104 
IDDINGS. 
veloped before the magma came to rest, for the groundmass 
exhibits flow structure, while it may be equally evident that 
all of the minerals at the center of the mass developed after 
the magma came to rest. 
(4.) The progress of a magma through its conduit may not 
be at a uniform rate. In fact, it could not be uniform if the 
shape and size of the conduit varied. It must travel faster 
through the narrower portions than through the broader. 
Moreover, its progress is often spasmodic, rising a certain 
distance and discharging the advance portion out at the 
surface, or into gaps in the superficial crust where planes of 
weakness divide the rock; then standing, or even sinking, in 
the conduit until another combination of forces causes it to 
rise still farther. 
The effect of a rapid increase in the rate of ascension in 
the conduit would relieve the pressure at a faster rate, which 
might gain on the rate of cooling to such an extent as to 
destroy the previous ratio between them. Its effect would 
be equivalent to an increase of temperature. If this ratio 
had previously been that which caused the supersaturation 
of the magma and induced crystallization, the newly dimin¬ 
ished ratio would have the effect of reducing the degree of 
saturation and of hindering further crystallization, or might 
even lead to the dissolving of the minerals already separated 
out—that is, of resorbing them. 
Thus it may happen that the porphyritical crystals ( pheno- 
crysts) in a magma may be partially resorbed during the 
ascension of the magma, not by an increase of temperature, 
but during its actual decrease, the cause of the resorption 
being the rapid diminution of the pressure. 
When the balance between the temperature and pressure 
oscillated about the point of saturation of the magma the 
development and retardation of crystallization might be 
often repeated, as it appears to have been in certain zonally 
built minerals with irregular, rounded zones of resorption. 
Before leaving the subject of the resorption of crystals it 
