II2 NEW YORK STATE MUSEUM 
Its Service 
This kind of petrology has already earned a place for itself in 
studies of ore deposits. It is more readily appreciated in that field 
than in most others, that origin and subsequent changes and present 
exact condition of the material are vital matters. On the develop- 
ment of these factors, dependent as they are on perfectly normal 
geologic processes, depends also the extent and quality and distribu- 
tion of the ore, as well as its changes in quality or value in depth or 
distance. No methods other than those of the new petrology have 
ever been successful in determining any of these fundamental 
questions. 
In that branch of the science of geology where great financial 
interests are often dependent on correct interpretation of conditions 
and origin, it is comparatively easy to show the practical value of 
methods that serve these ends; but, as a matter of fact, wherever a 
better understanding of the life history and the causes of the present 
condition of rock material is needed, there the new petrology — the 
interpretation form of rock study — is of real service. 
That kind of service is as broad as are the wants of geologic 
science. It is probable, indeed, that by petrographic methods 
designed to this end one could pry much further into the obscure 
secrets of many phases of geologic history than by any other means. 
It will do for structural history what paleontology does for stratt- 
graphic history and physiography does for the surficial history of 
the earth. 
It would be easy to illustrate some of these forms of service by 
a variety of practical problems in which such methods have been 
used. Two or three certainly will suffice. . 
1 It appears that a good zinc ore was being worked, and question 
arose about its-extent and probable distribution as:a basis for future 
development. ‘The field indications thus far had not been clear and 
there were several alternative possibilities. The ore was a mixture 
and occurred in limestone. It might be (a) essentially primary, or (b) 
simply an altered primary constituent in place, or it might be (c) 
introduced, or (d) altered introduced material. In the latter case 
the distribution of the original introduced mineral and its origin 
would be an important link in the history. It was a strictly practical 
interpretation problem. One needed only a working knowledge of 
the criteria of sedimentary and organic rock origin, of metallic 
mineralization, of replacement, and of mineral alteration. It could 
be shown then that the ore in question was nearly all secondary; 
