442 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



eral aspect determined by g{001), m(llO), and d{102). In this 

 type, ;?(324) and a(lOO) are absent, and in their stead there were 

 found the prism 7i(120), pyramid ^^(121), and domes Z(104) and 

 F. (1-0-14). 



The total of 13 forms found may be listed as follows: 



Pinacoids: a{100), h{010), c(OOl). 



Macrodomes: 6^(102), Z(104), /^i* (1-0-14). 



Brachydome: o(Oll). 



Prisms: m(llO), n(120). 



Pyramids: 2(111), [J(.(124), ;?(324), 25(121). 



The new dome /^i (1-0-14) was observed only once as a very nar- 

 row line face yielding a rather poor signal. The observed and 

 calculated angles are as follows: 



Measured Calculated. 

 c(OOl) AFi(l-0-14)=6°55' 6''41' 



CALCITE CRYSTALS FROM THE OVERLOOK MINE, PIONEERVILLE 

 DISTRICT, IDAHO. 



A specimen of ore from the Overlook Mine, Pioneerville district, 

 Boise County, Idaho, contains well-developed crystals of calcite. 

 Calcite is commonly present in the gold ores of many southern Idaho 

 mines, but where the mineral forms well-developed crystals these 

 almost invariably take the form of very j9at rhombohedrons. The 

 present crystals are so different from this prevailing type that they 

 merit a short note, although displaying no new 

 forms. This is especially true, since the writer has 

 long held the view that the crystallogi-aphic de- 

 velopment assumed by the omnipresent mineral cal- 

 cite is an important criterion in ore-bearing veins, 

 capable of indicating the temperature and to some 

 extent the composition of the solutions from which 

 it crystallized. 



The specimen in question consists of a breccia of 

 greatly bleached and altered angular fragments of 

 what appears to have been originally an acidic 

 lava or other fine-grained igneous rock, cemented 

 b}^ masses of coarse cleavable galena and pyrite 

 in a gangue of calcite and quartz. The calcite 

 crystals which line a small cavity are prismatic 

 in development and reach a length of 1 centimeter. They are trans- 

 parent and colorless. The aspect is like figure 10, the forms present 

 being identified as m(lOlO), a{lT20), <y(2131), r(ioll), 2/(3251), 

 and «(1341).i 



^When the drawing was made the form marked x (IsTl) was thought to be another 

 scalenohedron having the indices (3-9-12-4). Some small discrepancy has thus resulted, 

 the difference being Insuflicient to make it advisable to redraw the figure. 



Fig. 10.— Cetstal 



CALCITE. 



