CONTRIBUTIONS FROM THE MINERALOGIC LABORATORY 21 



Pyramids. The hemipyramids of the zone [ooi.iio] form a 

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

 (in), v (in) and c (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), /8 (121), 

 M(i22), 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. 110]. 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° o' 



45° 



X -. ._ - - 



102 



cx 



44 53 



44 Si 



V 



103 



cv 



33 46 



33 35 



c 



001 













Zone [c.b] 







b 



010 



bm x 



38° 15' 



38° 18J' 





Oil 



ntxm'x 



103 16 



io 3 23 



g 



012 



gg' 



64 37 



6 4 39* 



t 



013 



nt x g 



19 22 



19 22 



a 



018 



it 



9 26 



9 274 







t(l 



13 43 



13 53 







Zone [a.m] 







m 



no 



am 



32° 25§' 



32° 2 2 y 







120 



ao 



51 50 



5i 454 



r 



230 



or 



8 12 



8 10 



