584 JAMES ASTON 
apply the details of their working. ‘The igneous rocks are, as a rule, 
very complex, and may contain many component minerals or elements; 
also there is an additional complication in that solidification takes 
place very often under pressure. Pressure is of great importance in 
the consideration of vapors, and so difference of pressure may intro- 
duce additional complexity through the effect on the gases. Again, 
equilibrium conditions may not always be reached; for example, 
FIG: 
with rapid cooling of the magma, supersaturation may have occurred 
with a change in the order of deposition of the minerals, or differences 
in the microscopic structure. 
In the solidification of magmas, also, we have certain conditions 
which do not occur in the metallic alloys. We have discussed the 
composite nature of the eutectic structure. Among the metals, where 
the cooling is normal, this intimate mixture of the constituents is 
very common; with the slow cooling of the igneous rocks (especially 
when enhanced by their poor conductivity of heat) the long sojourn 
just below the temperature of transition may cause a segregation of 
