238 Penjield and Ford — Developments of Calcite Crystals. 



position would have the symbol 0'12'12'&, and is a little steeper 

 than the common rhombohedron f 0221, which truncates the 

 pole edges qfthe scalenohedron v. The pyramid of the_second 

 order y, S'8'IV>'3, was identified bv the measurement S'8'10-3 /\ 

 S-8'I6-S = 25° 40', calculated 24° 46', and, further, by its being 

 truncated by the positive rhombohedron 31, 4041. This rare 

 pyramid was first identified by vom Rath* on crystals from 

 Andreasberg in the Harz, and, as pointed out by the authors,f 

 this same pyramid is the prevailing form of the siliceous cal- 

 cites from the Bad Lands of South Dakota. On crystals from 

 Union Springs there is a tendency for the upper and lower 

 faces of the pyramid y to round into one another, owing to 

 vicinal development, and because of this rounding it was 

 impossible to obtain an accurate measurement between the 

 upper and lower y faces. 



On the majority of the specimens the crystals are not so 

 highly modified as the one just described, but as already stated 

 the variation in habit due to twinning and the unequal devel- 

 opment of certain faces, gives to the specimens a peculiar inter- 

 est. All the types to be described occur on a single specimen 

 having a surface about half the size of one's hand covered with 

 crystals. The crystals on this specimen were not suitable for 

 measurement and therefore no angles will be given, but the 

 forms were evidently like those identified on the small crystal 

 previously described. 



Scalenohedral type. — The scalenohedron v^ 7 # 4-ll-3, fig. 2, is 

 apparently very common at the locality. It should be stated 



2 3 



that this form has the same middle edges as the rhombohedron 

 r, 1011, and the common scalenohedron v, 2131, but is some- 

 what steeper than the latter form. A twinning about the 

 basal plane, fig. 3, is quite common. 



*Pogg. Atmalen, cxxxii, p. 521, 1867. fThis Journal, ix, p. 352, 1900. 



