342 ¥F. RUTLEY ON STRAIN IN CONNEXION WITH CRYSTALLIZATION 
We have in this very rock which we are now discussing the most 
distinct evidence that strain produces perlitic fission and that strain 
accompanies crystallization. 
The perlites of Hungary, Saxony, and other countries are filled 
with closely packed perlitic fissures; but the perlitic areas seldom 
environ one particular crystal. They cut indifferently across 
streams of microliths, and it is evident that the contraction which 
gaye rise to the perlitic structure is quite unconnected with crys- 
tallization, so far as we can see, being analogous to the spheroidal 
structure in basalts and phonolites, as pointed out long ago by 
Professor Bonney. I think if we regard the development of a 
crystal as the occasional means of augmenting strain round about 
it, or of developing strain where all was previously in equilibrium, 
we shall not be very far from the truth. 
Perlitic structure can be set up in a vitreous rock without the 
intervention of crystallization; but if perlitic structure and sporadic 
crystals be synchronously developed, an intimate connexion be- 
tween the one and the other will then, I think, be found to exist. 
Then, again, in the perlite of Schemnitz there are crystals of felspar 
and mica against which the perlitic fissures abut; but these fissures 
do not traverse the crystals, nor do we find any of these crystals 
lying within definite fissure-boundaries of their own. There is other 
evidence also which proves beyond question that these crystals 
existed in the rock before any perlitic structure was developed. In 
the porphyritic pitchstone of Spechthausen, near Tharaudt, in Saxony, 
we may meet with little fragments of rock and broken crystals, and 
these are very commonly closely wrapped round by minute curved 
cracks in the contiguous glass, which closely resemble perlitic fissures. 
Yet in such a case the cracks are more probably due to sudden 
cooling of the heated glass by contact with colder bodies, such as the 
fragments of crystals and rocks, which we must regard as volcanic 
ejecta or as rubbish taken up by the molten lava. 
We see that strains may occur in glass without the development 
of perlitic structure. 
It seems to be a question of the extent of the strain, whether or 
not actualrupture ensues. If the tension be tolerably equal around 
any particular point, there will be a surface of tension-limit which 
will be approximately a sphere; and when that limit is reached 
rupture will follow, and perlitic structure will be the result. 
Finally, when the strain is associated with the development of a 
crystal it may not be sufficiently intense to bring about perlitic 
structure, and may merely cause a nimbus; but, should it be 
powerful enough to induce rupture, then the perlitic fissure will 
encircle the erystal. 
In perlites, such as those of Schemnitz, where there is no relation 
between the perlitic structure and the minute porphyritic erystals, 
or between the perlitic structure and the fluxion-banding, we 
shall, I think, find, as I suggested in a paper long ago, and as I 
believe Professor Bonney did before me, that the perlitic fissures lie 
packed between rectilinear fissures along which we may assume 
