340 #. RUTLEY ON STRAIN IN CONNEXION WITH CRYSTALLIZATION 
25. On Srrary in Connexion with CrysTaLiization and the DEVELOP- 
MENT of Peruiric Structure. By Franx Rurzzy, Esq., F.G:S., 
Lecturer on Mineralogy in the Normal School of Science and 
Royal School of Mines. (Read March 5, 1884.) 
[Puate XVITTI.] 
In a paper on “ The Microscopic Structure of the Vitreous Rocks of 
Montana, U.S. A.,” read in connexion with a paper upon the mode of 
occurrence of those rocks by Mr. J. Eccles, and since published in 
the Quarterly Journal of this Society (vol. xxxviil. p. 391), I had 
occasion to allude to a peculiar phenomenon of depolarization in the 
little grains of obsidian constituting a rock which was very aptly 
called obsidian sandstone by the officers of the U.S. Geological Survey. 
I there pointed out the relation which the depolarization bore to 
the perlitic structure visible in the little obsidian fragments, and 
also indicated the probable connexion of these strains in the glass 
with the development of crystals. 
The following notes may serve still more clearly to verify the 
statements then made. 
The present observations were made upon a small but thick slice 
of obsidian from Java, given me some years ago by Prof. W. 
Chandler Roberts. On examining this section under the microscope, 
in plane polarized light, a considerable number of crystals may be 
seen, and in nearly every instance the surrounding glass, where it 
comes in contact with the crystal, shows very well-marked depolari- 
zation, which I think I might say, without any doubt, is due to the 
tension or strain produced in the glassy obsidian by the develop- 
ment of crystals or by a strain preceding the development of crystals, 
as I suggested in the paper already alluded to. 
In Pl. XVIII. fig. 1, for instance, we see portions of a crystal around 
which there is a nimbus of double refraction barred by dark radially 
disposed brushes. In drawing this and all the other figures the 
nicols were kept crossed in the position of a St. George’s cross (+). 
Fig. 2 shows a small opaque crystal resembling a cube or an octa- 
hedron, I could not ascertain which. Around this, again, there is a 
fringe of double refraction traversed by a dark cross. In fig. 3, 
again, the crystal is surrounded by a border of depolarizing glass, 
glass which depolarizes because it is in an abnormal condition of 
tension, and the dark arms, as in the preceding figure, are four in 
number ; but in this instance they do not form a symmetrical cross. 
Some of the strains, as evinced by the depolarization figures in the 
specimen, are very irregular, and the irregularity appears some- 
times to be caused by the proximity of one strain-begirt crystal to 
another. 
In fig. 4, we see a very interesting example of a crystal which is 
only about one third environed by a perlitic fissure. Where the 
