190 
IDDINGS. 
the geological age in which it was erupted. Durocher also 
realized the local character of volcanic action and introduced 
the idea of a local differentiation of part of the magma by 
a process of liquation such as takes place upon the cooling 
of certain metallic alloys. He also considered that varia¬ 
tions in composition may be affected by the accession of sea 
water to the molten magma. The independent action of 
neighboring volcanic vents suggested the idea that the vents 
do not connect directly with the general reservoir, but with 
branching fissures. 
A special form of synthetical hypothesis was conceived by 
Von Cotta. It consisted of one molten magma, which was 
basic, and a solid silicious crust, through which the magma 
was erupted and which it dissolved in various proportions. 
In 1868 Von Richthofen applied Yon Waltershausen’s 
theory to the occurrence of volcanic rocks in Tertiary times, 
and recognized the difficulty of its application to the occur¬ 
rence of trachyte and rhyolite, and also to the localization 
of volcanic phenomena, hut did not offer any satisfactory ex¬ 
planation of these anomalies. 
Ten years afterwards King elaborated the theory of local 
differentiation, having discarded the theory of a universal 
fluid zone of different specific gravities as incompatible with 
the constant repetition of series of rocks of variable composi¬ 
tion from pre-Cambrian time to the present. According to 
King’s view, a localized body of magma was rendered molten 
or fluid by a relief of pressure due to erosion. After cooling 
and crystallization had commenced in the magma a partial 
separation of the material would take place by the force of 
gravity. The character of the lava erupted would depend 
on the phase of separation at which the eruption took place. 
This differentiation, however, did not account for all cf the 
variations among the volcanic rocks of a region, but only 
for the modifications of each of the four principal groups 
which he considered had been satisfactorily established, 
namely, the propylitic, andesitic, trachytic, and neolitic, 
which latter embraced rhyolite and basalt. For each of 
