Dec, 1908. Mineralogical Notes 137 



the Albany material in the Museum is 10 mm. long, 10 mm. wide and 

 2 mm. thick. Some crystals seen must have been even larger than 

 this but they were broken in excavating. The crystals in cavities 

 are as a rule smaller than those attached to quartz, their average size 

 being 3x3x1 mm. The attachment of all the crystals is always 

 along an edge parallel with the vertical axis. They thus rarely show 

 more than half the faces belonging to the prismatic zone. Their out- 

 line tends to be rectangular or hexagonal, according as the lateral pin- 

 acoids or the prisms predominate. The orientation adopted for the 

 crystals for measurement was determined by the basal plane and by a 

 pinacoidal cleavage normal to this which was regarded as that of the 

 brachypinacoid, b (010). The distinctive characters of the base are 

 its pearly luster and striations || to a (100). In addition to the cleav- 

 age || to b (010), a prismatic cleavage giving angles of nearly 6o° was 

 occasionally observed. The faces of the crystals on casual inspection 

 appear bright and would seem to be well suited for measurement, 

 but on closer examination their surfaces are found as is usual with 

 bertrandite to be uneven and to give elongated reflections. This is 

 especially true in the prismatic zone, where nearly all the measure- 

 ments give variations between 2° and 3 . By taking the mean of 

 these, however, values were obtained which served for identifying the 

 faces. The crystals are not highly modified, only six forms being 

 observed, as follows: 



c (001) a (100) / (130) 



b (010) m (no) */ (203) 



Of these / (203) is new to bertrandite, its determination being based 

 on the measurement caI = (001) a (203) =33° 56'. The calculated 

 angle for this form, using the axial ratios of Penfield* is 34 59' or 

 using those of Urbaf is 34 48'. While the agreement of measured 

 and calculated values for this form is not as close as could be desired 

 it is all that can be expected when the imperfections of the planes are 

 considered. The measurements upon which the determinations of the 

 prisms were based are as follows, these being shown with the values 

 calculated from both Penfield's and Urba's ratios: 



Observed Calculated 



Penfield Urba 

 m a m" = (no) A (1I0) = 58 52' 59° 16' 59° 21' 



f A f = (130) A (T30) = 6i° 12' 6o° 44' 6o° 39' 



Cleavage a cleavage = (1 10) a (010) = 6o° 16' 6o° 22' 6o° 19' 



•Am. Jour. Sci., 1880, 3, 37, p. 215. tZs. Zr. 1880, 15, p. 194. 



