180 
IDDINGS. 
to have been fluid, as in the case of a dike whose walls have 
a different composition from the middle; hence it is prob¬ 
able that magmas were fluid when chemical differentiation 
took place. 
Since it has been assumed that the differentiation is due 
to differences of temperature in various parts of the magma, 
it follows that the magma must be located where it will 
experience differences of temperature; but in widely remote 
periods of time similar series of rocks have come to the sur¬ 
face of the earth; hence there must be a source of rock 
magma so situated that it has not been affected by differen¬ 
tiation during ages of the earth’s history. In other words, 
the general magma from which a series of differentiations 
originated in Tertiary times must have been similar in com¬ 
position to that from which a like series originated in pre- 
Cambrian time. Consequently the undifferentiated magma 
must have remained homogeneous through long lapses of 
time and must either have been situated beyond the zone of 
variable temperature—that is, in the region of constant tem¬ 
perature—or it must be solid and not susceptible to differ¬ 
entiation. It is necessary, therefore, to assume a stable 
magma, which at various periods of the earth’s history has 
passed into a condition of instability. It is in accord with 
the physical nature of rock magmas to assume that they 
may exist in a state of potential liquidity, though in fact 
rigid—an excessive pressure overcoming the effect of high 
temperature. 
Leaving the question of the condition of the interior of the 
earth to the physicists, we may start our speculations with a 
magma which has passed from a stable into an unstable con¬ 
dition. 
The geological history of regions of volcanic activity shows 
that the volcanic phenomena have followed and also accom¬ 
panied great orographic movements and profound disloca¬ 
tions of the earth’s crust, and that molten magmas have risen 
to the surface through fissures. It also shows that differen¬ 
tiation has commenced with the earliest eruptions and con- 
