SPHERULITIC CRYSTALLIZATION. 
459 
The synchronous growth of crystals of the same mineral 
with two distinct habits is a natural consequence of crystallo¬ 
graphic branching as distinguished from that due to the 
splitting or cracking of microlites.* Its occurrence in this 
rock indicates how slight may be the difference in the condi¬ 
tions under which either form of crystal is induced. This 
accords with the fact that we find no fixed order in which 
positive and negative spherulitic growths have been devel¬ 
oped. In the rock of Obsidian Cliff they alternate with 
each other, sometimes one having started first and sometimes 
the other. 
The Essential Characteristics of Spherulitic Growths. 
In expressing what seems to the writer to constitute the 
essential nature of spherulitic crystallization, there will be 
no attempt made to suggest a cause for its initiation. It is 
simply a statement of conclusions which have probably 
forced themselves upon all who have made a careful study 
of these kinds of microstructures. 
Just as the earliest definition of a crystal was confined to 
the outward form of the body, and rested upon its being 
bounded by plane surfaces, so the conception and derivation 
of the term spherulite rested on its spherical form. In both 
cases the definitions would not now be limited to a state¬ 
ment of the outward form of the bodies. It is, of course, un¬ 
derstood that a spherulite differs from a crystal in being a 
complex intergrowth of minute crystals of one or more 
species. The essential character of the spherulite must be 
sought in the internal arrangement of this intergrowth ; 
for we should not consider the essential character of the 
crystallization which produced a spherical body as having 
changed because in one instance the sphere had a smooth 
surface and in another a roughened one. In the latter case 
the composite crystals may have been larger. Nor is it 
necessary that a complete sphere should have been formed. 
* Lehman (O.). Molekular Ph} 7 sik. 8°. Leipzig, 1888, p. 378. 
