152 
IDDINGS. 
oxides have been inserted in a different type. Since it is 
the relative proportions of these oxides in each rock which 
are to be compared with one another, it is not necessary to 
reduce them by some common factor in each case so as to 
bring the sum totals to 100. In fact, it is more accurate in 
most instances to compare those in different rocks, as they 
have been determined to exist in the rocks, than to assume 
a theoretical proportion by omitting from the analysis a 
number of accessory ingredients and then readjusting the 
proportions of the essential ones. Moreover, it seems un¬ 
necessary to consider the constituents in the metallic state, 
since in all of the rock-making minerals they exist in the 
same state of oxidation, with the exception of iron, and the 
molecular proportions have the same relations, whether they 
are discussed as oxides or metals. In either case the results 
reached by their consideration would be the same. 
Absence of stoichiometric proportions .—Referring to the six 
tables of analyses already given, which represent the chem¬ 
ical composition of various members of six groups of geolog¬ 
ically related rocks, some of them in the Electric peak and 
Crandall basin groups representing different facies of one 
rock body, we observe the irregular variability of the molec¬ 
ular proportions of the constituent oxides. 
All attempts to calculate a stoichiometric relation between 
them is fruitless. It is possible, however, to express in very 
general terms certain relations which appear to exist between 
the constituents of the rocks of one group ; but other groups 
of rocks can be found in which the relationship is different. 
How indefinite such relationships actually are will appear 
when the diagrams of molecular proportions are examined 
in detail. 
Some of the generalizations for the groups of analyses 
under discussion are as follows : 
In those groups in which the alumina maintains a gener¬ 
ally uniform proportion and the alkalies increase gradually 
with the silica, the magnesia, lime, and iron oxide decrease 
rapidly as the silica increases. 
