CRYSTALLIZATION OP IGNEOUS ROCKS. 
89 
strata, in that portion of it where the grain of the rock varies 
rapidly within a few feet, and where the crystallization of 
the coarse-grained portion undoubtedly took place after the 
magma ceased to move, it is observed that the various modes 
of crystallization not only affected the magnetite, ferro-mag- 
nesian silicates, feldspar, and quartz, but even the apatite 
and zircon; for it is observed that in the fine-grained varie¬ 
ties apatite occurs in great numbers of minute, idiomorphic 
crystals. In the coarser-grained varieties it is in fewer and 
larger crystals, and in the coarsest-grained varieties it occurs 
in very much fewer individuals, which are irregularly out¬ 
lined, like all the other minerals in the rock. The zircons 
also are larger in the coarser-grained varieties of the rock. 
Hence it is evident that in this instance the oldest minerals 
in the rock crystallized very near the surface of the earth 
and under comparatively low pressure. The magma must 
have been entirely fluid when it came to rest, and was free 
from all kinds of crystals. 
There are cases also where it can be shown that the crys¬ 
tallization set in after the magma reached the surface of the 
earth. Such is the case in the spherulitic obsidian flow which 
forms Obsidian Cliff, in the Yellowstone National Park.* 
Relation of Rock Structures to Chemical Composition. —(1.) 
The degree or extent of crystallization of a rock also bears 
a very marked relation to its chemical composition. The 
basic rocks have a much greater tendency to crystallize than 
the acid and alkali rocks. Thus surface flows of basalt are 
almost always crowded with crystals, and pure basalt glasses 
are very rare, and are then in small bodies. The size of the 
individual crystals is generally larger than those of acid 
and alkali rocks of similar occurrence, and it is even prob¬ 
able that some coarse-grained gabbros may have been very 
large surface flows. 
On the other hand, rhyolitic lavas are often almost entirely 
glassy; the microscopic crystals in them are very minute, 
*Loc. cit., p. 248. 
10—Bull. Phil. Soc., Wash., Vol. 11. 
