REPORT OF THE DIRECTOR, I922 Iit 
them. Doubtless many of them are too obscure to be detected by 
any methods that we know how to use. Doubtless, also, the effects of 
the later happenings tend to obliterate or obscure those that existed 
before. It may even be that beyond a certain period everything is 
lost because of the very violence or completeness of the revolution. 
Such a case is where a rock suffers igneous fusion; or, again, 
where there is complete reconstruction, as in some forms of 
metamorphism. 
These difficulties and the severe limitations set by the nature of 
the forces and changes that one has to deal with, however, do not 
furnish sufficient excuse for avoiding the problem. The same kind 
of limitations surround all branches of geologic research. The 
inherent difficulties do not remove the problem, they simply empha- 
size the peculiar nature of the problems of the science. 
The great needs are standardized criteria. One must now assem- 
ble his own; and a little experience in assembling them will soon 
show how very scattering are the observations and comments that 
serve as reliable source records, and how much of a task it really 
is to organize a working scheme along any line of petrographic 
interpretation. 
For example, one needs criteria of origin, and criteria for source 
of original material, and for the order of appearance of the different 
constituents, for deformation and recrystallization, for introduced 
mineralization, for hydrothermal influences, for pneumatolitic action, 
for deep-seated alteration as contrasted with weathering or super- 
gene changes, for deuteric effects or end-product changes as con- 
trasted with entirely independent subsequent modifications, for 
secondary enrichment, for primary as contrasted with secondary 
constituents, for high-pressure-temperature as contrasted with 
low-pressure-temperature conditions, for sitlication, for cabona- 
tion, for replacement or metasomatism, for metamorplusm, or any 
other geologic process in its effects on rocks or for any quality or 
condition that might make them more readable and better under- 
stood. 
For a good many years the writer and his students and associates 
have been slowly assembling and trying out such criteria, and have 
been attempting to interpret rocks. Even the imperfect standards 
that have been used show beyond doubt that the results are service- 
able; but there is a long way to go to perfect a reliable working 
scheme of criteria. 
