CONTRIBUTIONS FROM THE MINERALOGIC LABORATORY 



21 



Pyramids. The hemipyramids of the zone [001.110] form a 

 regular series universally present as well defined planes ; of these n 

 (in), v (in) and e (112) are particularly well developed. The 

 hemipyramids A (113),^ (114) and K (115) are represented by 

 relatively narrow faces often showing natural etchings. In the 

 zone [001.120] the planes of the hemipyramids Q (122), (3 (121), 

 M (122), i (123) and a .(124) are represented by small narrow 

 faces beveling the edges between the planes of the clinodome zone 

 and those of the zone [ooi.no]. They were for_the most part 

 identified by zone equasions. The rare faces (148), (149) and 

 (1.4.10) have been already noted in the zone [001.140]. This 

 series of planes gave fair reflections and were measured on five crystals, 

 the results agreeing with theory within the limits of accuracy. 



Twinning was observed on one crystal 8 centimeters in length 

 on the b axis. This crystal which is shown in figure 40 is a pene- 

 tration twin parallel to a (100); having c for the twinning axis. 

 The larger individual is of the snowy white color, which is common 

 to the occurrence and which suggests the color of cryolite; the 

 smaller individual which is shown protruding from this last is 

 light greenish in color, transparent and resembles the typical 

 Bergen Hill datolite. 



A list of the occurring forms with the measured and calculated 

 angles is given below. 



SUMMARY OF OCCURRING FORMS, MEASURED AND CALCULATED ANGLES 



Zone [a. c] 



Measured Calculated 



