418 G. F. Kunz — Sapphires from Montana, 



color and miscalled rubies, cyanite, stream-tin, chalcedony, 

 limonite pseudomorphs after pyrite nodules, etc. At Ruby 

 Bar two facts of great significance were encountered, bearing 

 on the age of the gravel and the source of the gems. The 

 writer saw and measured a mastodon tusk three feet long, 

 embedded in the sapphire layer of the gravel ; and a dike was 

 found cutting the slaty bed-rock beneath ; in this dike were 

 crystals of sapphire, pyrope and sanidin. All these facts were 

 described by the writer in the Mineralogical Magazine (vol. ix, 

 p. 396, 1891), together with an account of the rock by H. 

 Miers (loc. cit.), who characterized it as a " vesicular mica- 

 augite-andesite," abounding in brown mica and porphyritic 

 crystals of augite, with a ground-mass of feldspar microlites 

 and brown glassy interstitial matter, with magnetite. 



Two years before, indeed, in 1889, the writer had seen some 

 specimens of a trachytic rock, enclosing well-defined crystals 

 of sapphire similar to those of Eldorado Bar, from a dike some- 

 what farther up the river. These facts, which were referred 

 to in the u Gems and Precious Stones of North America" 

 (p. 49), and the Appendix (p. 341), sufficiently showed the 

 source of the gems as coming from the erosion of dikes of 

 igneous rock. 



More recently sapphires have been found throughout a con- 

 siderable district lying some seventy-five to a hundred miles 

 east of the Missouri bars, the principal point being Yogo 

 Gulch, on the Yogo fork of Judith River near its head- 

 waters, in Fergus County, Montana, on the eastern slope of 

 the Little-Belt Mountains. The nearest town is Utica, fifteen 

 miles to the northwest, in the same county. The sapphires 

 occur over a somewhat extended area, which is being explored 

 and laid out in claims. They are imbedded in a yellow earthy 

 material, from which they may be washed out by sluicing, as 

 for gold, the heavy crystals gathering at the bottom. Mr. S. S. 

 Hobson, of Great Falls, Montana, the original discoverer of 

 the gems at Yogo Gulch, states that at that point there are two 

 veins (dikes ?) containing sapphires, which have been traced 

 for a distance of seventy-five hundred to eight thousand feet 

 in an east-and-west course, about eight hundred feet apart. 

 One of these is seventy-five feet wide, aud consists of a " yel- 

 low earth " (i. e. completely decomposed). It has been found 

 that what was supposed to be the end of the " vein " is really 

 a fifty-foot fault, and that the vein can be traced very much 

 farther. In working down to a greater depth, the unaltered 

 igneous rock has been reached, and its full description is given 

 in an accompanying article by Prof. L. Y. Pirsson. 



These Yogo Gulch sapphires have been referred to by the 

 writer in the 16th and 17th Annual Reports of the IT. S. Geo- 



