101 



The catapleiite crystals of type III, like those of the fore- 

 going type, are all twins with the base as composition -face. 

 On each individual a more or less distinct twinning-line is ob- 

 servable running along the faces of the fundamental prism round 

 the crystal complex. Such a twin formation would not, of 

 course, be possible, if the mineral were holohedral hexagonal. 



In a perfectly unaltered condition the catapleiite crystals 

 are colourless and clear. Often, however, they show a faint 

 brownish tarnish. Crystals that have been long exposed to 

 atmospheric action , are enamel-white and opaque. They often 

 exhibit an iridescent shimmer. 



Fig. 10, Plate V represents a section of a crystal of type 111, 

 cut parallel to the base. It has been cut so that the whole of 

 it is on the same side of the horizontal twinning-line. The 

 section consists of two portions meeting in an irregular line: 

 a isotropic, and Ъ doubly-refracting. The field a shows in the 

 conoscope a regular uniaxial interference figure of positive 

 character. The field Ъ shows in the orthoscope extinction pa- 

 rallel to the edge m'^. In the conoscope this field shows a bi- 

 axial interference figure. The axial plane is parallel with m^\ 

 and the axial angle, measured by means of a micrometer ocular 

 and Schwartzmann's scale, has been found to be 



2Е='Ж. 



The field Ь is traversed by a number of lamellae as thin 

 as a hair, orientated partly parallel to m\ partly to m'". These 

 lamellae are, however, so minute as not to aiïect the inter- 

 ference figure, which is sharply defined and regular. The cha- 

 racter of the double refraction is here also positive. 



Already at a temperature of 30° the double refraction of 

 this section is totally lost; on cooling it returns to its original 

 condition. The heating makes the whole section uniaxial positive. 

 The mimetic structure is, then, here less constant by far than 

 with the first two types. Even by being enclosed in the warm 

 hand the section becomes isotropic. This molecular rearrange- 



