12 THE ROMAN COMAGMATIC REGION. 
In the terms of the quantitative classification the preceding magmatic divi- 
sions, as far as subrang, are represented in the Roman Region. They include 3 
classes, 10 orders, 14 rangs, and 17 subrangs. There are probably a few more 
which are not discussed in this paper, but the amounts of these are probably small, 
and those given here undoubtedly embrace the great majority of the most impor- 
tant and most abundant of the Roman magmas. 
DESCRIPTIONS AND NAMES OF ROCKS. 
The rocks will be described under the heads of the various types to which they 
belong, a type being understood to mean a standard rock with a certain assemblage 
of characters, accurately and quantitatively described as regards the chemical com- 
position, norm, mode, texture, and the textural disposition of the constituent min- 
erals. Rocks will be considered to belong to the same type when they are quite 
or almost identical in these respects, so much so that they may be mistaken for one 
another, or might be parts of the same mass, as suggested in the publication of 
the Quantitative System (p. 179). As a practical matter the identity can not be 
absolute in all cases, and a certain amount of latitude, or very slight divergence 
from the type, must be allowed. Otherwise the number of types will be almost as 
great as the number of the occurrences or of the hand specimens collected. This 
latitude is only reasonable, and that it is allowable is shown by the practice in 
botany, where, for instance, individual plants are referred to the same species, 
although there are slight differences in minor and unimportant points, such as the 
lengths of the leaves, the heights in different habitats, etc. 
The name of a rock, then, according to the quantitative system, and as applied 
throughout this paper, will be considered to be the type name that is, the name 
of the subrang qualified by a typal adjective or one indicating the type to which 
the rock belongs. This typal adjective is formed according to the published sug- 
gestion by the use of a root derived from a geographical locality and the termina- 
tion al. Such a name expresses as concisely as possible, and without ambiguity, 
or at least with only very narrow limits of variation, all the inherent characters of 
a rock, the subrang name indicating its chemical character and systematic position 
in the classification, and the type adjective its color, modal and textural charac- 
ters, the textural disposition of the minerals, etc. 
For the purpose of correlation with the prevailing systems, and for the benefit 
of those who are unacquainted or unfamiliar with the quantitative system, the names 
by which the rocks would be called in the prevalent nomenclature are also used 
throughout the paper, in brackets [ ], while the symbols of the subrang will be placed 
in parentheses ( ) when it is deemed advisable to add them to the quantitative 
rock name. But it must be remembered that these symbols do not form any part 
of the name, and are used only to indicate the magmatic position on account of the 
unfamiliarity of the new terms. 
In assigning the names of the prevailing nomenclature, the usage of Zirkel in 
