54 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136- 
lowstone obsidians discloses a striking similarity in ordinary light — 
the same irregularly scalloped outline, the same central chain of clear 
spherules. With crossed nicols the close similarity vanishes, for in the 
ancient rocks the radial growth has utterly disappeared. The central 
clear chain consists now of a fine quartz mosaic. The dark borders, 
except- for the crowded magnetite globulites, can not be distinguished 
from the noncrystalline quartz feldspar groundmass. This clear cen- 
tral zone where the spherulites converged evidently furnished the plane 
of weakness and easy solution, along which silica was infiltrated and 
parallel to which the rock cleaves. 
The impurities which have entered the rock along this cleavage plana 
give rise to the central dark line mentioned in the macroscopic descrip- 
tion, while the silica forms the opaque white band on either side. The 
central zone is sometimes more than a millimeter wide. Where a feld- 
spar crystal lies across this plane of weakness with its longest axis at 
right angles to the latter, the strain has proved too great for the crystal, 
which has been broken apart and the break cemented by infiltrated 
silica (PI. XXIII, h). 
The chain spherulite structure is of more common occurrence in the 
aporhyolites of the Monterey district than any other form of spheruj 
litic growth. The acid rocks east of the Bigham copper mine show 
them in great perfection. llutley 1 has figured some similar chain 
spherulites in the felsitic lavas of England and Wales. In felsite of 
the Keweenaw series from the Minnesota shore of Lake Superior the 
writer has recently observed fine bands of silica so similar to the altered 
chain spherulites as to suggest a like explanation for them. 
Axiolitic structure. — Closely related genetically to the chain spheru- 
lites, but unlike them in being radial linearly rather than centrally, is 
the axiolitic growth. 2 
Axiolites are not particularly characteristic of the South Mountain 
aporphyolites. Curving, linearly radiating growths do occur, however, 
in specimens from more than one locality. PI. XXI, Z>, shows this 
structure. 
Rhyolitic structure. — The rocks in which the axiolites were observed 
arc noncrystalline, yet they exhibit most strikingly the characteristics 
of a glass. Flow and vesicular structures, stringers and shreds, and 
curved patches of a brownish-red color, forming what has been called 
the rhyolitic structure, abound. (PI. XXIV, a and b; PI. XXV, a.) 
This latter structure has been figured and described by Rutley, 3 Xor- 
denskjold, 4 and de la Vallee-Poussin, 5 and on a macroscopic scale by 
felsitic lavas of England and Wales: Mem. Geol. Survey, Gt. Brit., 1885, Pi. VII, figs. 11 and 12. 
: Zirkel, Microscopic Petrography : Geol. Expl. 40th parallel, p. 1G7. 
3 Rutley, On the microscopic structure of devitrified rocks from Beddgelert and Snowden: Quart. 
Jour. Geol. Soc. London, Vol. XXXVII, p. 406, figs. 1 and 2. 
'Xordenskjold, Zur Kenntniss des sogen. Halleflinta des nordbstlichen Smalands: Bull. Geol. Inst. 
Upsala, Xo. 1, Vol. I, p. 5, 1893. 
f Dela Vallee-Poussin, Les anciennes rhyolites, dites eurites, deGrand-Manil: Bull. Acad. Roy. Bel- 
gique, 3d series, Vol. X, 1885, p. 271. 
