CONTRIBUTIONS FROM THE MINERALOGIC LABORATORY 21 



Pyramids. The hemipyramids of the zone [ooi.no] 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), /? (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 [001. 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 



a 



100 



ax 



45° 



0' 



45° 







X 



102 



ex 



44 



53 



44 



5i 



V 



103 



cv 



33 



46 



33 



35 



c 



001 

















Zone [c.b] 











b 



010 



bwix 



38° 



*5' 



38° 



i8*' 



MX 



on 



m x m'x 



103 



16 



103 



23 



g 



012 



gg' 



64 



37 



64 



39i 



t 



013 



Wl x g 



19 



22 



1 9 



22 



a 



018 



gt 



9 



26 



9 



27* 







ta 



13 



43 



13 



53 







Zone [a.m] 











m 



no 



am 



32° 



,rl/ 



2 5s 



32° 



2 3¥ 







120 



ao 



5i 



So 



5i 



45i 



r 



230 



or 



8 



12 



8 



10 



