IGNEOUS ROCKS IN YELLOWSTONE PARK. 
217 
from a study of this diorite that much of its magma 
reached its position in the conduit in a completely 
fused condition, after which it crystallized into hyper- 
sthene, augite, hornblende, biotite, labradorite, 
alkali feldspars and quartz, with accessory magne¬ 
tite, apatite and zircon. We feel justified, then, in 
the further conclusions: 
VII. That the molecules in a chemically homogeneous, 
fluid magma combine in various ways, and form 
quite different associations of silicate minerals, pro¬ 
ducing mineralogically different rocks. 
VIII. In this region the .greatest mineralogical differences 
accompany the greatest differences in structure or 
degree of crystallization; hence, the causes leading to 
each are coexistent. 
IX. The causes of these mineralogical and structural differ¬ 
ences must be sought in the differences of geological 
environment, and these affect the rate at which the 
heat escapes from the magmas, and the pressure they 
experience during crystallization. 
Since it has been demonstrated by synthetical re¬ 
search that water- and other vapors are potent factors 
in the crystallization of quartz and other minerals 
that have not been produced artifically without their 
aid, and as there is ample evidence both in the ex- 
travasated lavas and in the coarsely crystallized 
rocks in the conduit that water-vapor was uniformly 
and generally distributed through the whole series of 
molten magmas, and no evidence that there existed 
in the magmas which stopped within the conduit any 
more or different vapors than those which existed in 
the magmas that reached the surface, we may con¬ 
clude that— 
X. The efficacy of these absorbed vapors as mineralizing 
agents has been increased by the conditions attending 
the solidification of the magmas within the conduit. 
