THE ORIGIN OF IGNEOUS ROCKS. 
101 
cooled or consolidated, or to the nature of those it pene¬ 
trated.” He remarks that the greater the depth the greater 
the heat, and that the more refractory minerals may be 
melted at the greater depths. The bases he considers as 
fluxes. He questions whether the more basic portions were 
present in the deepest seated molten masses, and were sepa¬ 
rated “on the first cooling and crystallization of the simple 
minerals,” or were acquired during its progress to the sur¬ 
face. He suggests that one portion might consolidate at 
one depth, a more basic might reach a higher level, and so 
on; “ so that from the very same stream of igneous matter 
proceeding from the interior to the surface of the earth, the 
more readily fusible portions might be successively squeezed 
out, as it were, as the infusible ones solidified and contracted 
in consequence of that solidification. This action might take 
place in spite of the greater specific gravity of the more fusi¬ 
ble minerals, since the difference of the specific gravity would 
probably be small compared with the power of the eruptive 
force.” 
Jukes also suggests that in place of the terms “ancient” 
and “ modern,” as applied to igneous rocks, be substituted 
the expressions “ deeply formed” and “superficially formed.” 
In accounting for the more basic varieties of rocks he 
says: “ These more readily fusible substances might be con¬ 
ceived either to have separated in liquid strings or veins from 
the consolidating rocks, below, or to have been acquired by 
the upper portion of the mass from the rocks it met with in 
its passage toward the surface, the substances thus added 
having acted as an additional flux to matter which would 
otherwise have solidified before it could have been poured 
out. 
“ Some such hypothesis as this seems to me less forced than 
one which obliges us to suppose separate deep-seated foci or 
reservoirs for every variety of igneous rock, those varieties 
frequently occurring in the same district and alternating one 
with the other over the same space of ground. 
