THE ORIGIN OF IGNEOUS ROCKS. 
189 
sition and character of such rocks varied with the geological 
age in which they were erupted—a belief which has been 
retained with great persistency by a large body of geologists; 
although combated from the first by Scrope, Dana, and others. 
From the time when Von Waltershausen enunqiated his 
theory on to the present the various conceptions of the origin 
of volcanic rocks have sprung from one of these early sources, 
with one exception. They may be designated as the theory 
of local differentiation, that of synthesis, and that of primi¬ 
tive universal differentiation. 
To the first class belong the views expressed by Lyell and 
Jukes. The former credits Scrope with a theory which ap¬ 
pears to be a special form of Dana’s hypothesis rather than 
an elaboration of Scrope’s ideas of 1825, since he did not 
then conceive of a fused magma. Jukes introduced the idea 
of an accession of material from the rocks penetrated by the 
fused magma, which was previously suggested by Bunsen. 
He enlarged upon the supposed effect of the physical condi¬ 
tions attending consolidation, combated the assumed rela¬ 
tionship between the character of rocks and the age of their 
eruption, and suggested that in place of the terms ancient and 
modern , as applied to igneous rocks, be substituted the ex¬ 
pressions deeply seated and superficially formed. 
The theory of Durocher, which followed that of Von Wal¬ 
tershausen by four years, resembles the latter’s theory more 
closely than that of Bunsen, although it emphasizes the idea 
of two magmas, which corresponded to those of Bunsen; but 
these were based on an estimation of the supposed quantita¬ 
tive relations of igneous rocks, and not on an assumed syn¬ 
thetical origin for them. The silicious and basic magmas 
formed two principal, universal zones within the earth, and 
were arranged according to their specific gravities, with a 
smaller zone of transitional character between; hence this 
much of the theory resembled that of Von Waltershausen, 
except in regard to the relative proportions of the different 
zones, and in the still more important fact that the zones 
remained fluid and the material erupted did not vary with 
