THE ORIGIN OF IGNEOUS ROCKS. 
117 
cooling. Thus an intermediate magma may become differ¬ 
entiated into a basic and an acid magma.” 
In 1889 Rosenbusch,* in a paper on the chemical rela¬ 
tions of eruptive rocks, adopts the theory of the differentia¬ 
tion of an originally uniform magma in order to explain 
the chemical differences of eruptive rocks. The subject is 
treated solely along the line of chemical investigation and is 
divided into three parts. The first deals with the question 
whether there are differences in the chemical composition of 
eruptive rocks of different geological periods, which is an¬ 
swered in the negative. The second, which constitutes the 
essential part of the paper, considers the character of the 
different phases of chemical constitution of the rocks. The 
third part consists of a single paragraph in answer to the 
question, How may the differences of eruptive masses at one 
and the same center of eruption be explained ? 
In his treatment of the second part Rosenbusch reviews 
Bunsen’s theory and points out Roth’s observation that the 
discordance between the theoretical and calculated anal¬ 
yses would be still greater than it is if some other con¬ 
stituent besides silica had been chosen as a basis of calcula¬ 
tion. He compares with it Durocher’s theory and emphasizes 
the latter’s hypothesis of liquation as applied to molten 
magmas, by which Durocher implied that liquation is a 
spontaneous separation of a chemically homogeneous molten 
magma into several chemically different liquid magmas 
whose differences depend on the circumstances of their 
solidification. This Rosenbusch interprets to mean that 
similarly constituted whole magmas must not necessarily 
separate into the same partial magmas; and he states that 
it is one of the objects of his communication to show the un¬ 
tenableness of this assumption, and adds that the “ gangge- 
steine ” furnish the sharpest weapons with which to refute it, 
for their different varieties are connected in the most persist¬ 
ent manner with certain “ tiefengesteine.” 
* Rosenbusch (H.) “ Ueber die chemischen Beziehungen der Eruptiv- 
gesteine.” Min. u. petr. Mitth. 8°. Vienna, 1889, vol. 11, pp. 144-178. 
