430 
CROSS. 
unappreciated, or because the physical laws controlling some 
recognized agent are too imperfectly understood. 
Among the spherulites studied by the writer there are 
two prominent classes : The first class embraces the distinct 
micropegmatite or “ granophyre ” * groups and the dense 
forms which may be considered their equivalent. It has 
been observed by various petrographers that between coarse 
micropegmatite and microspherulites of most delicate radi¬ 
ate structure there exist all intermediate stages. Iddings f 
has illustrated very beautifully the structure of certain 
spherulites formed by the interpenetration of feldspar crystals, 
each of which has quartz fibers regularly intergrown, and 
the manner in which these distinct groups grade into the 
very dense spherulites. He also calls attention to the occur¬ 
rence of single crystals or simple groups of orthoclase crys¬ 
tals with quartz thus intergrown among the phenocrysts of 
crystalline and vitreous rhyolites. While spherulites of mi¬ 
cropegmatite seem to form most characteristically in the 
early stages of consolidation, they are not thus limited, and 
occur at Silver Cliff as products of the last period. View¬ 
ing micropegmatitic spherulites as manifold interpenetra¬ 
tions of quartz-bearing feldspar crystals, it is evident that 
their size will be limited by the same general conditions 
which limit the development of single crystals in magmas. 
Micropegmatitic spherulites are composed of the two prin¬ 
cipal constituents of rhyolite, but it does not seem prob¬ 
able that the feldspar and quartz are necessarily present in 
the proportion in which they occur in the rock. The amount 
Granophyre is a term appropriately applied by Vogelsang (p. 160 of 
work before cited) to a porphyry with a granular crystalline ground- 
mass, as a companion to felsophyre and vitrophyre. Under the idea, now 
abandoned, that some glassy residue was essential to a porphyry, Rosen- 
husch (p. 31 of work before cited) diverted the term from its original 
use and inappropriately applied it to what is called micropegmatite by the 
French and by many English authors. It seems to the present writer that 
the usage of the term in the sense advocated by Eosenbusch is*, to be depre¬ 
cated both on historical and on etymological grounds. 
f Obsidian Cliff, Yellowstone National Park. Seventh Annual Report 
Dir. U. S. Geol. Survey. 8°. Washington, 1888, Plate XV. 
