THE ORIGIN OF IGNEOUS ROCKS. 
179 
eruption of the rocks in one region. In other words, there 
may be a recurrence of partial series of rock varieties in the 
whole series. 
It is also possible to find a recurrence of different varieties 
at one center of eruption, which may be accounted for by 
supposing successive supplies of magma from some depth, 
which differentiate into similar varieties before their final 
eruption. This is particularly noticeable in a region of 
neighboring volcanic vents, such as the Yellowstone Park, 
and is in contrast to the infrequent recurrence of different 
kinds of rocks in a series of fissure eruptions such as may be 
found throughout the region of the Great Basin of western 
America. 
There is often a great difference in the size of bodies of 
similar kinds of rocks which belong to the series which have 
been developed at various centers or belong to fissure erup¬ 
tions. 
It has already been remarked that the localized character 
of certain groups of rock varieties, together with their broader 
connection as members of the whole group of one region, and 
the consequent recurrence of similar rocks in the series repre¬ 
senting the order of succession in point of time are explained 
by the theory of the chemical differentiation of a general 
magma in consequence of differences of temperature in 
different parts of it. Let us consider how such a process 
may have taken place. 
Physical considerations .—The theory of differentiation pre¬ 
supposes a homogeneous fluid magma; homogeneous, because 
of the uniformity of the results in various regions, and fluid, 
because the evidence of volcanic lavas as well as the phe¬ 
nomena of crystallization* indicate that before crystalliza¬ 
tion commenced the magmas were fluid, whatever may have 
been their previous condition. Moreover, the differentiation 
of magma on a large scale has led to the same results as the 
differentiation of a small portion of magma which is known 
* Iddings (J. P.) On the crystallization of igneous rocks. Bull. Phil. 
Soc., Washington. 8°. Washington, 1892, vol. 11, pp. 65-113. 
