W. E. Fonl—RemarkaUe Twins of Atacamite. 17 



the close agreement between the measured and calculated 

 angles, as sirovvn in the tables below, leaves no doubt that it 

 has at least been correctly formulated. 



The law of twinning may be stated as follows : One of the 

 e planes of the twin individual is always parallel to one of the e 

 faces of the individual in normal position. This is readily seen 

 on inspection of the crystals, for one brachydome face of the 

 twin always " flashes" with one of the dome faces of the nor- 

 mal individual. Further, the twinned individual has been 

 turned on the pole to (?(011) as if on a twinning axis through 

 such an angle that the second e face of the twin falls in the 

 prism zone of the crystal in normal position. These relations 

 can be best explained by use of flgure 3, drawn, not from a 



Fig. 4. 







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crystal, but rather to illustrate the law. Face e of the crystal 

 in twin position (No. II) is parallel to e of the individual in 

 . normal position (No. I), while e' of No. II falls in the same 

 zone as the prism and pinacoid faces of the vertical crystal. 

 These facts were proven when the twin crystals were placed 

 upon the reflection goniometer. The signals from the e faces 

 of the two individuals coincided with each other, and the sig- 

 ual from e' of No. II fell in the same zone as the prism faces 

 of No. I. This was found to be true on all the crystals meas- 

 ured, with only such slight variations as the occasional rather 



Am. Jour. Sci.— Fourth Series, Vol. XXX, No. 175.— July, 1910. 

 2 



k 



