REVIEWS 757 
cal and physical behaviors of molten magmas is begun with the considera- 
tion of the chemical reactions which give rise to minerals, and which 
would probably take place, within the earth, between the elements occurring 
in igneous rocks. From these studies the laws governing the association 
of certain minerals in the rocks are deduced. 
In consequence of changes of physical conditions, the molten magma 
separates into solids, gases, and, to a small extent, liquids. ‘These processes 
are complicated by variable factors such as change of chemical reactions 
or degree of supersaturation under different conditions of temperature and 
pressure produced by the movements of eruption. The causes of this 
Separation, the order and the effect of supersaturation upon it, the effect 
of viscosity upon the rate, and the separation taking place in isomorphous 
compounds and eutectic mixtures, are discussed. 
Following this presentation of the subject of the separation of substances 
from solution, is a chapter on the crystallization and texture of igneous 
rocks, the terms used being those recently proposed by Cross, Iddings, 
Pirsson, and Washington.t The degree of crystallization of a rock magma 
and the amount of glass affect both the mineral constituents of the resulting 
rock and its texture. The extent of crystallization, the magnitude, shape, 
and arrangement of the crystals or amorphous parts, and the factors 
which influence texture are fully described and illustrated. 
The differentiation, or splitting-up of a homogeneous rock magma into 
chemically unlike portions, is next considered. The evidences of such 
separation are divided into two groups: (1) Igneous rocks differing in 
chemical composition in different parts of the same body; and (2) igneous 
rocks of different compositions associatéd in regular manner with reference 
to place of occurrence and time of eruption, and possessing such chemical 
characteristics that their previous existence as components of a homogeneous 
magma is clearly indicated. 
The agencies which are likely to affect the character of a rock magma 
are changes of temperature, pressure, or gas content, and Professor Iddings 
considers the way in which each of these may act upon certain physical 
or chemical characters of the magma. They may change the density of 
the liquid magma in certain parts and produce convection currents, or 
they may change the viscosity and modify the diffusivity. A change in 
osmotic pressure would produce a variation in the molecular concentration 
in different portions, a change in saturation or chemical equilibrium would 
affect the crystallization and the resulting minerals, and a change in the 
character of the solidification would affect the texture of the rock. The 
t Jour. Geol., Vol. XIV, 1906, pp. 692-707. 
