THE ORIGIN OF IGNEOUS ROCKS. 
167 
somewhat below the average and in which silica is low. It 
is well known to occur in those high in silica and potash, 
with but little iron and magnesia, being the chief ferro- 
magnesian mineral of many rhyolites. In magmas rich in 
titanic acid it is evident that with sufficient silica sphene 
may he formed, but with insufficient silica perofskite will 
take its place. It is also evident that it is in rocks low in 
silica and alumina and rich in lime, iron, and magnesia 
that melilite is to be found; while in rocks low in silica and 
high in alumina picotite occurs. 
The proportion of alumina in rocks affects the combina¬ 
tion of the alkalies and lime. If it is not sufficient to com¬ 
bine with these in feldspar molecules the alkalies enter into 
combination with ferric oxide in an acmite molecule and 
perhaps a riebeckite molecule which has the same ratio, but 
takes the form of amphibole, and the lime must enter into 
a pyroxene or amphibole molecule or, in rare cases, into 
melilite. An excess of alumina increases the aluminous 
members of the pyroxene and amphibole molecule. 
It is evident from the study of the molecular proportions 
of the essential oxides of various rocks and of their constit¬ 
uent minerals that by the differentiation of a magma of 
intermediate composition varieties of magma are produced 
in some of which silica is in excess of that required to form 
silicate compounds with the other constituents, and as a re¬ 
sult more or less abundant quartz crystallizes out, while 
in other varieties the silica is insufficient to combine with 
all of the other constituents, and the least silicious com¬ 
pounds are formed together with those without silica, such 
as picotite and perofskite. This also proves the absence of 
fixed silicates in the molten magma. 
ROCKS OF EXTREME DIFFERENTIATION. 
The continued differentiation of an extremely localized 
magma leads to the production of an intimately related 
series of rocks which have been successively erupted at one 
center of volcanic activity, such as Electric peak and Cran- 
