188 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



taken to mean an erosion unconformity or structural unconformity of the 

 Belt and Flathead beds by the amount of missing strata at any one or more 

 localities. 



The section which, according to Walcott, most clearly shows the extent 

 of the unconformity is that running eastward through the Spokane Hills. In 

 the diagram illustrating the relationships there, the Middle Cambrian beds are 

 represented by Walcott as conformably overlying the Helena formation both 

 at Helena and near White's canyon in the Belt mountains uplift.* These 

 two localities are twenty-four miles apart. Nearly midway between them, at 

 the Spokane Hills, the Middle Cambrian is represented as again conformable 

 on the Belt beds but this time resting on the Spokane shales. Thus fully 3,000 

 feet of strata, the thickness of the Helena and Empire formations, are con- 

 sidered as lacking beneath the Middle Cambrian at the Spokane Hills. 



The text accompanying the diagram does not state whether the fossiliferous 

 Cambrian at the Spokane Hills is the stratigraphic equivalent of the Flathead 

 sandstone. Mr. Walcott has, by letter, very kindly informed the writer that 

 the Middle Cambrian beds at the Spokane Hills are not only faunally but litho- 

 logically the equivalent of the Flathead. There thus seems to be an actual 

 failure of at least 3,000 feet of Belt beds at the Spokane Hills. Whether the 

 failure is due to a lack of original deposition or to the erosion of a local 

 upwarp of the Belt beds is apparently not an easy question to decide. Walcott 

 has taken the latter view for this locality. On the other hand, he himself 

 writes, concerning somewhat similar relations about the town of Neihart: — 



' Whether the shore-line conditions, which are known to have existed 

 near Neihart during the period when the Belt terrane was formed, causing 

 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.'! 



All workers on these Montana rocks have observed that, wherever the 

 Flathead sandstone is seen in contact with Belt formations, no important 

 angular discordance of dip can be seen. Such slight discordances as have been 

 described and figured by Walcott in his 1899 paper, can be explained either 

 by slight, perhaps submarine, erosion of the older surface, or by local and 

 very gentle warping of the surface just before the Flathead subsidence. 



As early pointed out by Peale, Flathead time saw a general, rapid, but 

 not very pronounced subsidence of the western Montana region. A large 

 supply of quartzose debris was thus brought from the drowning land and depos- 

 ited alike over Archean schists and the various lenses of the geosynclinal. In 

 this way a fairly homogeneous formation was spread over a sedimentary mass 

 which, in the nature of the case, must have been composed of many and varied 

 lenses all of which petered out toward mainland, island, or shallow. 



* See C. D. Walcott, Bull. Geol. Soc. America, Vol. 10, 1899, p. 211. 

 flbid, p. 210. 



