SYMMETRICALLY DISTORTED CRYSTALS FROM WEST AUSTRALIA. 333 



presence of a large percentage of tin was proved by Mr. J. Verge, 

 b.a., and Mr. C. F. de Jersey Grut, b.a., b.e., by blowpipe tests. 

 After this it was thought the mineral was probably a dimorphous 

 form of Sn0 2 . Mr. Davies kindly lent me a number of crystals 

 to enable me to work out the crystallographic form. It was then 

 seen that some of the crystals were apparently monodinic, others 

 apparently rhombic. As will be seen immediately from the photo- 

 graph, the most perfect of the crystals is apparently monoclinic, 

 with prism faces [110] and clinodomes [Oil]. The other pseudo- 

 monoclinic crystal shows, apparently, the prism [110], clinodomes 

 [Oil], positive (1) and negative orthodomes [101] [101]. The 

 pseudo-rhombic crystal shows apparently (if the long axis be held 

 vertical), the prism [110], macrodomes [101], basal planes [001], 

 macropinacoid [100], brachypinacoid [010], and pyramid [111]. 



A measurement of the angles of the pseudo-monoclinic crystal 

 with the most brilliant faces at once explained the anomaly and 

 proved that the edges between the faces 1-2, 2-3, 3-4, 4-1 were 

 crystallographically similar, and that therefore the crystal 

 possessed the symmetry of the tetragonal system. 



The faces developed are those of the tetragonal pyramid [HI]. 

 Traces of oscillatory combination with the first order prism [110] 

 are expressed by striation parallel to the long edges of the crystal. 

 The readings of internormal angles measured over polar edges are 

 very concordant. The mean of seven readings, making due 

 allowance for the relative perfection of the images in different 

 cases, is 58° 43'. The mean readings over the lateral edges is 

 92° 38'. The temperature at the time of reading was constant 

 at 18° C. 



The measurements were made by means of a Babinet type of 

 goniometer (horizontal circle) by Fuess, the Websky's slit being 

 used as a signal. 



These numbers agree well with those given in the sixth edition 

 of Dana's "System of Mineralogy/' where the corresponding angles 

 are 58° 19' and 92° 53' respectively. From the observed angles 



