NO. 4 CAMBRIAN AND PRE-CAMBRIAN AT HELENA 285 



Belt Mountains — Helena section. — By oversight Rothpletz failed 

 to realize that my diagrammatic section to which he refers so fre- 

 quently was not drawn to illustrate the actual minor unconformities 

 at Helena, but to show the great unconformity indicated by the 

 absence in the Spokane Hills of the Empire shales, Helena lime- 

 stone, and Marsh shale beneath the Flathead quartzite of the 

 Cambrian. 



The Flathead quartzite rests on the Spokane shales in the Spokane 

 Hills and also to the southeast on Deep Creek east of Townsend, 

 Montana. The diagrammatic section is here reproduced as figure 

 10 (p. 263). 



Of the unconformity l^etween the Belt terrane and the Cambrian 

 I w^rote in 1899 : ' 



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 gides 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 unconformity 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 

 (Archean) crystalline schists. West of Neihart it rests on the Neihart 

 (Algonkian) quartzites. On O'Brien Creek, a few miles southwest of the 

 town, it rests on black shales (Chamberlain Algonkian 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 (Algonkian) Grayson shales, 

 while along the stage road up Sawmill Creek it is superimposed on the red 

 (Algonkian) 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 Grayson 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 dur- 

 ing 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-Cambrian 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 valley, the Cambrian 

 is seen resting on the Belt terrane, which at this locality does not show its 

 typical development, but consists largely of coarse. sandstones and grits com- 

 posed of Archean debris. In the south end of this range, but a few miles 

 distant from the former exposures, the Belt terrane is entirely wanting, and 

 the Cambrian rests directly on the Archean schists, as it does at Neihart. The 



^ Bull. Geol. Soc. America, Vol. 10, pp. 210-215. 



