CONSTITUTION AND ORIGIN OF SPHERULITES. 437 
surrounding magma is not perfectly homogeneous the secre¬ 
tion of the colloidal substance will probably not proceed 
uniformly and the common hemispherical or otherwise 
laterally developed forms are a natural result. If a lateral 
growth is once started by some primarily trivial cause, its 
continuation in that direction may be attributed to the thus 
localized influence of some factor, such as the heat liberated 
in rapid crystallization, which may promote further secre¬ 
tion of the colloidal matter in the adjacent portion of the 
magma. In this way bushy spherulites extending in all 
directions through a homogeneous magma may be explained. 
Many irregularities of growth to be observed in spherulites 
seem to be natural results of such a process as that outlined 
above. For example, it is common to find spherulites of 
regular form whose radiate crystallizations spring from 
several points not symmetrically related to that form. There 
is often a concentric zonal structure about each point of 
origin and the growths meet on sharply defined planes, but 
gradually a controlling force brings all into harmony and 
the outer zones are regular shells of the sphere. Such a 
structure may be explained by supposing that the secretion 
of the colloidal substance began simultaneously at several 
neighboring points. As the spheres came into contact they 
would at first be separated by planes such as those between 
drops of viscous liquid, and the accompanying crystalliza¬ 
tion would be limited by those planes. Gradually these 
additional zones would become concentric to a common point 
and might coalesce. The manner in which a spherulitic 
growth regains its symmetrical relationship to the outer form 
after curving around a foreign body is directly analogous to 
the case just considered. 
That the colloidal substance formed prior to the crystalli¬ 
zation of the feldspar needles cannot in all cases have been 
simply hydrous silica is indicated by several observations, 
three of which will be mentioned : 
1st. In the supplemental spherulitic growth which has 
been described (p. 424) and is illustrated by Fig. 2, Plate 5, 
