bascom.] APORHYOLITES. 51 
structure in the basic volcanic, the evidence in the aporhyolites of its 
being subsequent to fluidal lines or to perlitic parting, the indications 
that in many cases it is subsequent to a spherulitic crystallization, all 
denote a secondary origin for this structure in the South Mountain rocks. 
Spherulitic structure. — There are two sorts of spherulitic crystalliza- 
tion in the aporhyolites. They differ in no essential respect, but are 
unlike in appearance. The most numerous spherulites are also the 
simplest and smallest. They arc colorless, microscopic spheres, scarcely 
or not at all perceptible in ordinary light, but between crossed nicols 
showing a distinct dark cross. Spherulites in every respect similar 
have been described and figured by Professor Iddings from the rhyo- 
lites of the Yellowstone National Park. 1 Similar spherulites also occur 
in the rhyolites of Hungary and in the felsites of the Boston Basin. 
These radial growths are grouped in bunches and along lines, and are 
composed of positive fibers. Further optical determination of the fibers 
could not be made. Their positive character indicates either a prismatic 
section of quartz elongated in the direction of the vertical axis or a 
clinopinocoidal (010) section of orthoclase elongated in the same direc- 
tion. In the latter case the extinction would be slightly inclined. 
This is impossible, however, of determination. 
While it is not impossible that some of these spherulites are second- 
ary, in some cases there is evidence of their primary character. One 
such case of spherulites whose formation was coincident with the con- 
solidation of the rock occurs in an aporhyolite from a cut made by the 
Gettysburg Railroad north of Toms Creek trestle. Minute colorless 
spherulites are embedded in a base which suggests in every way its 
former glassy condition. In ordinary light there is no evidence of 
crystallization except the porphyritical. 
The groundinass is traversed by irregular, angular cracks, evidently 
the result of crushing. These cracks, which are cemented by epidote, 
pass through the spherulitic aggregates (seen with crossed nicols), some- 
times cutting directly across a spherulite, portions of which appear on 
either side of the crack. Between crossed nicols the field breaks up 
into a mosaic of quartz and feldspar. The granular crystallization 
disregards the cracks, filling the spaces left by the cementing epidote. 
It seems fair to conclude from these observations that the spherulitic 
crystallization is prior to the cracking, that the granular crystallization 
is subsequent, and that the cracking occurred in an already solidified 
glass. 
The secondary character of the granular crystallization admitted, it 
is easy to suppose that it is due to devitrification continued through 
lapse of time. The spherulites, on the other hand, being prior to 
the crystallization of the groundmass, and prior to the cracking, are 
doubtless primary and contemporaneous with the consolidation of the 
glass. 
1 Obsidian Cliff': Seventh Ann. Rept. U. S. Geol. Survey, Pi. XVII, p. '276. 
