IGNEOUS ROCKS IN YELLOWSTONE PARK. 
205 
comprehended by comparing their molecular proportions. 
This has been done graphically as in the accompanying dia¬ 
gram Table VIII, in which the molecular proportions of 
the principal oxides are plotted as ordinates, those of the silica 
being taken as abscissas. The point of origin of the latter is 
some distance to the left of the diagram. 
The first impression derived from the diagram is that of 
the irregularity of the variations in all of the oxides besides 
silica, magnesia being the most erratic. The second im¬ 
pression is that of the apparent independence of the varia¬ 
tions of the different oxide molecules. But this independ¬ 
ence disappears upon closer study and certain evidences of 
sympathy become very marked. The most striking evi¬ 
dence of this exists between the ferrous and ferric oxides ; 
they are noticeably inversely proportional to one another, 
an increase of ferrous oxide being accompanied by a decrease 
of ferric oxide. The total amount of iron varies irregularly, 
decreasing from the basic to the acidic end of the series. 
Each of the iron oxides seems to be quite independent of the 
magnesia. The most regular variation is that of the lime, 
which decreases steadily from the basic to the acidic end of 
the series. The magnesia drops very rapidly at first, and is 
very irregular in the more siliceous end of the series. The 
alumina, though quite irregular between certain limits, main¬ 
tains a uniformly high position, being largely in excess of 
any one of the other oxides, except silica. The alkalies are 
most like the alumina in their variations and remain very 
nearly uniform, increasing slightly toward the siliceous end 
of the series. The soda molecules are more than twice as 
numerous as those of the potash. In the basic end of the 
series the alkalies vary together in the same direction, while 
in the more siliceous end they vary in opposite directions. 
There is a marked accordance between the soda and 
alumina, both varying in the same direction, with one ex¬ 
ception, though not to the same extent. There is an equally 
marked discordance between the alumina and magnesia, 
which, with one exception, vary in opposite directions. 
