THE ORIGIN OF IGNEOUS ROCKS. 
95 
augite and feldspar and of basalts and feldspathic rocks is 
pointed out, and the ground is taken that supposed differ¬ 
ences of age can have had no influence on the character of 
the rocks, “ for heat and pressure and slowness of cooling, the 
requisite material being' present, are the prominent influ¬ 
encing causes determining the formation of particular min¬ 
erals or rocks,” and the requisite material is shown to have 
existed from ancient times. 
“ These feldspathic rocks are apparently an essential part 
of the volcanic dome during its whole progress, as they occur 
along the center, often to the very summit.” He does not 
think they were inserted after the dome was completed, nor 
thrown up before hand, but concludes that “ the main body 
of the mountain and its feldspathic center were in contem¬ 
poraneous progress at least as long as there were summit 
eruptions.” 
As to the view presented by Darwin, that a separation of 
the basaltic material by gravity may have taken place, he 
argues that Darwin supposes the feldspars and augites rise or 
sink as crystals of these minerals; “ but if anything is in 
fusion it is feldspar or augite (supposing these to be the con¬ 
stituents of the lava), and while in fusion there are no crystals, 
as crystallization is the first step in the process of solidifica¬ 
tion. If there is any sinking, therefore, it must be a sinking 
and separation of these materials in the fluid state, or, at 
least, this must be the case with the augite, which is the 
more fusible of the two minerals, and will be the case with 
both at certain depths, wherever the temperature is that of 
the fusion of feldspar. The impossibility of there being 
any crystals of feldspar will farther appear from the fact 
that the material of clinkstone is generally without a dis¬ 
tinct crystalline texture.” He also shows that the eruptions 
from the summit of Kilauea have a high specific gravity, 
which would controvert Darwin’s hypothesis. 
The explanation offered by Dana is based on the presence 
of vaporizable substances and the unequal fusibility of 
feldspar and augite. 
