436 
CROSS. 
the frequent development of opalline silica with the spheru- 
lites, has led to the idea that this amorphous substance plays 
an important role in the formation of these bodies. Michel- 
Levy* has expressed the same belief, founded on the observed 
facts, first, that opal characteristically assumes the globular 
form; second, that chalcedony is often developed in spheru- 
litic form, and on the erroneous supposition discussed above, 
that all positive rays of spherulites are quartz. 
The hypothesis to be presented has been suggested by 
numerous observations tending to show that the crystalliza¬ 
tion of the feldspars in radiating branching forms was but 
one act in the history of the spherulite, and that it was pre¬ 
ceded by another more important act, which provided the 
conditions necessary to that peculiar growth and directed 
and limited it. This precedent act is conceived to be a local 
change in the character of the magma, in that, within the 
area of each spherulite, there first developed a colloidal sub¬ 
stance. There are several ways in which this general idea 
can he elaborated and applied to different phases of the 
problem in hand. 
The simplest conception is to suppose that the magma 
splits into its two chief components, hydrous silica and feld¬ 
spar, through the separation of the former in its character¬ 
istic globular form, and that the individualization of the 
latter takes place simultaneously. Starting with the initial 
globule of opalline or colloidal silica, containing in it the 
elements of feldspar, the idea is that the degree of viscosity 
and the tension which must be assumed induce and direct 
the arborescent feldspathic growth. Such a globule once 
formed might serve as a nucleus for successive secretions from 
the surrounding magma. If this secretion proceeded at a 
uniform rate the structure of the resulting spherulite would 
be simple. But if the opalline matter were added in distinct 
periods a concentric zonal structure might be emphasized 
by slight variations in the development of the feldspars, or 
in other ways actually observed in spherulites. Where the 
* Michel Levy (A.). Structures et classification des roches eruptives. 
8°. Paris, 1889, p. 22. 
