(210) 



(301) 



О 



о 



о 



(301) 



(12о; 



Fig. 29. Etchi ng figures 



on {210} 



produced by alkalies. 



82 



placed horizontally, across the longi- 

 tudinal direction of the prismatic 

 faces ; otherwise the form may be 

 somewhat varying. 



Л {2 10/ is more slightly corroded 

 than the preceding face. 



(Fig. 29) The form of the figures 

 is an isosceles triangle, sometimes 

 approaching very much an equilateral 

 one; as in the preceding face they 

 have also here their greatest extent 

 horizontally. 



с Natural etching figures. 



In a few cases I have succeeded in pointing out natural 

 etching figures on the ilvaite. if a common face of the mineral 

 is examined under the microscope by means of incident light, 

 differently shaped, sometimes quite regular and distinctly bounded 

 figures are seen. Generally, when the crystal does not other- 

 wise appear to have been influenced by a solvent, there will 

 be no reason to suppose such figures to be etching figures; 

 but it is more probable that they have been produced by irre- 

 gularities in the growth of the crystal. 



On the other hand, we find numerous instances, in which 

 the crystals have originally been imbedded in calcite, which 

 has later been dissolved by the influence of the water of the 

 atmosphere. This fact is generally recognised by all the faces 

 being quite dull, while in other crystals, or even in other parts 

 of the same crystal still in contact with the calcite, they are 

 quite bright and shining. Such is especially the case with 

 crystals of type XIV, of which a particularly rich material is 



