CRYSTALLIZATION OF IGNEOUS ROCKS. 101 
magma about to rise from a great depth through its conduit 
toward the surface of the earth. Whether it contains any 
crystal secretions at this stage we have no means of judging, 
for it has been observed that all of the rock-making minerals 
are capable of crystallizing at or near the surface of the earth, 
and none demand an unusual depth for their formation. On 
the contrary, it has been observed that molten magmas fre¬ 
quently reach the surface or to near the surface without 
having commenced the crystallization which often leads to 
their complete separation into minerals. 
(1.) As the magma rises through its conduit the 'pressure 
will diminish in proportion to the vertical distance traversed. 
The temperature will also diminish, but according to the loss 
of heat by conduction through the walls of the conduit. 
This loss will be more variable than that of the pressure, 
and will proceed at a different ratio to the distance traversed. 
As it depends primarily on the temperature of the walls of 
the conduit and on their conductivity, it is readily seen that 
the rate of decrease of temperature will be much less at first, 
when the hotter rocks near the original seat of the magma 
are being traversed, than when the cooler rocks near the 
surface of the earth are passed through. Hence the loss of 
heat will be slow at first and more rapid toward the end of 
the eruption, while the decrease of pressure will be more 
nearly uniform. 
Now, since the effects of heat and of pressure upon fluid 
rock magmas have been assumed to be the opposite of one 
another, though not necessarily in a directly inverse ratio, 
it is seen that a rapid decrease of pressure might more than 
offset a slower loss of heat, and the result would be equiva¬ 
lent to an increase of heat. Thus the magma might at first 
become more fluid and farther from the point of saturation 
and crystallization than before its eruption. 
If we assume that the rate of ascension in the conduit is 
uniform, it is apparent that the ratio between the loss of heat 
and the decrease of pressure will increase as the magma 
