THE ORIGIN OF IGNEOUS ROCKS. 
181 
tinned to the close of volcanic activity. Hence the chemical 
instability of the original magma is coeval with volcanic 
action and succeeds orographic movements; from which we 
may conclude that the instability of the magma is a result 
of changes in the position of the earth’s crust, whose greatest 
effect must be on the pressure of the underlying portion of 
the globe; so that the stability of the original magma must 
he a function of the pressure under which it exists. 
Fissures as reservoirs of magma .—Fissures of vast dimen¬ 
sions must have penetrated to the seat of molten magmas in 
order to permit the escape of the enormous volumes of lava 
which form some massive eruptions in western America. It 
is self-evident that fissures, through w T hich lavas rise to the 
surface, must open downward—that is, they must be in the 
nature of synclinal cracking, and will be large at the bottom 
and narrow at the top. They w T ill undoubtedly be very 
irregular in shape, and often branched. 
A great complex system of fissures having been formed, 
and having been filled with molten magma, it is evident 
that the magma will experience very different conditions in 
different parts of the system. In places it may reach the 
surface at once, and then be shut in. At other places a 
constant connection with the surface may he established, 
while in other directions it may not reach the surface, or 
may reach it at different. times, as subsequent movements 
extend the fissures. The temperature in various parts of so 
irregular a reservoir would vary greatly because of the 
variable size of the bodies of magma and of the various rates 
of cooling due to the differences of conductivity in different 
kinds of enclosing rock. The differentiation of the magma 
will therefore progress at various rates and to different ex¬ 
tents in different places. 
Dynamical considerations .—It is to be remarked that be¬ 
yond the fact of a chemical differentiation of rock magmas 
and the probability that it is the result of variations in 
temperature in different parts of the magma, nothing is 
known of its rate of action nor of the laws affecting a com- 
