CURRENT PRE-CAMBRIAN LITERATURE 845 
The Cherry Creek beds occupy an area of thirty to forty square 
miles in the foothills immediately west of the Madison River and a few 
miles north of the southern boundary of the quadrangle, and also a small 
area on the east side of the Madison valley, at the western edge of the 
Madison range. The rocks are marbles and interlaminated mica-schists, 
quartzites, and gneisses. Between Cherry Creek and Wigwam Creek, 
on the west side of the Madison valley, Cambrian strata rest uncon- 
formably upon the upturned edges of the Cherry Creek beds. Before 
the deposition of the Belt formation, the Cherry Creek beds suffered 
extensive deformation. 
The Belt formation occurs in the northern portion of the district 
—in the foothills of the northern portion of the Bridger range, in the 
hills north of the Gallatin and East Gallatin rivers, and in the rugged 
hills of the Jefferson canyon. In the lower portion of the formation 
are coarse sandstones and conglomerates, in the central part appear 
argillites and siliceous limestones, and in the upper part sand stones 
predominate. The Belt formation is overlain by the Flathead (Cam- 
brian) quartzite. It is possible that further investigation may result 
in the reference of this formation to the lower part of the Cambrian. 
At present, however, it is referred provisionally to the Algonkian. 
The Flathead and Gallatin formations (Cambrian) rest with marked 
unconformity upon the Archean for three fourths of the district ; for 
the remainder of the district they rest upon the Algonkian, and the 
unconformity, if it exists, is very slight. 
Weed and Pirsson map* and describe the geology of the Castle 
Mountain mining district of Montana. The Belt group of rocks, 
assigned to the Algonkian, occupies large areas in the district. The 
series presents no definite lithological horizons, but there is a general 
sequence, from the base upward, as follows: 
Alternating shales and sandy beds. 
Dark gray, laminated, thinly-bedded limestone. 
Pearl-gray sericitic shales. 
Sandy shales, with thin beds of ripple-marked sandstone. 
Red shales and slates. 
The series has thus far yielded no fossils. It attains a thickness of 
8000 feet. Basic and acid intrusive rocks penetrate the Belt forma- 
tion very freely. 
* Geology of the Castle Mountain mining district, Montana, by W. H. WEED and 
L. V. Pirsson. Bull. U. S. Geol. Survey, No. 139, 1896, pp. 165. With geol. map. 
