THE ORIGIN OF IGNEOUS ROCKS. 
187 
Nineteen years later, when Charles Darwin turned his at¬ 
tention to the problem of the origin of igneous rocks, the 
idea of the differentiation of a homogeneous mass took a 
somewhat different form. The undifferentiated mass became 
a molten magma, and a state of partial crystallization was 
considered the condition in which a separation of its constit¬ 
uents could take place by the force of gravity. He, however, 
added to this hypothesis another, which conceived the par¬ 
tially crystallized molten magma to be capable of draining 
off the more liquid portions into open fissures and abysses. 
In contrast to the crudeness of these conceptions are the 
expressions of Dana, published in 1849, five years after 
those of Darwin. Dana appreciated the gradual transition 
in composition between the various kinds of rock forming 
the volcanoes of the Pacific islands, and also the transitions 
in crystalline texture from glassy to granular forms. Pie 
stated clearly his opinion that the crystalline character of 
all igneous rocks depended solely on the conditions of press¬ 
ure, temperature, and slowness of cooling under which mol¬ 
ten magmas had solidified. He did not attempt to account 
for the diversity in composition of igneous rocks in general, 
but confined his speculations to the differences which were 
supposed to exist within the body of a volcano. At that 
time it was considered that the central part of a volcano was 
usually composed of more feldspathic rock than the surface 
lava flows. 
Although subsequent methods of research may have modi¬ 
fied our knowledge of the petrological character of the mass 
of a volcano, the conception which Dana had of the process 
by which the material of lavas became differentiated is note¬ 
worthy. It was a differentiation which antedated crystalli¬ 
zation and took place in a fluid molten magma. It was 
based on the assumption that as the magma began to cool 
the feldspathic constituents would grow T more viscous than 
the pyroxenic, and the vaporizable ingredients would tend 
to inflate the mass as it approached the surface and would 
lower the specific gravity of the more liquid portion, which 
