422 
CROSS. 
it seems both unnecessary and unwise to suppose that in¬ 
distinguishable bodies may occur in the same association and 
of the same condition of origin which are either made up of 
a wholly hypothetical substance, such as petrosilex or micro- 
felsite, or of a known mineral, as chalcedony, whose forma¬ 
tion in such circumstances is chemically unlikely. The 
assumption for other cases, that the optical properties are 
due to tension in an amorphous substance, should have some 
special basis to rest upon. 
Dismissing from further consideration these very dense 
and very minute spherulitic forms, the main part of the fol¬ 
lowing descriptions will have reference to a distinctly differ¬ 
ent class of spherulites, the nature of whose growth permits 
them to reach a size and assume forms scarcely possible for 
the others. The spherulites in question occur in several 
generations of growth in the rocks of Custer county, and 
each growth possesses characteristics suggesting some local 
name for use in study and description. In using these con¬ 
venient terms in this article the writer must not be under¬ 
stood to suggest that similar types, developed in other lavas 
under somewhat different conditions, are likely to appear in 
the same order of formation, or in the same formal relation¬ 
ships. A perfect coincidence in products of two occurrences 
would imply a perfect coincidence in numerous varying con¬ 
ditions, something very improbable for the entire series 
though it might be partially realized in many cases. 
The first spherulites of this series are commonly charac¬ 
terized by internal cavities, large with reference to the whole 
spherulite and of very irregular shape. Movements of the 
stiff viscous mass soon after these hollow spherulites were 
formed crushed many of them and the cavities were filled 
by liquid matter. Fig. 1, Plate 5, shows in natural size a 
rock in which the hollow spherulites are abundant. Most 
of them have been crushed, but the dislocation of fragments 
was evidently slight. 
The solid shells of hollow spherulites are chiefly made up 
of colorless needles of feldspar radiating from many different 
