133 



of the second pinacoid; this is mostly dependiiii,^ on the com- 

 paratively great breadth of the faces belonging to the form x. 

 The faces of the fundamental pyramid are dull, generally giving 

 only shimmer reflexions, but they are never drusy. J\or is any 

 part of the crystals of type II darker in colour than the mineral 

 in general. 



Below are given the angular values obtained by measure- 

 ments, together with the corresponding calculated values. 



Observed Calculated 



a:b =(100): (010) = 89° 5;V 90° 



11 :n = (120) : (120) =*100° 47^ — 



и:« = (120):(Г20) == 79° 12' 79° 13' 



ш:« = (110):(120) = 18° 52' 19° 15' 



ж:и = (1.12.0):(120) = 31°33' 31° 45' 



p:m= (ill): (110) = *55° 13' — 



jj:6 = (in):{010) = 72°32' 72° 55' 



p:p = {\\\):{\\\) = 34°35' 34°19' 



^j:^j = (11|):(Г1|) = 58°14' 58° 27' 



p : n = (1 1 1) : (120) = 57° 45' 57° 25' 



:w = (23l):(l20) = 32° 52' 32° 34' 



: = (23l):(23l) = 78° 28' 78° 20' 



: = (23l):(23l) = 70° 31' 70° 40' 



in colour the two crystal types difïer somewhat from each 

 other. Type 1 is almost colourless. Only a faint tinge of brown 

 is observable. That, however, the crystals of this type show a 

 sort of dark impregnation on their unattached ends, has already 

 been pointed out. The crystals of type II, on the other hand, 

 are far less transparent. This is not exclusively due to the 

 circumstance that these crystals are thicker than the former; 

 the principal cause is the greater intensity of the brown colour. 

 A tinge of violet is also perceptible in these crystals. 



