106 NEW YORK STATE MUSEUM 
to be regarded as a rock type at all, may carry more readable mean- 
ing than all the rest of the surrounding material. An ore or a vein 
matter or a contact product are as suitable material for the petrog- 
rapher as are the igneous rocks or the sediments. 
For. this kind of petrology the term rock requires a revised 
definition. 
A rock is anatural mineral aggregate of sufficiently definite com- 
position and character to be representative of some structural unit 
or of some process or condition, to justify separate consideration in 
arriving at a working understanding of the life history and meaning 
of the physical unit to which it belongs. 
How large these units may be, or how small, depends on the detail 
of the study and the significance of the material in contributing to 
the solution of the problem involved. 
Any petrographer who has tried to make practical use of his 
petrology and of his own skill in any more systematic or connected 
way than that of simply naming or classifying separate fragments 
of rock, knows that it is very easy indeed to overstate the require- 
ment of constancy of mineral makeup. ‘Two pieces taken from the 
same physical unit in close proximity, often vary enough to require 
classification as quite different rocks. In many comparatively sim- 
ple igneous bodies this variation is, in many cases, still greater and a 
whole series of different types could be secured. If one is content 
with classification, such classification as we now have, these condi- 
tions may easily lead to confusion. There is no indication in the 
terms usually used of the fact that these varieties are all one geo- 
logical unit of a very simple history and meaning. Emphasis on 
these classification differences, therefore, may be very misleading. 
Many difficulties have arisen where the geologist or the engineer 
has tried to find in the field as many structural units as the petrog- 
rapher who described the rocks called for. 
The fault has been, in the past at least, that the petrographer dealt 
largely with mechanical distinctions, whereas the field man had to 
deal with genetic relations. The time has come now, surely, when a 
real petrographer must also be a field geologist, or he must at least 
appreciate the point of view of a field investigator. 
The Object 
In the past our best petrographers have lent the weight of their 
influence and the fruits of their labors to the field of tabulated detail 
and to niceties of discrimination and to schemes of classification, as 
