52 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 
The other class of spherulites correspond to those figured by Professor 
Iddings. 1 They are much larger than those that have just been 
described. The smallest is easily discernible by the naked eye, while 
the largest is 12J cm. in diameter. They are spherical, hemispher- 
ical, cylindrical, fan-shaped, oval, or irregular in form. While they 
all possess a clear-cut, conspicuous outline in ordinary light, in many 
sections they completely disappear between crossed nicols. The fresh 
spherulites (PI. XXII, 6), which still show in polarized light a radi- 
ating structure, are usually colored red by a finely disseminated iron 
oxide. The globulites of hematite are distributed homogeneously 
throughout the spherulite, or they are grouped in radial and concentric 
lines. These lines are most dense near the margin of the spherulite, 
and may be separated from the central portion by a narrow clear zone, or 
it is the outer rim of the spherulite which is clear. Iu the central por- 
tion of the spherulite the coloring matter shows a tendency to collect 
in bunches that correspond with areas which extinguish as individuals. 
Between crossed nicols the field is broken up into the minute areas 
which were referred to on page 50 as forming a structure approaching 
the micropoikilitic. Feldspar phenocrysts usually occupy the center of 
the radiating crystallization, and two or more spherulitic centers may 
be included within a single outer zone. In specimens from Raccoon 
Creek 2 these radial growths were remarkably well preserved and occur 
in a groundmass which retains the characteristics of a glass in great 
perfection (PI. XX, a; PI. XXI, a). It bears a close resemblance to 
the groundmass of some of the Colorado rhyolites, and in ordinary light 
would certainly be mistaken for the base of a fresh glassy lava. Deli- 
cate perlitic parting, Avhich because of its delicacy is easily obliterated, 
is here preserved in wonderful detail. It is evidently subsequent to 
the radial crystallization to which it accommodates itself. The pres- 
ence of innumerable globulites accentuates the perlitic and rhyolitic 
structures. With crossed nicols the groundmass at once betrays its 
noncrystalline character (PI. XX, b). All glassy structures disappear, 
to be replaced by granular quartz and feldspar, a crystallization which 
closes the perlitic parting and thereby completely obliterates it. 
The porphyritic feldspars still show inclusions of a glassy base. It is 
impossible by any description to produce that definiteness of conviction 
as to the original glassy nature of the groundmass which the character 
of the sections justifies. To one who has studied these sections in both 
ordinary and polarized light there can be no question as to the secondary 
character of the holocrystalline groundmass. One can not escape the 
conviction that the rock originally consolidated as a spherulitic perlite 
and has become holocrystalline by a process of devitrification. 
The sequence of events, as revealed by microscopic study, is as fol- 
ows : There was first the intratelluric development of the porphyritic 
1 Op. cit., p. 277, PI. XVII. 
2 In Franklin County, Pa., east of Kocky liidge and south of Graeffenburg. 
