AND THE DEVELOPMENT OF PERLITIC STRUCTURE. 343 
that cooling has taken place. The main point demonstrated in 
these notes is, that when a crystal is developed in a vitreous magma, 
the strain accompanying its development influences the adjacent 
vitreous mass, reducing it to a condition of permanent tension for 
some distance around the crystal, and that if this strain be insuf- 
ficient to cause rupture in the glass, the tension gradually decreases 
and fades away, while if it be strong enough to produce a perlitic 
fissure, the permanent tension will end abruptly at the fissure, the 
glass which surrounds the perlitic area showing no trace of tension. 
It seems therefore probable, if not certain, that perlitic structure 
may be developed in three ways, in each case strain or tension being 
the primary cause of the perlitic structure, whatever may be the 
cause of the strain. 
lst way. Cooling along minute fissures. This seems to be the 
usual way where the perlitic structure pervades the whole 
rock, as in perlites. 
2nd way. Cooling around included fragments taken up in a lava 
flow. This seems to be merely a marginal structure connected 
with the fragments, and not affecting the rest of the rock. 
drd way. By the tension in the surrounding mass when a crystal 
is formed ; the formation of a crystal not necessarily involving 
perlitic fission. 
Of course there is always the question whether the crystals were 
developed before, at the same time as, or after, the solidification of 
the glass. Iam inclined to think that a crystal which has a de- 
polarizing nimbus has been developed at the time when the sur- 
rounding vitreous matter was in the act of solidifying. 
Before, however, expressing a positive opinion upon this point, it 
is desirable that a large number of observations should be made 
upon thick slices of vitreous rocks. 
At p. 66 of the late Hermann Vogelsang’s ‘ Krystalliten’ will 
be found a very clear record of his observations on the depolarization 
around certain crystallites which he found in samples of window- 
glass from the Stolberg works. 
With regard to the development of structures in glass along 
fissures, the following observations may possess some interest. 
Fifteen or twenty years ago a house was burnt down in the market- 
place at Dover ; a small piece of plate-glass from one of the windows 
is seen, when closely examined, to be traversed by minute, irregular 
cracks like those in the artificially cracked, carmine-stained quartz 
of the French jewellers, known as rubace. When examined under 
the microscope, between crossed nicols, these cracked surfaces appear 
to be studded with great numbers of beautiful little spherules with a 
radiating crystalline structure, each traversed by the usual dark 
cross. That they follow the cracks is evident, because one has to 
focus down to them in tracing the crack from the upper surface of 
the glass downwards. They are, however, really circular crystalline 
films like little flat wheels lying on the cracked surfaces of the glass. 
