762 WHITMAN CROSS 
almost on the intermediate line. The extreme range from I to L 
is less than one-half that of Order 4. 
In regard to rang, expressing the quantitative relation of alkalic 
to calcic feldspars, the new symbols further emphasize the relation- 
ship of most of these averages. The range from the most alkalic 
rock, L, to the most calcic, I, is somewhat less than that of a full 
rang. 
In the relation of potassic to sodic feldspars, a large majority 
are found to be transitional between dosodic and sodipotassic 
subrangs, although but two are of the latter. The average of 
Italian analyses would represent a sodipotassic magma, but even 
in this case falling on the sodic side of the centerpoint of Subrang 3. 
The next most richly potassic average is that of Colorado analyses, 
but it is to be noted with regard to this average that a large number 
of analyses of phonolite and other sodic rocks from the small 
Cripple Creek center somewhat obscure the characteristic relative 
abundance of potash which the writer has long recognized in Colo- 
rado-rocks. 
The use of these new symbols by the writer has convinced him 
that they afford a good means of expressing in a concise way close 
relationships or marked differences which it is otherwise necessary 
to explain in many words. It is to be hoped that petrographers 
using the quantitative system may become impressed by the status 
of the transitional rock. It is just as important as any other type, 
but unless the transitional character is expressed by name or symbol, 
erroneous impressions may be given. A rock specimen shown by 
analysis to occupy a transitional position may have come from a 
mass which as a whole belongs on the opposite side of the division 
line. It is evident that special care is desirable in the calculation 
of the norm and the ratios of transitional rocks. 
In closing, it seems well to point out to petrographers that it is- 
bad practice to name any division of the quantitative system after 
the occurrence of a type which is transitional in any respect. The 
authors of that system have themselves erred in a few cases, but 
some subrang names recently proposed by other petrographers 
are particularly unfortunate. It would be well in future, in the 
writer’s opinion, to base a name only on a type falling within the 
central half of each division of the system. 
