264 OBSERVATIONS ON THE STRUCTURE OF ROWLEY RAG. 
It is important to notice the difference between this 
transparent perfectly colourless glass and the dark, almost 
opaque, glass which has resulted from the rapid cooling of 
masses of similar composition, and which goes by the name 
of Tachylyte. For purposes of comparison I have a slide cut 
from the artificial tachylyte which Messrs. Chance, of Old¬ 
bury, produced many years ago by melting and casting in 
moulds the rock under discussion. The difference is evidently 
owing to the fact that the oxides of iron to which the colour 
is due had, when these residual glassy patches solidified, been 
almost all removed by the previous separation in a crystalline 
state of magnetite and augite, so that the remainder is quite 
colourless. 
When the dust mentioned above is examined with a high 
power it is seen that many of the individual specks are very 
irregularly shaped masses of some brown substance, and in a 
very few cases I believe there are circular clear bubbles in the 
inclusion, but the bubbles are immovable. In the description 
of a “Trachyte Pitchstone” from Cantal, in Central France, 
quoted from Von Lasaulx in RosenbusclTs “ Mikroskopische 
Physiographie der massigen Gesteine,” p. 162, the occurrence 
of patches of brown glass with bubbles in the midst of the 
colourless glass of the rock is mentioned as an unusual 
circumstance. 
In the red veins the structure is exactly the same, but the 
glassy base is replaced by a fibrous radiating mineral, though 
whether it is an original product of the consolidation of the 
vein or a result of subsequent alteration of the glass it is not 
easy to decide. 
There are a few brownish grains, of which I do not know 
the nature. They are slightly dichroic, have bright colours 
in polarized light even in very thin section, and stand out 
with the peculiar bold relief which shows a high refractive 
index. One of them shows one line, apparently a cleavage 
line, parallel to the sides of the section, and this one 
extinguishes parallel to the line. In another case the substance 
fills the angle between two felspar crystals; in another, a minute 
grain is shut into an augite crystal. The dichroism is not 
strong enough for mica, nor does the structure seem fibrous 
enough. 
The colourless glass is by no means only found in the 
veins of which I have been speaking. Some of the slides 
which are exhibited show considerable quantities among the 
other constituents of the rock, especially in the parts where 
the crystals are large, and it appears always to be filled with 
a fine dust similar to that in the grey veins. 
