Palache and Wood — Crystallographic Study of Millerite. 347 



and the angles rounded and striated. ~No terminated crystals 

 of this type were found, all showing terminal planes of cleav- 

 age only. A striking and hitherto unknown character of the 

 mineral was first seen and interpreted on one of the triangular 

 cleavage fragments shown in Ho;. 2. This crystal consists of 

 three segments, the upper and lower ones parallel, the inter- 

 mediate portion in twin position to the other two, a face of 

 the form e, (0112), being the twinning plane. 1011 ^ 1010 

 (twin), measured 21° 23% calculated 21° 25'. The formof 

 the twin suggested that it might be due to the presence of a 

 gliding plane and produced by pressure ; and experiment soon 

 showed this to be the case. A crystal held firmly and sub- 

 jected to shearing stress can be offset readily. The same 



result may be obtained by placing the crystal on a yielding 

 support, and pressing upon it transversely with a knife-edge, 

 in which case a fine lamella is thrown into twin position. The 

 gliding takes place w T ith equal ease, of course, parallel to each 

 of the three rhombohedron planes. With very slender crystals 

 it is difficult to produce the twinning, the pressure breaking 

 the brittle substance to pieces, and this probably accounts for 

 the failure to recognize this remarkable property of millerite 

 hitherto. Careful tests on millerite crystals from a number of 

 localities were uniformly successful, and showed it to be a 

 general property of the mineral. In view of the ease with 

 which this gliding twinning can be obtained, and of the strik- 

 ing effect produced, it seems that millerite should take equal 

 rank with calcite as an illustration of this interesting pheno- 

 menon. 



Highly perfect cleavage is present parallel to the unit 

 rhombohedron, r, (1011), and also to the negative rhombohe- 



