238 
POPULAR SCIENCE REVIEW. 
slag, glass, or lava structure, from its being so characteristic 
of these substances also. 
Everyone has probably noticed the existence, especially in 
glass of inferior quality, of certain lines which more or less 
injure the transparency of the glass, although in themselves 
quite transparent. In the portions .of glass remaining attached 
to the bottom and sides of old or broken glass pots these lines 
are still better seen, being commonly rendered distinctly visible 
by alternating lines or layers of glass possessing different tints 
of green with others white or colourless, and often presenting 
a parallel structure of great beauty, especially when these lines 
are seen to be contorted and crumpled up into all manner of 
shapes. The same structure is seen in the vitreous slags 
from iron furnaces, the colours being usually shades of blue, 
yellow, green, and grey ; whilst the slags from the copper- 
smelting furnaces often show extremely beautiful alternating- 
striations of a deep red and black colour. 
In obsidian or the so-called volcanic glass, an exactly iden- 
tical striped or banded structure is extremely common, and in 
PL LXXIII., fig. 7, which represents a piece of obsidian from the 
Lipari Islands, an attempt has been made to convey an idea of 
this appearance. 
This structure appears to be caused by the movements of 
the different parts of a viscid molten mass, which flow over 
one another at different rates of progression, whilst its existence 
is usually denoted by the bands or stripes in the rock either 
differing in colour or in the shades of some one colour. Some- 
times, especially in obsidians and traps, this structure is 
accompanied by an innumerable number of small air-bubbles 
(? gas or steam also), which, being drawn out or elongated 
from their original spherical shape by the progressive move- 
ment of the molten mass, develop a parallel structure of very 
peculiar appearance. In the older lavas these cavities often 
become, through infiltration, filled up with carbonate of lime 
or other minerals, and thus form what is termed amygdaloidal 
trap or lava. 
When such lavas, glasses, or slags cool quickly, they retain 
their vitreous character and the striated structure already 
described ; when, however, the cooling takes place very slowly, 
or when, by natural or artificial agencies, they have been again 
heated and kept so for a considerable time, devitrification com- 
mences to take place, the crystalline or stony structure develop- 
ing itself first along the lines of striation. In PL LXXIII., 
fig. 2, which depicts a section of a fragment of greenish 
plate-glass from the St. Helen’s works, an attempt has been 
made to illustrate the gradual development of crystallisation 
(or foliation, in other words) along the lines of striation, origi- 
