44 COKXJNDUM^ ITS OCCURKENCE AND DISTRIBtJTION. 
exposures of the dike project through this covering of loose material. The 
rock of the dike is composed chiefly of feldspar and is white in color, being 
thus in marked contrast to the darker rock mass which it cuts. 
Professor Lawson regards the corundiferous rock as a separate 
dike cutting the peridotite and as a new type of rock, for which he 
proposes the name plimiasite^ from Phnnas County, in which it 
occurs. Without having made any personal examination of this 
occurrence, but judging from hand specimens of the feldspathic 
portions of the rock containing the corundum, from specimens of 
the serpentine and of the peridotite rock, which were kindly sent to 
me by Mr. Edman, and from the descriptions of Professor Lawson, 
I find this oligoclase-corundum rock similar to the zone of corundum 
and oligoclase feldspar which is believed to have separated from the 
peridotite magma at Buck Creek, Clay County, N. C. 
CORUNDUM IN ANDESITE. 
The occurrence of corundum in andesite in the United States was 
first described by Mr. G. F. Kunz, in the case of a dike of andesite at 
Ruby Bar, near Eldorado Bar, on the Missouri River, 12 miles north- 
east of Helena, Mont.* As described by him, this rock is in a dike 
cutting the slates of the country and is a vesicular mica-augite-andes- 
ite, which is made up of a groundmass of feldspar microlite and a 
brownish glass in which are many particles of biotite and crystals 
of augite. 
A similar occurrence has been observed by myself on the river 6 
miles above Eldorado Bar, at French Bar, which is nearly 12 miles 
due east of Helena. At this locality a narrow dike, 3 to G feet wide, 
was found cutting through the slates of this section. The trend of 
the dike is N'. 5° to 10° E., and it dips about 45° E. It was encoun- 
tered by miners who were working the gravels of the bar for sapphires, 
and it has been exposed at but one .point, so that its extent is not 
known. 
The rock is fine grained, of a rather light-gray color, and a decided 
basic appearance. Biotite is the most conspicuous mineral, and 
occurs in small, flat, tabular plates, sometimes with distinct crystal 
outline, and up to a millimeter or tw^o in diameter. In some speci- 
mens of the rock there are nodules, 5 to 10 mm. in diameter, that ap- 
pear to be partially decomposed feldspar. The augite, which is prom- 
inent in the thin section, is not very apparent in the hand specimen. 
Prof. L. V. Pirsson, of Yale University, has kindly examined thin 
sections of this rock for me, and says that the rock is an altered 
augite-mica- andesite. It contains unaltered phenocrysts of a clear 
« Am. Jour. Sci., vol. 4, 1897, p. 418 ; Mineral. Mag., vol. 0, 1891, p. 396 ; Seventeenth, 
Eighteenth, Nineteenth, Twentieth Ann. Repts. U. S, Geol. Survey. 
