628 MOTONORI MATSUYAMA 



start from the angular points of these zigzag boundaries. The 

 direction of the parallel straight lines was not the same in different 

 crystals. Sometimes apparently two systems of these straight 

 lines developed in one crystal, nearly perpendicular to each other. 

 When the unbent portion of the same bar was examined in the 

 same way, the crystals were found to be bounded by very smooth 

 boundaries and no straight lines in their sections were visible. In 

 another case, the same crystals were identified before and after 

 bending took place. The same contrast of the disturbed and 

 undisturbed crystals was observable in this case. 



At the time of these observations, the equipment for petro- 

 o-raphic photography was not available for the writer. He was 

 obliged to content himself with very careful sketches of these 

 phenomena as they appeared to him. These are shown in Figures 

 12 and 13. Since these figures represent the sections nearly parallel 

 to the base, the straight lines in the sections must be considered 

 to show the development of a system or two of parallel planes 

 parallel to the optic axis. Uniform extinction was generally 

 observed throughout each individual crystal, but in some crystals 

 portions divided by the straight lines showed slight difference in 

 extinction. 



Bearing upon the question of straight lines in the section, Tarr 

 and Rich^ describe one case in which phenomenon of the sort were 

 observed. They regarded it as notable, however, that this was 

 the only one of their experiments in which bending took place by 

 shearing. Their experiment was about the bending of a single 

 crystal of glacier ice whose optic axis was parallel to the supporting 

 edge. In the present investigation, the same phenomena was 

 observed whenever the constituent crystals were perpendicular to 

 the bending plane. It has already been stated that deformation 

 either by torsional, or by bending, stress took place least easily 

 when the force was applied parallel to the basal section of the 

 constituent crystals. This was thought to suggest that in the 

 deformation of an aggregate of parallel crystals, the contact sur- 

 faces between adjacent crystals probably have played an important 



' R. S. Tarr and J. L. Rich, Zeits.f. Glets., Vol. VI (1911-12), p. 243. 



