CRYSTALLIZATION OF IGNEOUS ROCKS. 
85 
crystalline groundmass of orthoclase and leucite; long slen¬ 
der crystals of apatite occur in great abundance in the feld- 
spathic minerals and are absent from the olivines and au- 
gites. The comparatively great length of these apatite 
needles, one being three hundred times longer than wide; 
their straightness, together with the total absence of fluidal 
or other orderly arrangement, indicate that they did not 
exist in the magma when the olivines and porphyritical au- 
gites were formed, nor while the mass was in motion. They 
must have crystallized after the olivine and augite, and after 
the magma came to rest, prior to the crystallization of the 
feldspar and leucite. The manner of occurrence of apatite 
needles in certain diorites also indicates that they were 
among the later crystallizations in these rocks. 
(2.) In rocks where the same species of mineral occurs as 
the result of more than one period of crystallization—that 
is, where the crystallization has been interrupted—it is evi¬ 
dent that the interruption may happen when only one or 
two of the mineral species have commenced to crystallize, 
or when all have progressed to different extents. In the 
second period of crystallization several or all of the minerals 
of the first generation may crystallize again in the ground- 
mass. This is particularly the case with the iron ores, ferro- 
magnesian silicates, feldspars, and quartz. 
It is sometimes observed that the minerals belonging to 
the second generation differ from those of the first generation 
when they belong to isomorphous series. Thus it frequently 
happens that the feldspars composing the groundmass are 
more alkaline than those occurring as porphyritical crystals 
(. phenocrysts ). A difference is sometimes noticeable between 
the porphyritical hornblendes or pyroxenes and those taking 
part in the groundmass; but it often happens that no differ¬ 
ence can be detected optically, and many minerals appear to 
be identical in both modes of occurrence. This is particu¬ 
larly true of magnetite, biotite, leucite, nepheline, and quartz. 
The porphyritical leucites and the microscopic ones of the 
