pratt] MODES OF OCCURRENCE. 29 
component, possibly leucite, perhaps analcite. Some calcite in agglom- 
erated granules is also seen in the section. 
This calcitedoes not appear to be of secondary origin, and is proba- 
bly duo to fragments of limestone that were picked up as the igneous 
mass forced its way up through the limestones and were converted 
in1o calcite. 
As has been indicated by Professor Pirsson ' in his paper, the amount 
of biotite in this rock shows that it is closely related to the minettes, 
and although it is lacking in feldspar it lias the biotite and pyroxene 
of these rocks. It is a more basic type of these and is also similar to 
the shonkinites from the Highwood Mountains, Montana, described 
by Weed and Pirsson. - 
The sapphires, which are all of some shade of bine, occur but rarely 
in the rock, and from their sharp, distinct crystals and their general 
distribution il is very evidenl thai they have crystallized oul of the 
molten magma at the time of its intrusion, similarly to crystals 
of feldspar in porphyry. The alumina of the sapphires was not 
an original constituent of the magma, but, as has been shown by 
Pirsson, 1 was due 1<> inclusions of clay sediments taken up from the 
strata through which it passed. The Bell formation, consisting of 
clay shales of great but unknown thickness, undoubtedly underlies t he 
limestones, and the included fragments of these shales were the source 
of the alumina of the sapphires. These included fragments would be 
dissolved by the molten magma and thus form local areas that would 
be very rich in alumina. As 1 he magma began to cool corundum crys- 
tals would separate out in these alumina-rich areas. 
CORUNDUM IN ANDESITE. 
The occurrence of corundum in andesite in the United States was 
first described by Kunz. 4 He described a dike of andesite at Ruby 
Bar, near Eldorado Bar, on the Missouri River, L2 miles northeast 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-andesite, 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 6 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 encountered 
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. 
1 Am. Jour. Sci., 4th series, Vol. IV, 1897, p. 422. 
2 Idem, Vol. I, 1895, p. 467. 
3 Idem, Vol. IV, 1897, p. 423. 
4 Idem, Vol. IV, 1897. p. 418; Min. Mag., Vol. IX, 1891, p.396; Seventeenth, Eighteenth, Nineteenth, 
Twentieth Ann. Repts. U. S. Geol. Survey. 
