the position of any magma with respect to these 
divisions a scale of ratios is given in Fig. 1. Magmas belonging 
to intermediate divisions may be described in terms of the varia- 
tions indicated by the symbols; as, for example, I’, 3, 2’, 3 (teha- 
mose) is a femic, calcic tehamose; more precisely, a more femic, 
more calcic tehamose than the central varieties, since these also 
contain femic and calcic components. 
Transitional magma names.—It was pointed out in the first 
publication of the Quantitative System, p. 166, that a rock whose 
magma belongs so near a boundary line between named divisions 
that it should be considered a transitional variety should receive 
a compound magma name, made by uniting the names of the two 
divisions concerned and connecting them with a hyphen. The 
name of the division within which the magma belongs is placed 
last. This has been done in numerous instances, but no definite 
statement has been made as to the limits of the magmas that may 
be called transitional. It is here proposed to fix the limits half-way 
between the boundary between named or numbered divisions, and 
the boundaries of the intermediate divisions described in the pre- 
ceding paragraphs. This conforms to the general principles of 
divisions in the Quantitative System and establishes a still smaller 
division of the petrographic series. It does not appear desirable 
to extend the use of compounded names to the whole range of 
intermediate magmas, which are sufficiently designated by symbol. 
In case a rock is transitional in more than one respect, as in 
subrang and in rang, or in rang and order, or in class, order, and 
rang, or in all possible respects, it is necessary for convenience to 
select two magmatic names to form the compound name, and 
express the remaining transitions by descriptive terms or symbols. 
Since the magmatic name commonly employed in designating a rock 
is that of its subrang, the name to be compounded with it should 
be that of the subrang toward which it is transitional through the 
nearest classificatory division in which it is transitional. Thus, 
in the case of a rock whose symbol is II.(3)4.3(4).4; that is, one 
