REBORTEOEL DUE DIRECTOR LO2Z2 TT5 
phism, replacement, silication, silicification, carbonation, and metal- 
lic mineralization. Without discovering criteria by which rock 
material of this history, both original limestones, sandy limestones, 
and sandstones, could be distinguished one from the other, the deter- 
minations could not be done at all. With extreme caution such 
criteria can usually be discovered and used, as was done in this 
case. Even in samples where a rock that was originally a lime- 
stone now carries as much free quartz as is usual in a quartzite, it 
was generally possible to distinguish the original limestone repre- 
sentative from the quartzite; and, incidentally, help to solve an 
important, practical stratigraphic and economic problem. 
Its Discoveries 
The chief discoveries of the new petrology are: 
1 That it is both practicable and very practical ; 
2 That it is very much more interesting and attractive as an 
instruction subject than is the old form; 
3 That it yields beautifully to the application and teaching of 
research methods under simple and controllable circumstances ; 
4 That petrographic nomenclature is sadly deficient in descriptive 
terms and names for this particular kind of rock description. 
The first two items require no additional comment, but perhaps 
the last one does. It is the chief direction, in addition to the estab- 
lishing of reliable criteria, for important contributions. 
In the new view, genesis and history, or origin and subsequent 
changes are fundamental factors. In some way they must be kept 
in the foreground both in description and in classification. Other 
features need not be neglected but are, for this purpose at least, of 
secondary rank. The larger groupings must be on these more impor- 
tant factors where the greater significance lies. i 
When one tries to apply these revised ideas to his own work it is 
discovered, in the course of time, that there are very many rock 
types and rock conditions and process effects for which there are no 
corresponding petrographic terms at all. Petrology is a branch of 
science which is thought by many to have too many names and terms 
now, and with this criticism we may all agree in principle. Many 
of the terms are too mechanical in their conception and too unpro- 
ductive in their significance. Yet this can scarcely be counted 
against new ones if they can demonstrate their usefulness. 
_. My only purpose is to point out a few discoveries of this kind and 
venture the opinion that within a very few years suitable terms will 
