IIo NEW YORK STATE MUSEUM 
supposed criteria and learn how to use them. One must discovei, 
or learn how to apply, new criteria—the criteria that aid in 
interpretation. 
How shall I determine that a rock is igneous in origin if I do not 
know that fact already from some other circumstance or relation : 
It is not an easy thing to do, yet most petrographic work starts with 
that as an assumption. 
How shall I determine that the minerals I now see in the 
makeup of the rock have been in part introduced long since the rock 
was first made? It is not an easy thing to do, but it may well be the 
most important thing about the whole history. 
How shall I determine that the ore values that I can see in a por- 
phyry copper ore are primary or secondary? And, if they are mixed, 
as they well may be, what are the evidences of their distribution ? 
How shall I determine that a specimen of rock has come from a 
formation that has been deformed and recrystallized and invaded by 
igneous material and subsequently mineralized and secondarily 
enriched until it is a workable ore? This is much more important 
than its name. 
How shall I determine that the former constituents have been 
replaced, that the composition the rock now has is wholly new; and, 
if I can determine that, how shall I ever find out what the original 
was? 
In a historical direction, and for practical and applied purposes, 
these are some of the things that one must find ways of doing. 
These and many other questions one will sometimes solve. These 
and many others one will many times fail to solve. Sometimes, no 
doubt, the fault or cause of failure is lack of sufficiently critical 
training in observation; sometimes it is an incomplete command of 
the principles of geologic processes; sometimes it is little facility in 
geologic reasoning or too great facility in loose reasoning ; and some- 
times it is a lack of properly standardized criteria. 
Perhaps, therefore, a word about these needs would not be out of 
place. The last of these reasons,— the lack of properly standardized 
criteria,— is a strictly petrographic question; the others are in 
large part individual and personal. 
Difficulties and Needs 
I suppose that every rock has impressed on it, and to some degree 
it preserves, traces of its origin and every subsequent stage in its 
history. The difficulty is in reading these traces and interpreting 
