554 STUDIES FOR STUDENTS 



ingston ranges."^ The two districts described above are about 150 

 miles apart in a general north-northwest and south-southeast direc- 

 tion; and as the intermediate region has not been studied in detail, 

 Willis does not pretend to correlate closely the several formations 

 described by him in the northwest with those described by Walcott 

 from the district near Helena, but the similarity of sequence is suffi- 

 ciently striking to warrant placing them in juxtaposition as is done 

 below. 



Belt Formation, Helena Region — Algonkian or Northwest Montana, 

 Walcott Lewis and Livingston Ranges — 



WnLi5 



Thickness in feet Thickness in feet 



Marsh shales ^oo \ ^intla argillite 800 -h 



^ / Sheppard Quartzite . . . 700 ± 



Helena limestone 2,400 Siyeh limestone 4,000 



Empire shales 600 



Spokane shales 1,500 Grinnell argillite . 1,000 to 1,800 



Greyson shales 3,000 Appekunny argillite .... 2,000 -f- 



Newland limestone .... 2,000 Altyn limestone 1,400-)- 



Chamberlain shales .... 1,500 (Bottom of limestone not ex- 



Neihart quartzites and sandstone 700 posed) 



12,000 9,900 to 10,700 



Brief descriptions of these formations are quoted as follows, those 

 of the equivalent formations of the two localities being placed together : 



Neihart quartzite and sandstone Helena region, Little Belt Mountains. — In 

 this formation are included the reddish, coarse sandstones, with interbedded 

 dark greenish layers of fine-grained sandstone and shale, beneath the Chamber- 

 lain shales. The lower 400 feet of the formation is a massive, sometimes cross- 

 bedded quartzite, which, in some of its members, where unaltered, is a compact, 

 hard sandstone. The prevailing color is pinkish-gray on the freshly exposed 

 surface, with dark and iron-stained weathered surface. Occasional layers 

 of a fine conglomerate occur in some portions near the contact with the gneiss.^ 



About 300 feet above the base the character of the formation changes. The 

 pink and white pure quartzites are replaced by more thinly bedded rocks, no 

 longer of pure arenaceous material, but containing an admixture of greenish 

 mica, which higher in the group forms the layers of mica shales interbedded 

 with the quartzite. The higher strata are still more impure and the qtiartzite 

 beds are but six to twelve inches thick, blackened by carbonaceous material that 

 now forms a prominent feature of the intervening shales, becoming increasingly 

 abundant until the latter rocks are true black shales in which the green mica no 



I "Stratigraphy and Structure, Lewis and Livingston Ranges, Montana," 

 ibid.. Vol. XIII, pp. 316-24. 



^ Bulletin Geological Society of America, Vol. X, p. 204. 



