86 
IDDINGS. 
groundmass are often characterized by the same inclusions 
and the same twin lamination. 
The importance of the identity of the minerals of first and 
second generation rests in the light it throws upon the influ¬ 
ence, or the apparent lack of influence, of certain physical 
conditions on the crystallization of the rock-making miner¬ 
als. It may be assumed that the large porphyritical crystals 
( phenocrysts ) were formed much earlier than the microscopic 
individuals in the groundmass, and therefore that they were 
formed at much greater depths and under far greater press¬ 
ure. But when it is observed that the small crystals in a 
holocrystalline groundmass, which were probably formed at 
or near the surface of the earth, possess exactly the same 
characters as those of crystals assumed to have been formed 
at very great depths, it becomes evident that in their case, at 
least, extremely great pressure is not necessary to their crys¬ 
tallization. The point is illustrated by the following occur¬ 
rence : A holocrystalline dacite in the Yellowstone National 
Park occurs as a surface flow of lava connected with breccias. 
Its groundmass crystallized completely to a microcrystal¬ 
line aggregation of quartz and feldspar, which undoubtedly 
formed after it came to rest at the surface of the earth. The 
porphyritical crystals consist of hornblende, biotite, plagio- 
clase, and quartz. Some of these quartzes are idiomorphic; 
others are rounded. They carry numerous glass inclusions. 
The microscopic quartzes of the groundmass are nearly all 
idiomorphic, and exhibit the characteristic dihexahedral 
forms, interfered with slightly by the individuals of feldspar, 
which must have crystallized at about the same time. These 
minute quartzes carry dihexahedral glass inclusions like 
those in the large quartzes. From this it is evident that 
quartz with the same extraneous characteristics, such as out¬ 
line and inclusions, may crystallize under comparatively 
slight pressure and also under very great pressure. 
It oftener happens that the outline or shape of the min¬ 
erals of second generation differs from that of the porphy¬ 
ritical individuals of the same species. This is because the 
