Review of Recent Geological Literature. 
183 
volcanics (eovolcanics). (2) According to the second view the differ¬ 
ences of habit just mentioned are due to alterations secondary to the 
consolidation of the rock. This view seems the more probable, because 
palseovolcanic rocks which have undergone dynamic metamorphism re¬ 
semble the eovolcanics, while those that have not undergone such met¬ 
amorphism are inseparable from the neovolcanics. Even though this view 
were true, the author thinks it is fitting to separate these rocks by such 
a name as eorhyolite until the correctness of the view and the exact 
character of the metamorphism is established. He recognizes the 
difficulty at once met with in consistently carrying out this nomencla¬ 
ture, viz., the impossibility of separating the younger quartz-porphyries, 
which have been altered by dynamic metamorphism, from the eorhyo- 
lites. The author states that the most important change which these 
rocks have undergone is devitrification (Entglasung) on a large scale. 
This was the case with the corresponding rocks of the South mountain, 
and was the occasion for the proposal of the name aporhyolite.* 
If devitrification were a function of time only and demanded for its 
consummation such long periods of time as to confine its occurrence to 
the eorhyolites, i. e., to pre-palseovolcanics, that name could fitly cover 
all rhyolites showing this phase of alteration. The reviewer’s idea is 
that glassy lava is in a condition of unstable equilibrium; that glass al¬ 
ways tehds to become stony, that while this process may take place 
slowly during considerable periods of time, it may also take place with 
comparative rapidity if the glass be subjected to heat or pressure; in 
short, that devitrification is a function of both time and conditions of 
heat and pressure, and that the time called for varies with the composi¬ 
tion of the glass, and may not be nearly so long as is implied by confin¬ 
ing devitrification to eorhyolites. 
The tendency of glass “ to degenerate ” is a common experience in 
chemical laboratories. It has been observed in the chemical laboratory 
of Bryn Mawr College that glass tubing which has been kept on hand 
more than three years, will, upon slight heating (not to fusion) instant¬ 
ly become opaque and rough. The microscope shows the roughness 
and opacity to be due to a fine network of cracks, within and along 
the edges of which incipient crystallization has taken place. It is be¬ 
coming a recognized fact among chemists that certain chemical combin¬ 
ations, which were formerly supposed to take place only at very high 
temperatures, will also take place at much lower temperatures, but very 
slowly. The converse is equally true. Fresh glass tubing remains per¬ 
fectly clear under the action of heat, but during three years it had 
plainly passed into a state of unstable equilibrium. Under the action 
of heat it rapidly passed into a more stable form, a condition which it 
would not have reached for a long time without that aid. That this 
period of time is not of indefinite duration is shown by the well known 
devitrification of window glass, nor is devitrification unknown in post- 
Tertiary glassy lavas. The rhyolitic lavas of the Rosita hills, Colorado,* 
show devitrification in residual areas around the spherulites. In this 
"The Journal of Geology, vol. i, no. 8, Nov.-Dec., 1893, p. 328. 
