THE NAMING OF ROCKS 689 
Influence of rock classification on naming of rocks—The broad 
question of rock classification I do not intend here to discuss. 
However, it is necessary to mention some of the criteria which 
are recognized in the classification"o0f rocks, since these are also 
factors in the naming of rocks. By most petrographers, chem- 
ical composition, mineral constituents, and rock textures and 
structures are controlling considerations; but by different 
petrographers these are placed in different orders of importance. 
By some petrographers the distinction between plutonic and 
volcanic rocks is given weight in classification. In making a 
classification of rocks it is necessary to consider to what extent 
the altered rocks shall be recognized, and whether such textural 
terms as porphyry and obsidian shall be used in naming rock 
species. Doubtless’ different petrographers would decide these 
points in various ways. 
Now the above factors, which are controlling considerations 
in the classification of rocks, are of necessity secondary factors 
in the naming of rocks. If a new rockibe found, which, in 
regard to these secondary factors, is so different from any pre- 
viously known rocks that it cannot be grouped with any of them 
by the plan given below, it may be entitled to a new family 
name, even if not abundant. : 
But to return to the matter of abundance. After a system 
of classification shall have been worked out by a petrographer, 
he must decide what rocks shall have independent names. It is 
my contention that at this point abundance shall be recognized 
- as having the place of first importance. Names are tools by 
which we avoid the circumlocutions of descriptions. Since, as 
already shown, there are everywhere gradations between rocks, 
not every phase of rock can have an independent name, else the 
number of names would be infinite. Since fevery rock phase 
cannot be assigned a name, what kinds shall be selected for 
such names? Manifestly those which occur most abundantly. 
For the common things, the common kinds of rocks, as a matter 
of convenience, I repeat, we must have names. 
As above noted, it has been supposed that a rock might be 
