222 ALEXANDER N. WINCHELL 
It should be remembered that rock names are not always used 
with the same meaning; therefore some of the averages presented 
herewith include analyses of rocks not classified in accordance with 
recent practice; this is especially true of the average for granite, 
which certainly includes some analyses of rocks which would now 
be classified as quartz monzonite, and probably includes also a few 
alkaligranites. The average for basalt probably includes analyses 
of trachydolerites. It is to be hoped that averages for these rocks 
will be prepared from analyses of rocks whose correct classification 
cannot be questioned. 
If.the average quantitative mineral composition of the various 
igneous rocks were known, it would be as useful as the average 
chemical composition. Of course the ‘‘normative mineral com- 
position” can be calculated directly from the average chemical 
composition for each rock type. But the “normative mineral com- 
position” is much less important than the ‘‘ modal” mineral compo- 
sition; it is merely the result of a distribution of the oxides among 
a certain group of minerals somewhat arbitrarily selected. Ina 
few cases the actual (modal) mineral composition can be calculated 
at least approximately from the chemical composition. As our 
knowledge of the composition of the mineral constituents of each 
rock type increases, the mineral composition can be calculated for 
an increasing number of rocks. 
A few calculations of this kind have been made; they are much 
facilitated by the use of Mead’s circular slide rule‘. For the 
alkaligranite all the ferric iron is assigned to riebeckite; alkaligranites 
often have, in place of riebeckite, either arfvedsonite, acmite, barke- 
vikite, lithia mica, or two or three of these minerals. All the mag- 
nesia of the quartz monzonite is assigned to biotite and hornblende 
(including augite) in the ratio (about 3 to 1) required to prevent 
surplus of either lime or alumina in forming feldspars. The biotite 
and hornblende are assumed to have the composition obtained by 
analysis of those minerals from quartz monzonite from California 
(analysis d, Table XIV, and a, Table XIII, Quant. Class. Ign. 
Rocks). In calculating the mineral composition of the monzonite 
sufficient magnesia is assigned to biotite and hornblende to prevent 
tW. J. Mead, Econ. Geol., VII (1912), 136. 
