446 
IDDINGS. 
The general conclusion arrived at by the former study 
was summed up in an article published in 1887, and is 
briefly as follows: “ It seems highly probable that the differ¬ 
ences in consistency and in the phases of crystallization ” 
(lithophysae, spherulites, and allied structures) “ producing 
the lamination of this rock were directly due to the amount 
of vapors absorbed in various layers of the lava and to their 
mineralizing influence.” * 
The writer’s conception of the action of an absorbed vapor, 
like that of water, upon the crystallization of a molten 
siliceous magma is derived from the following consideration. 
Since the water-vapor does not enter into the chemical com¬ 
position of the resulting minerals in most cases, though it 
may affect the oxidation of the iron in some instances and 
lead to the formation of mica and hornblende,f its action is 
most probably physical rather than chemical; but in the 
case of other vapors, as fluorine, the action may also be 
chemical. The occurrence of a completely crystallized 
portion of magma, such as a spherulite, within a wholly 
glassy rock indicates that while the greater part of the molten 
magma cooled and consolidated so rapidly that it formed 
amorphous glass, a certain portion of it passed through 
different conditions, which permitted it to crystallize com¬ 
pletely. It is evident from the nature of the case that 
any considerable mass of the magma within the body of the 
rock must have cooled at a uniform rate, and therefore the 
portions forming the spherulites must have cooled at the 
same rate as the magma immediately surrounding them, 
which solidified as glass. The crystallization of minerals 
in a molten magma requires a certain amount of molecu¬ 
lar mobility within the fluid at the proper temperature— 
that is, between the point of fusion of the minerals developed 
* Iddings (J. P.). The nature and origin of lithophysae and the lamina¬ 
tion of acid lavas. In Amer. Journ. Science. 8°* New Haven, 1887, Jan. 
vol. 33, p. 45. 
j-Iddings (J. P.). The mineral composition and geological occurrence 
of certain igneous rocks, etc. In Bulletin Phil. Soc. Washington. 8°. 
1890, Yol. XI., p. 209. 
