58 VOLCANIC ROCKS OF SOUTH MOUNTAIN. [bull. 136. 
spherulitic, and fluxion structures, so clearly indicated in ordinary 
light, in a homogeneous noncrystalline mosaic is one of the strongest 
evidences for the secondary character of the crystallization. 
There are also instances where the nature of the crystallization is 
distinctly proved. On page 51 it was shown to be subsequent to the 
cracking which must have occurred in a solid rock. Pages 52-53 
describe the replacement of the radiating crystallization of the spheru- 
lites and chain spherulites by a granular crystallization which is homo- 
geneous with a granular groundmass. Finally, on pages 49-50, the 
secondary character of the micropoikilitic crystallization has been 
indicated. One or another of these indications of secondary crystalli- 
zation is almost invariably present in the rocks which have been 
included under the name aporhyolite. 
The exceptional occurrences, where these structures are absent, show 
genetic relationship in the field to typical aporhyolites. The deter- 
mination of the character of the groundmass in the cases described 
thus practically determines it for all the aporhyolites. 
The secondary character of the holocrystalline groundmass once 
admitted, and the indications of an original glassy base recognized as 
such, one is forced to conclude that the former was developed from the 
latter by a process of devitrification. 
That the processes of crystallization do not necessarily cease with 
the solidification of a magma is well known, for experiment has x^roved 
that crystallizing forces are active in a glass as well as in a molten 
magma. 1 This action is exceedingly sluggish, and requires, unless 
accelerated by heat and moisture, an immense amount of time. Devit- 
rification has been considered the result of dynamic action only; 2 but 
while dynamic action undoubtedly accelerates the process, if it does not 
initiate it, devitrification may also take place independently of dynamic 
action, as was the case in the famous example of the old cathedral win- 
dow glass 3 and the ancient devitrified glass from Nineveh investigated 
by Sir David Brewster. 4 The nature of the process is in no way differ- 
ent from the process of crystallization in a fluid magma save in the 
rapidity of the action, and is of both a physical and a chemical character. 
The devitrification which has occurred in the South Mountain apo- 
rhyolites is not attributed to dynamic action, of which there are many 
evidences of another nature in the South Mountain, but to statical 
metamorphism. The former would, by shearing, obliterate the original 
structures of a glassy rock and produce a slate, while the latter might 
be an important initiatory and accelerating factor in the process of 
devitrification of the glassy rocks. 
'Daubree, Geologie experimental, 1879, p. 158. 
'-De la Vallee Toussin, Les eurites quartzeuses (rhyolites anciennes) <le Xivelles et ties environs: 
Hull. Acad. roy. sci. lett. et des beaux arts <le Belgique, 57 annee, 3d series, Vol. XIII, No. 5, 1887, 
pp. 521-522. T. G. Bornemann, Der Quartzporphyr von Heiligenstein und seine Fluidalstructur: 
Zeitschr. Deutsch. geol. Gesell., Vol. XXXIX, 1887, p. 793. 
3 British Ass. Ptept., 1840. 
"Trans. Royal Soc. Edinburgh, Vols. XXXII, XXXIII. 
