302 C. ANDERSON. 



Segments. Meas. b a b 



I, IV 62 44 "\ 



II' IH 62 49 f 0alc - for ^-twiiis 62° 16' 



V, VII 62 35 ) 



Thus all the segments are united by the two laws and 

 the deviation from the exact angle of twinning is nowhere 

 more than 11'. 



Group VIII (Plate LIV, fig. 3, Plate til, fig. 5, Text 

 Fig. 5). — This instructive group consists of thirteen 

 individuals, some of which are represented by several 

 crystals in parallel position, and finely illustrates the com- 

 plicated twinning and reticulated or dendritic structure of 

 the mineral. It also exemplifies well the manner in which 

 'secondary' and 'tertiary' twins are apt to form in the 

 re-entrant angle of the 'primary' twins and so to fill up 

 this angle. The individuals I, II and III are much larger 

 than the others, I having a length of about 5^ cm.; these 

 three may be described as 'paragenic' twins, that is they 

 formed an embryonal triplet and grew up together, while 

 the others are 'metagenic' twins which came into existence 

 after I, II and III had attained some size. A small, typical 

 arrow-head twin IV and V is planted on I; it has the forms 

 b (010), r (130), x (012), k (011), y (102), p (111), s (121). 

 This r-twin pair is not in any exact relation to the main 

 group except that the direction of its vertical axis is the 

 same. Two small crystals VII and XII, which are not 

 exactly twinned to one another (angle 63° 14', calc. 62° 46'), 

 are attached to the IV and V pair, but, like the latter, 

 these are not in any precise twin relation to any of the 

 others, although VII is very nearly parallel to IX and XII 

 to VIII; the other twin relations are well established. 

 The drawing (Plate LII, fig. 5) is partly diagrammatic; 

 the stippled portion in the bottom left corner of the figure 

 belongs to a twin group whose vertical axis is not parallel 

 to that of Group VII. 



