AND THE DEVELOPMENT OF PERLITIC STRUCTURE. 341 
fissure comes, there the depolarization ends abruptly, but the re- 
mainder of the depolarizing area shades off gradually. 
Fig. 4a represents the same crystal as seen by ordinary trans- 
mitted light. The extent of the perlitic fissure is shown on the 
right-hand side of the crystal. This crystal, like some others in the 
section, has a rounded or spheroidal form. 
I may here mention that, in a piece of plate glass given me by 
Mr. W. Douglas Herman, a distinct depolarization nimbus may be 
seen around a group of crystallites when the nicols are crossed ; and 
this phenomenon around crystallites in ordinary artificial glass is 
well known. 
In fig. 5 we see a crystal completely surrounded by a perlitic 
fissure. ‘The perlitic area depolarizes, but the depolarization ends 
abruptly at the fissure. In examining this section of obsidian with 
crossed nicols, one can instantly detect the position of the perlitic 
fissures by the sharp boundaries of the areas of depolarization. 
Fig. 5a is the same. as fig. 5, when viewed by ordinary transmitted 
light. The perlitic fissure is shown completely surrounding the 
crystal. 
I think that these observations tend to show a definite sequence 
of events, which we may classify in the following manner :— 
A. 
1st. Development of Strain. 
2nd. ie Perlitic Structure. 
Thus we may meet with perlitic structure, without any enclosed 
erystals, but with depolarization within the perlitic area. Instances 
of this are seen in the obsidian sandstone of Montana, U.S.A. 
B. 
1st. Development of Strain or development of strain and 
2nd. £ Crystals crystals at the same time. 
Examples of this are seen in figs. 1, 2, and 3, where the obsidian 
around the crystals shows depolarization, but no trace of perlitic 
structure. 
C. 
1st. Development of Strain or development of strain and 
2nd. 3 Crystal crystal at the same time. 
ord. = Perlitic Structure. 
In such instances we find crystals with surrounding depolarization 
areas bounded by perlitic fissures as in fig. 5, while fig. 4 represents 
the incipient development of the perlitic fissure. 
The question may be asked, Why in a depolarizing perlitic area 
may we often find no crystal developed if the strain be so intimately 
connected with the development of a crystal ? 
This isnot quite an easy question to answer. 
