THE ORIGIN OF IGNEOUS ROCKS. 
163 
liarity of having more potash than the main portion of the 
group or the rocks of Crandall basin. The soda is about 
the same in both groups, and so is the alumina in the more 
silicious end of the series. 
This group of exceptional rocks has a great range of 
silica, including intermediate amounts. At the least silicious 
end of the series there is a great diversity in alumina and 
magnesia for rocks with nearly the same percentage of silica. 
With all this variability of composition, these exceptional 
rocks are united by their association in the field, and by the 
time of their eruption, as well as by a high percentage of 
potash, into what might be called a sub-group. They 
represent the differentiation of a magma which is connected 
by its geological occurrence and by petrological characters 
with the less potassic rocks of the main group. It must be 
the result of a differentiation of the magma of the main 
group, for it is of small volume and is followed by im¬ 
mensely greater masses of magmas of the normal type for 
this region. The more potassic magma is not confined to 
one center of activity, but occurs at numerous centers and 
represents a modification of the magma which took place at 
each of these centers. 
This proves the existence of a number of successive orders 
of differentiation which undoubtedly took place on a larger 
and smaller scale. Thus the separation of a large body of 
magma into a series of normal partial magmas, also pro¬ 
duced a partial magma with an increased percentage of 
potash; this was followed by a more complex differentiation, 
which produced the extremely magnesian varieties and the 
highly feldspathic varieties of the potassic magma. This 
closed one phase of eruptive activity, and the more volumin¬ 
ous eruptions that followed reverted to the average magma 
of the region. 
The difference between the chemical composition of the 
main group and sub-group in this instance corresponds to 
that which exists between some geographical groups, so that 
the inference may be drawn that the cause of the difference 
