SPOKANE, EMPIHE, HELENA, AND MARSH SHALES 207 



Spokane shales. — The Spokane shales oceiir as massive beds of silicious 

 and arenaceous shales of a deep-red color. The arenaceous shaly por- 

 tions frequentl}' thicken up into thin laj^ers of sandstone. The shales 

 break down on exposure, but they are usuall}^ sufficiently firm to resist 

 erosion and form strongly nrarked slopes and cliffs. In nearly all of the 

 contacts between the Belt terrane and the Cambrian they form the upper 

 .member of the Belt ; in but one case known to me do the subjacent Grey- 

 son shales come intocontact with the Cambrian sandstones; this is on 

 Belt and O'Brien creeks, near Neihart. In some localities, as at Sawmill 

 canyon, there is but a comparativel}' thin band of the Spokane shales be- 

 tween the Greyson shales and the Cambrian. 



The most characteristic localit}' of these shales is in the Spokane hills, 

 15 miles east of Helena, although the base of the formation is not there 

 exposed. The estimated thickness of the Spokane shales in Whites 

 canyon, 10 miles east of the Spokane hills, is 1,500 feet. 



Empire shales. — These are greenish gra}', massively bedded, banded, 

 silicious shales, forming the basal portion of the formation above the 

 granite in the vicinity of Empire and at Mar^^sville. Th-ey are finely 

 exposed in the Drum Lunnnon mine, at Marysville, and along the ridge 

 north of Empire, between Lost Horse gulch and Prickly Pear creek. 



The type localities are on the ridge north of Empire and in the canyon 

 walls just below Marysville. The estimated thickness is 600 feet. 



Helena limestone. — The Helena limestone formation is composed of 

 more or less impure bluish gray and gra}^ limestone, in thick layers, 

 which weathers to a buff and in many places to a light gray color. Irregu- 

 lar bands of broken oolitic and concretionary limestone occur at various 

 horizons. Bands of dark and gra.y silicious shale and greenish and pur- 

 plish argillaceous shale are interbedded in the limestones. These bands 

 are from half an inch to several feet in thickness. There are also beds 

 of thinner bedded limestones, especially toward the top of the formation. 



The name " Helena limestone " is given on account of the occurrence 

 of the limestone in the upper part of the city of Helena and on the hill 

 slopes to the east, where the estimated thickness is 2,400 feet. 



Marsh shales. — At Helena there is a thickness of about 250 feet of shales 

 and thin bedded sandstones of the Belt terrane above the Helena linie- 

 stone and beneath the Cambrian sandstones. The same bed, on the north 

 side of Mount Helena, is reduced to 75 feet in thickness, but to the north- 

 west the formation increases in thickness to 800 feet or more. The reddish 

 shales are prominent on Greenhorn mountain and beyond to the canyon 

 of Little Prickl}^ Pear creek, and also to the north at Marsh creek. On 

 the ridge between the two creeks there is an excellent section, and the 

 name " Marsh shales " is given to this, the topmost formation of the 



XXXI— Bull. Geol. Soc. Am., Vol. 10, 1898 



