150 FIELD CoLUMBIAN Mus—EumM—GEo .oey, VoL. III. 
MIMETITE 
EUREKA, UTAH 
FIGS. 4 AND 5, PLATE L 
On several specimens from Eureka, Utah, obtained from Maynard 
Bixby, mimetite occurs in acicular form. In one of these specimens 
(Mus. No. M 8384), the crystals are in the form of minute white 
needles occurring in great abundance coating pyramidal crystals of 
anglesite. On another specimen (Mus. No. M 8385), the crystals 
are larger, reaching a length of 1 cm. with a thickness of .75 mm. 
These crystals are transparent and colorless. Many of them show a 
termination in which it is possible to recognize definite crystal planes, 
and examination with the reflecting goniometer permits identifica- 
tion of the unit prism m (1olo), the unit pyramid x (1olr) and the 
basal plane ¢ (ooo1). Fig. 4, Pl. L, shows the characteristic devel- 
opment. In another specimen (Mus. No. M 9383), the mimetite 
exhibits the same habit, but the crystals are somewhat shorter and 
have a wine-yellow color. These crystals have an average diameter 
of .6 mm. and reach a length of 5 mm. The forms of which they 
are. composed are similar to those previously mentioned, but the 
basal plane is more extensively developed as shown in Fig. 5, Pl. L. 
No doubly terminated crystals were found. Neither the colorless 
nor the yellow crystals exhibit noticeable absorption or pleochro- 
ism in polarized light in the direction of the vertical axis. On 
heating, the yellow crystals change to a smoky color. 
OCTAHEDRITE 
JEQUITINHONHA RIVER, BRAZIL 
- FIGS. 24, PLATE LI 
Several crystals of octahedrite were presented to one of the authors 
by Olaf E. Ray, Esq., an official of the Chicago Brazilian Diamond 
Company. These crystals were obtained from washings of the dia- 
mond-bearing sand of the Jequitinhonha River, near Diamantina, 
Brazil. The crystals have the typical pyramidal character of octahe- 
drite and range from 5 to 8 mm.in length. Their color is the 
typical brownish-black of the mineral showing greenish-yellow by 
transmitted light. Aside from striations the planes are splendent. 
The edges are somewhat rounded from stream rolling, but otherwise 
the crystals are well developed and give excellent signals with the 
