78 
IDDINGS. 
small crystals in the groundmass, great quantities of which 
are enclosed in the porphyritical ones. 
In holocrystalline porphyritic rocks the outlines of the 
porphyritical crystals ( phenocrysts ) are sometimes straight- 
edged, usually when the groundmass is very fine-grained. 
But in the coarser-grained varieties the sharp outline of the 
porphyritical crystals ( phenocrysts ) disappears. The surface of 
the crystals is then rough, and is bounded by the small crys¬ 
tals of the groundmass, which are more or less irregular. 
This indicates that the crystallization of the large crystals 
continued while the multitude of small ones were growing, 
and continued until they interfered with one another. 
In much coarser-grained rocks the form of the component 
crystals is still more irregular, but generally differs for differ¬ 
ent mineral species. Some may possess their proper crys¬ 
tallographic form completely; they are idiomorphic. Others 
may have quite irregular forms, which have been imposed 
upon them by adjacent minerals. These are said to be allotrio- 
morphic. As already remarked, the mineral that possesses its 
proper form when in conjunction with another that does not 
is older than the second. Hence the idiomorphism of a min¬ 
eral is an indication of its age with respect to the minerals 
contiguous with it. It indicates that the idiomorphic min¬ 
eral ceased to crystallize before the others reached it. 
There are cases, however, where one mineral finished grow¬ 
ing before another came in contact with it, but where the 
first one does not possess a sharp crystal form. The original 
crystallization appears to have been irregular, as noted in the 
fine-grained varieties. In these cases the relative age is rec¬ 
ognized by the partial enclosure of one of the minerals by 
the other. 
Two minerals that grow together at the same time often 
penetrate each other irregularly, so that they mutually in¬ 
close portions of each other in such a manner that all the 
scattered parts of one mineral have the same crystallographic 
orientation, and all the parts of the other mineral have 
another orientation, which may be parallel to that of the first 
or not. 
