394 WEED AND PIRSSON HIGH WOOD MOUNTAINS OF MONTANA. 



of a break in the continuity of the plain is given until within a few feet 

 of the canyon wall. 



About the mountains there are occasional remnants of higher levels, 

 forming bench-like steps. To the north the extension of the gentle out- 

 ward slope of the platform awa}^ from the range is interrupted by the 

 Shonkin Sag, an old drainage-way which marks the southern limit of 

 the moraine of the Laurentide glacier. The open country between the 

 mountains and the Belt range is a very wide, shallow valley rather than 

 a plain. The strata composing the platform are in general horizontal or 

 but slightly inclined away from the mountains. The exceptions are the 

 beds just east of Highwood gap, at the debouchment of Shonkin creek, 

 and in Davis creek valley, in each of Avhich places local flexures occur. 

 The strata west of the mountains dip genty away from the peaks. 



The rocks themselves consist of sandstones and clayey shales belong- 

 ing to at least two groups of the Cretaceous. The lowest series is exposed 

 in this vicinity only in Belt creek, and consists of rather firm, well 

 cemented sandstones, alternating with purple and red clays and shales. 

 These beds are classed as Kootanie. Along the flanks of the Little Belt 

 range this series rests in apparent conformity upon the fossiliferous 

 Jurassic beds. It is the series containing the well known coal seam of 

 the Great Falls field. The strata vary rapidly, sandstone beds ending 

 abruptly and being replaced by shales. All the evidences show shallow- 

 water conditions and shifting stream currents. The sandstones are rarely 

 conglomeratic when the pebbles are of black quartzite, but the sand grains 

 are angular and show little alteration by attrition. Plant remains from 

 this series found near Great Falls at approximately the horizon of the coal 

 seam were determined by the late Dr J. S. Newberry ^ as Kootanie, a 

 later paper by Professor W. M. Fontaine f taking the same view. A 

 small collection of plant remains collected by one of the authors from 

 the coal seam south of the Highwoods proves to be of Kootanie or Lower 

 Cretaceous age upon examination by Professor F. H. Knowlton. 



The coal seam is the most important single seam in the state, and at 

 Belt and Armington it is being extensively mined. The seam is from 

 6 to 8 feet thick, with several minor partings and a persistent layer of 

 sandy shale which separates it into an upper and lower bench. The 

 character of the coal from these two benches differs quite materially, the 

 upper being a free-burning bituminous variety with high percentage of 

 ash, the lower a coking coal with higher fuel ratio. [j; The latter alone is 

 mined at Belt for the Butte smelters. 



About 200 feet above the coal seam is a limestone layer, or more gener- 



* Am. Jour. Sei., vol. xli, 1891, p. 191. 



t Proc. U. S. Nat. Mus., vol. xv, 1892, p. 487. 



X Weed : Bull.Geol. Soc. Am., vol. 3, 1892, p. 301. 



