226 Allen, Crenshaw, Johnston, and Larsen — 



elongated along the a axis. Several of them are - 5 mm in 

 length. Out of seven crystals measured, one is twinned after 

 the law : twinning plane (021) ; three are twinned after the 

 law: twinning plane (023); two after both laws, and one 

 crystal is an untwinned hexagonal tablet. The crystals resem- 

 ble the untwinned crystals of lot 3 in that the dominant forms 

 are {001}, {010}, and {111}, but {021}, {011}, {0.1.20}, {110}, 

 and { 112} were also recognized. Fig. 20 represents one of these 

 crystals twinned after (121). Fig. 21 is the stereographic 

 projection of crystal U, which is twinned after both (023) and 

 (021). One of the individuals of the twin after (023) is poorly 

 developed on the part of the crystal shown. 



Fig. 21. 



Fig. 21. Stereographic projection of a crystal of a-pyrrhotite twinned 

 after both (021) and (023). The half of the crystal represented on the pro- 

 jection shows few of the faces of one of the individuals after (023). The 

 faces of the crystal twin after (021) are underscored once, those of the twin 

 after (023) twice. 



Table X gives a summary of all the crystals of a-pyrrhotite 

 which were measured. The designation of the crystal is given 

 in column one and the number of the preparation from which 

 it came in column two. Column three lists the faces recog- 

 nized on the crystal, and column four gives the average of the 

 angle between the faces (110) and (010). The eight succeed- 

 ing columns give the weighted averages of the measured inter- 

 facial angles each followed by the value of p Q computed from 

 that angle. Next to the last column lists the weighted aver- 

 ages of the constant p Q which is equal to the vertical axis, c. 

 The final column gives all measured angles not listed in any of 

 the previous columns. 



