50 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [mux 136. 
oval spaces, the spherulitic and the microfelsitic. The former does so to 
the exclusion of a glassy groundmass, and must have been formed prior 
to the consolidation of the rock. The latter shares the space with the 
glass into which it gradually passes and of which it may be a molecular 
alteration. Turning to the South Mountain aporhyolites (particularly 
the spherutaxites), we find a similarity in the development of faint gran- 
ular lines forming irregular oval meshes and giving to the section the 
network appearance figured by Zirkel. These lines maybe traces of a 
former perlitic parting or of a microrluidal structure. The meshes are 
now filled either by the quartz areas which condition the micropoiki- 
litic structure or by the vestiges of a spherulitic crystallization. 
The latter represents a primary structure, as in the rhyolites; the 
former may represent the molecular rearrangement of a spherulitic 
crystallization or of a glassy groundmass. In the last case it is the 
direct result ot devitrification and infiltration, processes more readily 
initiated along the borders of the perlitic cracks, as in the rhyolites of 
the Washoe district. 
A comparative study of some sections 1 of the rhyolites of the Rosita 
Hills, Colorado, and of Obsidian Cliff, Yellowstone National Park, 
side by side with the aporhyolites under discussion, also suggests the 
secondary character of the micropoikilitic structure with reference to 
the spherulitic crystallization. In the trichitic spherulites of the mod- 
ern rhyolites there is an appearance analogous to the micropoikilitic 
mottling, caused by the breaking up of the radiating spherulitic fibers 
into irregular areas which extinguish differently. An altogether simi- 
lar phenomenon occurs in some of the spherulites of the ancient rhyo- 
lites. It is indicative of an intermediate stage between the spherulitic 
and a completely micropoikilitic crystallization. This change from the 
spherulitic to the micropoikilitic structure is carried still further in 
some sections, notably in the case of a specimen crowded with minute 
red spherulites. Each spherulite extinguishes as an individual filled 
with inclusions of feldspar and iron oxide (hematite). The host can be 
determined by optical tests to be quartz. The shape of the spherical 
bodies in the hand specimen and in thin section, their tendency to form 
bands and chains, and their uniform color put their original spherulitic 
character and the secondary nature of the micropoikilitic structure 
beyond doubt. It is not supposed that a prior spherulitic crystalliza- 
tion always existed where now the aporhyolites show a micropoilitic 
structure, but these evidences of the derivation of the structure from 
spherulites establish a presumption for its secondary origin in other 
aporhyolites, where it may be the direct result of devitrification or 
may be due to the subsequent alteration, by infiltration, of a granular 
crystallization. 
On the whole, the plainly secondary character of the micropoikilitic 
1 Sections of material from these localities wore kindly loaned the writer by Dr. Cross and Professor 
rddings. 
