184 



University of California Publications. 



[Geology 



The form (241) is common to the crystals of all three habits, 

 and the forms (223) and (263) are common forms on Habits 2 

 and 3, but were not observed on crystals of Habit 1. The steep 

 negative pyramid (661) is represented by one broad face on a 

 brown crystal. The other pyramids are fairly common. 



Referring to the gnomonic projection of the forms and zones 

 as shown in plate 31, figure 6, it will be seen that the two zones 

 of pyramids are missing where the values of p u = -+-1 and — 1, 

 whereas in colemanite these two zones are well developed. On 

 the other hand, the zone with p = -\-2 occurs on neocolemanite 

 and is missing on colemanite. There is also the additonal zone 

 with p = — f , on these crystals. 



Measurements of the crystals. — The two-circle goniometer was 

 used for the measurements, and practically all of the readings 

 were good. It was evident from the first crystal measured that 

 the angles (j> and p for the clinodomes and base were greater than 

 those for colemanite, indicating that the vertical axis was longer 

 and the forms steeper. This variation was constant for all 

 crystals and the lowest values of <j> and p were higher than the 

 greatest values for the corresponding faces on colemanite. 



Ten good crystals were chosen for the measurements and 

 below is a table containing the number of measurements of each 

 form, the averages of the readings, and the calculated values from 

 the axial ratio determined. For comparison the calculated values 

 of <f> and p for colemanite for corresponding forms as determined 

 by the writer 3 and by Goldsehmidt 4 from Jackson's measure- 

 ments, are also included. 



3 Loc. cit. 



* V. Goldsehmidt. Krystallographische Winkeltabellen. 



