274 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 64 



Moon Pass, which is in the southern central section of the Big Snowy 

 Mountains. 



Belt scries. — Of this series Calvert writes as follows: "The 

 oldest strata exposed in the area are correlated with the upper part of 

 •the Belt series, of Algonkian age, named from the Little Belt Moun- 

 tains where studied and first described by Weed/ In the Big Snowy 

 Mountains these rocks were actually seen only in Half Moon Pass 

 amphitheatre, but they are believed to be exposed also in connection 

 with several other similar topographic features at the heads of Blake, 

 Careless, and Timber creeks. A thickness of about 300 feet of these 

 strata is exposed near Half Moon Pass, where they consist of dark, 

 limy shale highly indurated and approximating a slate in physical 

 condition. No fossils were found in these rocks, but their similarity 

 to strata in the Little Belt Mountains of definitely determined 

 Algonkian age, together with the marked angular discordance be- 

 tween them and the overlying Cambrian quartzite, seem sufficient 

 to justify their assignment to the Belt series of the Algonkian. At 

 only one locality was the unconformity noted, namely, just to the 

 east of Swimming Woman Creek, in what, if surveyed, would prob- 

 ably be the S. E. ^ of Sec. 9, T. 11 N., R. 19 E. At this locality the 

 Algonkian shale dips south at a ip° angle, zvhereas the overlying 

 quartzite lies practically horizontal, so that an angular unconformity 

 is apparent. 



" It is not known definitely whether the calcareous nature of the 

 shale is due to original content or to later infiltration of descending 

 waters charged with lime from the Paleozoic strata higher in the 

 section. The latter assumption is given weight by the presence of 

 many joint planes and a number of fault zones filled with calcite 

 undoubtedly of secondary origin." 



Cambrian formations. — " Lying unconformably on shale of the 

 Belt series is a sandstone 75 feet thick composed mainly of pure 

 quartz. Although indurated, the sandstone is not a quartzite, as 

 stratification is distinct, and cleavage along bedding planes is marked. 

 It is evidently a shore deposit, as cross-bedding is abundant and ripple 

 marks .are beautiftilly developed. The shore phase is also attested 

 by abundant worm trails, the only evidence noted of life existing at 

 the time of deposition of the sandstone. Layers of quartz con- 

 glomerate are of frequent occurrence in the sandstone with pebbles 



'Weed, W. H., The Little Belt Alountains Folio, Geol. Atlas of the U. S. 

 No. 56, 1899. 



