bascom] APORHYOLITES. 53 
crystals, followed by tlie emergence of the magina, the development of 
globulites and fluxion structure, the commencement of radial crystalli- 
zation, and finally the consolidation of the magma as a glass and the 
development of perlitic parting. Subsequent to all this, and extending 
over a much longer period of time, the process of devitrification took 
place. 
Associated with a holocrystalline groundmass, which bears less 
marked evidence of an original glassy character, are the more altered 
spherulites. Their circular outline in the thin section and their spher- 
ical shape in the hand specimen testify to their former presence. 
In both hand specimen and thin section a threefold zonal arrangement 
is often clearly defined by the distribution of red or black iron oxide. 
With crossed nicols these boundaries become inconspicuous, and the 
field of the microscope shows only a uniform quartz-feldspar mosaic, 
or the former radial growth is indicated by zones of a finer-grained 
crystallization than the groundmass (PI. XXIII, a), or a micropoikilitic 
structure is present within the spherulitic boundaries when absent in 
the groundmass. Occasionally, vestiges of a radiate structure still 
remain. 
In the specimen already referred to on page 50 and figured in PI. 
XXII, a, the spherulites are colored red by a uniform dissemination of 
hematite particles, and are not more than one-half mm. in diameter. 
They traverse the rock in rows and chains, which are in turn grouped in 
bands about 2 inches wide. The rock has been sheared, a ready cleav- 
; age has been produced, and sericite has developed around each spher- 
I nlite. As has already been mentioned, the spherulitic individuals have 
t become micropoikilitic individuals. Another specimen shows spheru- 
lites which have been rendered almond-shaped through shearing or, 
less probably, by the fluidal movement of the magma during this 
I consolidation. That the former was probably the case is indicated by 
the gradual passage of the rock into a slate and the development of 
biotite. 
Spherulites which, like these, have been replaced by a fine-grained 
! mosaic of quartz and feldspar have been described by Klockmann. 1 
Chain spherulites. — The arrangement of spherulites in layers and along 
planes so that a cross-section shows a chain of spherulites has been 
j described on page 43 and figured on PI. XVIII, b, as it appears in the 
I hand specimen. In ordinary light the microscope discloses somewhat 
sinuous or straight dark bands, with scalloped borders sharply outlined, 
inclosing an irregular clear chain which also has scalloped edges. 
Frequently there are detached clear spots with circular outline. 
These bands sometimes spread out so as to include phenocrysts and 
sometimes curve around them. (PI. XVIII, b.) A comparative study of 
these spherulitic chains and the chains of fresh spherulite of the Yel- 
' Die Torpbyre ; Der geologisobo Aufbau des sogen. Magdebnrger Uferrandes niit besonderer Berttck- 
sicbtiguiig der auftreteudeu Eruptivgesteine: Jabrbucb K. prrnss. geol. Landesaustalt, Vol. XI, 
1890. p. 179. 
