210 ('. I). WALC'OTT I'RE-CAMBKIAN K(JSSIL1FER0US FORMATIONS 



usually washed clean by the sea as it advanced on the land, not a sand 

 of river and tidal deposition mixed with mud, such as forms many of 

 the sandstones of the Belt terrane, notably those of the Spokane shales 

 formation. 



Absence of lower Cambrian rocks and fauna is accounted for b}- the 

 fact that that portion of the continent now covered by the Belt and asso- 

 ciated middle and upper Cambrian rocks was a land surface during 

 lower Cambrian time. 



VNCONFORMITY BETWEEN BELT TERRANE AND CAMBRIAN 



Extent and character of recognized contacts. — The contact of the Flathead 

 Cambrian sandstones with the rocks of the Belt terrane may be observed 

 along a great extent of outcrop on the eastern, southern, and western 

 sides of the Little Belt and Big Belt mountains. Fully 200 miles or 

 more of outcrop may be followed, along which frequent contacts may be 

 observed. On the eastern side, in the vicinity of Neihart, the uncon- 

 formity between the Cambrian and underlying Belt terrane is clearly 

 evident, though the angular unconformity is generally slight and has 

 been recognized only on Sawmill creek. Four miles north of Neihart 

 the Cambrian rests on a nearly level surface of crystalline schists. 

 West of Neihart it rests on the Neihart quartzites. On O'Brien creek, a 

 few miles southwest of the town, it rests on black shales (Chamberlain 

 shales), of which there is less than 300 feet in thickness between the 

 Cambrian and the top of the Neihart quartzite. On Chamberlain creek 

 and upper Belt creek, 6 miles southeast of Neihart, the Cambrian rests 

 on the Greyson shales, while along the stage road up Sawmill creek it 

 is superimposed on the red Spokane shales. The only exposures on the 

 eastern slope of the Little Belt range, those of the south fork of the 

 Judith, show the Cambrian resting on the drab Greyson shales. These 

 are the only instances known where the red Spokane shales are wanting 

 beneath the Cambrian. Whether the shoreline conditions, which are 

 known to have existed near Neihart during the period when the Belt 

 terrane was formed, caused a wedging out of the beds to the north, so 

 that the Cambrian rests on different horizons at this locality, or whether 

 pre-(^ambrian erosion was extensive enough to pare down the exposed 

 edges of the beds, is not certain from the evidence, though the latter 

 view seems improbable. Similar conditions prevailed southward in the 

 Bridger range. 



In the north end of the Bridger range, east of Gallatin valle}^ the 

 Cambrian is seen resting on the Belt terrane, which at this locality does 

 not show its tjqncal development, but consists largely of coarse sand- 

 stones and grits composed of Archean debris. In the south end of this 



