GEOLOGICAL IMPORTANCE OF SEDIMENTATION 555 



longer shows. At the same time the quartzite beds decrease in thickness and 

 purity, while the interbedded shale increases in thickness and purity, so that 

 an arbitrary line must be drawn separating the two formations.^ 



Chamberlain shales, Helena region, Little Belt Mountains. — This formation 

 is composed of a series of dark silicious and in places arenaceous shales. Rip- 

 ple-marks, mud-flows, and sun-cracks were occasionally seen, but no traces 

 of life were observed. The dark shales frequently form low cliffs along the 

 canyon side, near the beds of the streams.^ 



Newland limestone, Helena region, Little Belt Mountains. — At the typical 

 locality on Newland Creek the limestones are thin bedded, the layers averaging 

 from two to six inches, with shaly partings of variable thickness between them. 

 In the section of Sawmill Canyon, near Neihart, the layers are somewhat thicker, 

 more impure, and with a greater number of beds of interbedded shale. The 

 prevailing color of the limestone is dark bluish-gray on fresh fracture, and buff 

 to straw color on the weathered surface.'' 



Altyn limestone, Lewis and Livingston Ranges. — Limestone, of which two 

 members are distinguished; an upper member of argillaceous, ferruginous 

 limestone, yellow, terra-cotta, brown, and garnet-red, very thin-bedded; thick- 

 ness about 600 feet; .... and a lower member of massive limestone, grayish- 

 blue, heavy-bedded, somewhat silicious, with many flattened concretions, rarely 

 but definitely fossiliferous ; thickness, about 800 feet.^ 



Greyson shales, Helena region. Little Belt Mountains. — Dark-colored, 

 coarse, silicious, and arenaceous shales, passing above into bluish-gray, almost 

 fissile shale, which, when broken up, weather to a light gray fissile shale, resem- 

 bling a poor quality of porcelain. These in turn are succeeded by dark gray 

 silicious and arenaceous shales, with interbedded bands of buff-colored sandy 

 shales and occasional layers of hard, compact, greenish-gray and drab silicious 

 rock. At the base of this series, in Deep Creek Canyon, a belt of quartzites 

 occurs, interbedded with shales, the base of the quartzites showing ten feet of 

 interformational conglomerates, composed of sand and pebbles up to eight 

 inches in diameter, and derived from the subjacent Belt rocks. ^ 



Appekunny argillite Lewis and Livingston Ranges. — The Appekunny argil- 

 lite is a mass of highly silicious, argillaceous sediment approximately 2,000 feet 

 in thickness. Being in general of a dark gray color, it is very distinct between 

 the yellow limestones below and the red argillites above. The mass is very 

 thin bedded, the layers varying from a quarter of an inch to two feet in thickness. 

 Variation is frequent from greenish-black argillaceous beds to those which are 

 reddish and whitish. There are several definite horizons of whitish quartzite 

 from fifteen to twenty feet thick. The strata are frequently ripple -marked, 

 and occasionally coarse grained, but nowhere conglomeratic.^ 



1 W. H. Weed, Geology of the Little Belt Mountains, pp. 281, 282. 



2 Bulletin Geological Society of America, Vol. X, p. 206. 



3 Ibid. 4 Ibid., Vol. XIII, p. 317. s Ibid., Vol. X, p. 206. 

 (>Ibid., Vol. XIII, p. 322. 



