22 



total obliteration of the original form. In the first stage of the 

 process the etching figures are very distinct. The rhombohedral 

 faces seem to offer the greatest resistance against the decom- 

 posing agents. The pits etched in them remain for a long time 

 few in number and of small size. They form isosceles triangles 

 with their points turned downwards. (Fig. 5). They are con- 

 formable with the rhombehedral faces, but turned in the opposite 

 direction. 1 have not found any difference between the etching 

 figures on the positive rhombehedron and those on the nega- 

 tive one. 



On the prismatic faces the etching proceeds much more 

 energetically than on the rhombohedral faces. The pits on the 

 prismatic faces are generally very large and distinct. They are 

 wedge-shaped and arranged in relation to a prismatic combina- 

 tion-edge in such a manner that the wedges on both sides of 

 it turn either their points or their bases towards the edge. The 

 points of the etching figures are always turned towards the 

 combination-edge at the end of which the form s occurs. There- 

 fore these figures present a good means of distinguishing right- 

 handed and left-handed crystals from each other. On the 

 prismatic face wich is below the positive rhombehedron the 

 points of the wedges on a right-handed crystal are turned to 

 the right and on a left-handed crystal to the left. On the pris- 

 matic faces wich are below the negative rhombohedral faces 

 the position of the etching figures is reversed. Also the twin 

 formation appears most distinctly by the position of the etching 

 figures. 



Molengraaff has described ^) natural etching figures on 

 quartz crystals form Carrara which perfectly resemble those 

 mentioned above. 



As the process of corrosion proceeds, the etching figures 

 grow larger and more numerous, till at last they meet. 



Zeitschrift für Krystallographie, Vol. U. p. 199. 



