Pirsson — Corundum-hearing Rock from Montana, 421 



Aet. XLV. — On the Corundum-bearing Rock from Yogo 

 Gulch, Montana ; by L. V. Pirsson. 



The corundums whose occurrence and character have been 

 described in the foregoing paper, are found in a dike of igne- 

 ous rock cutting the sedimentary beds near the entrance of 

 Yogo Gulch. While the corundums, which are washed out as 

 gems, occur in that portion of the dike which has been highly 

 altered and decayed, comparatively little altered material is 

 also obtainable, and the opportunity to study a specimen and 

 some sections cut from it the writer owes to the kindness of 

 Mr. W. H. Weed of the XL S. Geological Survey. 



In the hand specimen the rock is of a dark gray, basic 

 appearance and has an uneven fracture. It contains light 

 green or white included fragments which form its most con- 

 spicuous feature, and these angular inclusions are probably 

 pieces of limestone broken off and carried upward by the fluid 

 rock in its ascent. They vary in size from those of micro- 

 scopic dimensions to some that are a centimeter across. Many 

 of them consist entirely of calcite, while others appear to be 

 made up wholly of a pale green mineral which is probably a 

 pyroxene. The largest inclusions show a reaction rim of the 

 same green pyroxene, the rim being about one millimeter 

 thick, while the entire center is of calcite with scattered. prisms 

 of the same green pyroxene. The rock itself shows only a 

 few scattered tablets of mica two or three millimeters in diam- 

 eter as phenocrysts, while the groundmass glitters with minute 

 flecks of biotite, and considerable pyroxene is seen. 



It is in this rock that the sapphire occurs imbedded in large, 

 distinct, well-formed crystals as described in the previous 

 paper. They show the corroded, etched surfaces characteris- 

 tic of this occurrence and often have traces of a blackish crust 

 upon them. 



Microscopical. In thin section the rock at once shows its 

 character as a dark, basic lamprophyre, consisting mainly of 

 biotite and pyroxene. There is a little iron ore present, but 

 its amount is small and much less than is usually seen in rocks 

 of this class. The biotite is strongly pleochroic, varying 

 between an almost colorless and a strong, clear, brown tint. 

 It occurs in ragged masses, rarely showing crystal outline, and 

 it contains a large amount of small apatite crystals. The 

 pyroxene is of a pale green tint with the habit of diopside 

 and is filled with many inclusions, now altered but probably 

 originally of glass ; in some crystals these inclusions are so 

 abundant as to render the mineral quite spongy. The grains 

 sometimes show crystal form but are mostly anhedral and vary 

 in size, though the evidence is not sufficient to show two dis- 

 tinct generations. 



