CRYSTALLIZATION OF IGNEOUS ROCKS. 
87 
smaller ones continued their growth until they mutually 
interfered and produced the same structure as that of evenly 
granular rocks, which may be granitic, ophitic, or a combi¬ 
nation of the two. The character of the inclusions is some¬ 
times different in the minerals of first and second generation, 
or in the porphyritical individuals and in the so-called gra¬ 
nitic ones of the groundmass. This indicates differences in the 
manner of crystallization in the two cases; but the identity 
of the mineral species is not affected by these extraneous 
differences. This is particularly well illustrated for so simple 
a mineral as quartz, where the difference between its porphy¬ 
ritical occurrence and its so-called granitic occurrence is con¬ 
fined to its outward form and the nature of its inclusions. 
For example, in a quartz-porphyrite occurring as an intru¬ 
sive body near the surface of the cretaceous strata at Electric 
Peak,Yellowstone National Park, the finest-grained varieties 
of the rock carry small porphyritical quartzes in an extremely 
fine-grained groundmass. The quartzes are mostly idiomor- 
phic and dihexahedral. Some are rounded. They carry 
crystallized inclusions resembling the groundmass, which 
also penetrates some of the individuals in pockets or bays. 
In the coarser-grained varieties of the same rock the porphy¬ 
ritical quartzes are larger and their outline more irregular. 
The inclusions of groundmass are coarser-grained. In the 
coarsest-grained varieties of the rock the quartzes are still 
larger. Many of them no longer exhibit even a rude dihexa¬ 
hedral form, but are quite irregularly shaped and inclose 
grains of feldspar. Some quartzes still exhibit a rude idio- 
morphic form. They have in many cases inclusions shaped 
like those in the quartzes of the finest-grained varieties, but 
the inclusions are coarser-grained and appear to be feldspar 
for the most part. The outer portion of these quartzes may 
be called granitic. 
In this occurrence it is evident that the crystallization of 
the porphyritical quartzes in the finest-grained varieties of 
the rock was suddenly checked, and the groundmass formed 
as a distinct act of crystallization; but in the coarser-grained 
