THE ORIGIN OF IGNEOUS ROCKS. 
153 
In groups in which the last mentioned oxides maintain a 
nearly uniform proportion, the alumina decreases as the 
silica increases, and the alkalies may or may not do the same. 
In the Etna group they decrease with the alumina, while at 
Pantelleria they remain nearly constant. 
The alkalies maintain no fixed relation to one another, 
though they generally behave in somewhat the same manner. 
Their relative proportions in different groups of rocks is quite 
varied and characteristic. 
Owing to the uncertainty attending the determination of 
the oxidation of the iron in rock analyses, little reliance can 
be placed on most determinations of it; but in the very care¬ 
fully executed analyses of the rocks from the region of the 
Yellowstone Park the different oxidations of the iron are 
more than usually accurate. In these groups of rocks the 
reciprocal behavior of the ferrous and ferric oxides is one of 
the most marked chemical features. They are undoubtedly 
connected by some definite relationship. When considered 
separately they exhibit no connection with any other con¬ 
stituent, but when combined as ferrous oxide they fall in 
place with the lime and magnesia; so that it seems highly 
probable that during the differentiation of the magma all of 
the iron existed in the ferrous condition and was acted on as 
ferrous iron, and that subsequently it was in part more 
highly oxidized, so that the more ferric oxide was produced, 
the less ferrous remained. 
As just remarked, when all of the iron in the rocks is re¬ 
duced to ferrous oxide it is found to vary in a general way 
with the lime and magnesia; but these constituents do not 
always vary to the same extent. In the three groups from 
the Yellowstone region the magnesia is the most variable 
oxide, attaining its highest degree of variation in the basic 
rocks of Crandall basin and Absaroka group, while the iron 
oxide is the least variable. The same is true for the Etna 
group, though in this case the magnesia is subordinate in 
quantity to the lime and iron oxide. In the Vesuvian 
group the lime is most variable and the iron oxide the least 
23—Bull. Phil. Soc., Wash., Vol. 12. 
