144 
IDDINGS. 
will be more thoroughly investigated, in order that we may 
discover whether there exists any connection between the 
different types of rock groups which occur over great areas 
of the globe and the geological history of those regions. 
DIFFEKENTIATION OF MOLTEN MAGMAS. 
GEOLOGICAL EVIDENCE. 
It having been demonstrated, that the igneous rocks in 
one region, or about one center of eruption, which belong to 
an era of volcanic activity, exhibit characteristics which 
serve to identify them as belonging to a connected group; 
and it having been shown that they pass into one another 
by gradations of chemical composition, and are $lso con¬ 
nected physically by their geological occurrence in many 
cases, it remains to be discovered how these relationships 
came into existence or what is the origin of the variation 
among the rocks of one group. 
In a great many regions where the geological occurrence 
of all of the igneous rocks belonging to one eruptive epoch 
has been thoroughly studied, a definite order of eruption has 
been observed. The result of extensive observation in many 
parts of the world has been summed up by von Richthofen 
in his well-known order of succession for massive eruptions. 
In consequence of the subsequent development of microscopi¬ 
cal petrography, the understanding of the terms in which he 
expressed this law has been modified, but when the rocks 
to which the terms were applied are kept in mind it still 
has its value as a broad generalization, especially when it is 
applied to the regions particularly studied by von Richt¬ 
hofen, namely, Hungary, China, and the Great Basin and 
Pacific coast of the United States. 
Von Richthofen’s law of succession does not apply to all 
regions and occurrences of volcanic rocks, especially to erup¬ 
tions from a center, as in a volcano, where frequent repeti¬ 
tions of whole or partial series occur. Nevertheless it ex- 
