200 



University of California. 



[Vol. 3. 



rapidity in different directions. Many of the crystals are 

 hollow, the inside being entirely dissolved away. Occasionally 

 portions of the prism faces are also dissolved, leaving merely 

 shells. 



Hitherto, our knowledge of the crystallography of pisanite 

 has been limited to the results obtained by Des Cloizeaux, who 

 derived his elements from three interfacial angles. The method 

 of measurement, with the two-circle goniometer is peculiarly well 

 adapted for the determination of the axial elements since all of the 

 readings can be used. From the average values of p'o, q'o and e' , 

 and the readings on the prism faces, the following axial ratio 

 was obtained by the writer: 



a :!>:<■ = 1.1670 : 1 : 1.519:3; fi = 105° 11'. 



The ratio does not vary much from that obtained by Des 

 Cloizeaux, namely: a-.h-.c = 1.1609:1:1.5110; /8 = 105° 22', but 

 is believed to be nearer the true one. The crystals were readily 

 adjusted in true polar position, as usually all four of the prism 

 faces were present and all gave excellent reflections. 



In all, seventeen forms were observed, of which ten are 

 new. These forms are given below in two columns, those in the 

 second column being the new ones. 



Letter. 



Symbol. 



Letter. 





Symbol. 





oat. 



Miller. 





Gdt. 



Miller. 



C 







001 



a 



ooO 



1(1(1 



b 



o« 



010 



h 



2<x> 



210 



m 



QO 



110 



f 



l-oo 



320 



w 



+io 



103 



1 



00 9 



120 



t 



—10 



Tol 



V 



+ 10 



101 







01 



Oil 



.'/ 



— so 



205 



TT 





T12 



)' 



+ 1 



111 









E 



3 



335 









D 



2 



221 









a 



—12 



121 



The angles measured, together with those calculated from the 

 elements obtained by the writer, for the forms, are given in the 

 table below. 



