CRYSTALLIZATION OF IGNEOUS ROCKS. 
77 
many instances. These are generally found in rock glasses, 
and appear to be the result of very rapid growth. 
The porphyritical crystals sometimes occur in angular, 
irregular fragments, which are parts of a once perfect crystal 
that has been fractured while the magma was molten. This 
is particularly characteristic of the quartz and sanidine of 
many rhyolites. The outline of the porphyritical crystals 
( phenocrysts ) is at times rounded, so that no vestige of a crys¬ 
tallographic form is visible. In most of these cases it is 
evident that the rounding is not the original form of the 
crystal, but is the result of a partial resorption of the crystal 
by the magma. 
Irregularities of form, however, occur with porphyritical 
crystals ( phenocrysts ) in glassy rocks, which are undoubtedly 
produced at the time of growth, so that they are not bounded 
by crystallographic planes. 
When the crystals have started close to one another they 
have often grown together and have interfered with the de¬ 
velopment of perfect crystallographic form. Their line of 
juncture is quite irregular in most cases. This indicates 
that the two were growing at the same time. When one 
individual crystal has its perfect crystallographic form along 
the line of juncture of the two, and the second simply ter¬ 
minates against the faces of the first, it is evident that the 
first crystal was completed before the second reached it. It 
is therefore older than the second. 
Porphyritical minerals in glassy rocks are frequently found 
in juxtaposition. When they are, their relative age or time 
of crystallization can be observed. But when they are not 
found in contact with one another their relative age remains 
in doubt. The younger mineral may entirely surround the 
older one. 
The relative size of minerals is no indication of their rela¬ 
tive age. Porphyritical minerals are larger than those of the 
groundmass and are generally older. But there are instances 
where certain porphyritical minerals are younger than the 
