THE MINERALS OF IDAHO 303 



diameter of pitchy brownish-black garnet with pale greenish to 

 grayish-brown outer layers. The outer layers grade into brown and 

 cinnamon-brown material which abuts against vugs as fine little 

 cinnamon-brown crystals up to 1 cm., across which are striated 

 trapezohedrons with some small faces of the dodecahedron and 

 other forms. The vugs contain later vivid-green columnar epidote r 

 colorless cpiartz, and pale brown calcite. The inner pitch-black 

 material has an index decidedly above 1.82 — possibly 1.85 or higher — 

 and is transparent grayish-brown and isotropic under the micro- 

 scope. The outer grayish crust is porous, soft, and easily crushed in 

 an agate mortar. Under the microscope it varies in index from a 

 minimum of about 1.795 to near 1.820. Many of the grains show a 

 notable birefringence, especially those having an index of refraction 

 between 1.800 and 1.805. Some of these show a dividing plane like 

 a twinning plane, probably a fragment of an original sector structure. 

 Many yield sweeping extinction, but a few give uniform extinction 

 and what appears to be a negative biaxial figure with large axial 

 angle. The lustrous cinnamon-brown crystals show very fine lami- 

 nated zoning parallel to the crystal faces and vary in index from 

 about 1.803 to perhaps 1.835. The material is in part isotropic 

 and in part feebly birefracting. One specimen from this locality 

 is a large crystal 3 cm. in diameter, which is perfect in outer form, 

 being a dodecahedron modified by faces of the trapezohedron. The 

 outermost layer, which is rich brown-red, is 0.25 to 0.50 mm. in 

 thickness and is easily peeled off, revealing a likewise perfect lustrous 

 surface of greenish-gray color. Toward the center of the crystal 

 the layers become thicker and less easily separable and alternate 

 brown and grayish in color. 



A specimen from the Arkansas dump consists largely of deep 

 red-brown garnet containing only a little epidote which is apparently 

 later than the garnet. The specimen is vuggy and against small 

 cavities the garnet forms imperfect crystals of dodecahedral form up 

 to 1 cm. in diameter. The powder is reddish buff. Under the 

 microscope the grains are pale transparent brown and most of them 

 show a feeble birefringence. "The refractive index is decidedly above 

 1.82, estimated between 1.83 and 1.84. This garnet so resembles 

 almandite in color that a determination of ferrous iron was made upon 

 it which indicated a content of only 1.42 per cent FeO. A second 

 specimen from the Arkansas dump consists of greasy grayish-brown 

 masses and imperfect dodecahedral crystals averaging 1 cm. or less 

 in diameter, scattered through coarsely crystalline white calcite-, 

 which also contains scattered fine-grained pale green masses of 

 diopside. The garnet is apparently rather uniform in composition 

 and yields a pale brown powder which is colorless in section with an 



