PETROLOGICAL ABSTRACTS AND REVIEWS 369 
this is) but the freedom such a condition would give to the broader and 
often conflicting generalizations of petrologists and geologists concerning 
the various deep-lying relations of igneous and other rocks. Picture the 
future of systematic petrography if based on “provisional hypotheses”’ 
and ‘‘successive approximations to the truth”’! 
The critical reader of this address cannot fail to be impressed at the 
opening and again at the close of the discussion by an inconsequence in 
the use of the terms petrology and petrography which has its counter- 
part in other places and leads to doubt as to the logic of the reasoning 
in certain important particulars. The distinction between petrology as 
the broad philosophical science of rocks, and petrography, as the descrip- 
tive or systematic part of petrology, is well-nigh universal today. This 
usage is both approved and violated by Harker. In his introductory 
remarks he acknowledges that the idea as to the limitations of petrography 
“correctly denotes its purely descriptive nature” and yet asserts that 
“‘ petrology is at the present time in a state of transition .... froma 
merely descriptive to an inductive science. ... . ” Has the work of 
Bunsen, Durocher, Rosenbusch, and Brogger—not to mention many 
others—been “merely descriptive”’ ? 
Again, in his summary, Harker deplores the attitude of those who 
would abandon efforts to place petrography upon a genetic basis, which 
would be to renounce its claims as ‘“‘a rational science,’’ while he, in 
contrast, takes ‘‘a more hopeful view of the future of petrology”! There 
are many petrologists who hold the former view and yet join enthusi- 
astically with Harker in the latter outlook, having regard for the dis- 
tinction which he at times seems to lose sight of. 
The discussion of the ‘‘ Problems in Petrology”’ by Iddings is different 
from those of Harker and Loewinson-Lessing in that it is a statement of the 
problems of today, the ones which present knowledgemakes most profitable 
for the labors of petrographer and petrologist, rather than of the ultimate 
questions, or even those dependent on provisional generalizations. It is 
the problems of the rocks themselves and those to be formulated from 
the closer study of petrographical provinces which Iddings specially con- 
siders, though the results are of interest also through their bearing on 
still more remote and fundamental relations. 
The “actual mineral composition of igneous rocks”’ is first taken up 
as “a great field of research, imperfectly cultivated, capable of yielding 
immediate returns of the first importance for the solution of other prob- 
lems connected with the mineral composition of these rocks.’ The 
