WM. H. HOBBS 
N 
The modern petrographical microscope, with its accessories, 
has introduced great refinement into the methods of study, so 
that descriptive petrology, or petrography, has become a very 
exact science. It is now possible to describe a rock in so many 
ways (in respect to so many of its attributes, such as mode of 
occurrence, texture, mineral composition, chemical composition, 
alterations, genesis, etc.,) that the difficulties in the way of bring- 
ing the results of the study into an orderly classification have 
been greatly increased. Nor is there reason to hope for any 
immediate remedy for this condition, since the largest and most 
representative body of petrologists ever assembled —the Seventh 
International Congress of Geologists, at St. Petersburg — was 
almost unanimous in the conviction that it would be useless to 
attempt to harmonize the nomenclature of the science by any 
‘early action of that body. The view was, however, expressed 
that something might be accomplished through the labors of a 
representative committee, which should, by frequent and careful 
deliberations, arrive at a tentative scheme for presentation to a 
future congress. 
Undoubtedly the greatest obstacle in the way of reaching an 
understanding in the matter is that different values are assigned 
by different petrologists to the same attribute in rock classifica- 
tion. Some would lay greatest stress upon the mode of occurrence 
in the field ; others would give the first place to mineral constitu- 
tion, still others to texture, chemical composition, etc. 
LAE FIELD) GEOLOGIST Vs> Li PEDROLOGISE 
In deciding what shall be given first place as a basis in any 
system of rock classification, it should be realized, it seems to 
me, that the igneous rocks are not the sole property of the 
petrographer. The field geologist or the ‘‘naturalist,’’ whatever 
be his special line of work, has need to make determination of 
igneous rocks, and he has a right to ask of the petrographer, 
who from his greater familiarity with rocks is charged with 
arranging them in an orderly system, such a classification that 
the geologist’s determination in the field shall be zxcomplete rather 
