40 Y. KIKUCHI 



these sets of fissures is more perfect and distinct than the other, as in 

 fig. 10, where the crystal has been ])roken along one of the directions 

 of the fissures. G. Tschermak* has observed and photographed an 

 analogous series of fissures found in an Anorthite-crystal within the 

 tufaceous Meteorite (Eucrite) of Stannern. ]Jut in the crystals from 

 Miyake, they are more numerous and distinct, A third series of 

 fissures parallel to ooPoo , shown in Tschermak's figure has not been 

 observed in the Miyake Anorthite. An anomalous optical phenome- 

 non is observed in the crystals showing this fissured structure. 

 When a slide of such a crystal is examined under crossed. Niçois, the 

 extinction of light takes place in some parts, as usual at a direction 

 making -38° to -40° with the trace of M, but the entire field never 

 becomes dark ; the dark portion being distinctly marked off from the 

 luminous part. The boundary of the two parts is formed by zig- 

 zag lines which correspond to the directions of the fissures and to the 

 cleavage-trace of M. Fig. 1) represents the section of a crystal formed 

 by the parallel intergrowth of two individuals. The upper part, 

 almost devoid of fissures, is separated by an irregular boundary from 

 the lower half, in which very characterestic series of fissures are to 

 be seen. The crystal is twinned according to the Albite-type, consist- 

 ing of two principal twin-individuals aa and hh, which extinguish 

 the light symmetrically on both sides of the trace of M at an angle of 

 38°. The part marked cc never becomes dark in any position, only 

 showing a light bluish-violet tint when the portion aa becomes entirely 

 dark. Sometimes patches of such luminous parts are found scattered 

 M'ithin the dark field, assuming a damasked appearance. The phe- 

 nomena just described may probably be due to a strain resulting from 

 the sudden contraction experienced at the moment of eruption. In 

 fig. y, the mode of attachment of the upper and the lower halves 

 shows that the upper went on growing in the direction of the lower 



* Mikroskopische Beschaffenheit der Meteoriten. Taf. II, fi<^. 1. 



