346 ON STRAIN IN CONNEXION WITH CRYSTALLIZATION ETC. 
Fig. 5a. The same crystal and crack seen by ordinary transmitted light: x 32. 
6. Perlitic obsidian devitrified by spherules, Coniston-Limestone horizion, 
Till’s Hole, Long Sleddale, Westmoreland: x 77. 
Upper portion seen between crossed nicols. 
Lower portion seen by ordinary transmitted light. 
For the sake of clearness the cracks are slightly intensified in the 
upper half of the drawing. 
7. Depolarization around a crystal of ilmenite in obsidian, Mount Shasta, 
California. 
This crystal had not been observed at the time when the paper was 
read. ‘The figure is now added as an additional illustration of what 
is seen in fig. 2. By reflected light the crystal is seen to be partly 
altered into leucoxene. 
Discussion. 
Mr. Bavrrman asked if there were any evidence of marked dis- 
similarity in composition between the glass and the crystal in the 
centre. If there were none, the strain might be the result of sur- 
face-tension about the crystal. He remarked on the value of the 
use of microscopic sections of considerable thickness. 
The Prestpent had no doubt that in these porphyritic obsidians 
the crystals were formed before the consolidation of the glass, and 
were simply floating init. He thought that perlitic structure can be 
produced, without the existence of any preexisting cracks, by the 
contraction of a uniform mass. The contraction of a perlitic 
granule, with or without a crystal in its midst, would result in a 
state of stress within it. Spherulitic structures are of two different 
kinds, namely, with and without definite boundaries ; and he pointed 
out the circumstances under which these would be likely to be pro- 
duced. 
The AvtHor believed that in some cases there might be much 
truth in the opinions expressed both by the President and by Mr. 
Bauerman, and was quite ready to admit that similar results might 
possibly be brought about in various ways. 
