REVIEWS 457 



Weed' maps and describes the pre-Cambrian rocks in the Fort 

 Benton and Little Belt Mountains quadrangles of Montana. 



The Archean rocks are found only in the Little Belt range in the 

 southwestern part of the Fort Benton quadrangle and in the north- 

 western part of the Little Belt Mountains quadrangle. They are 

 gneisses and schists of various kinds, and of somewhat uncertain 

 origin. They are, in part at least, of igneous origin, and none of 

 them show any traces of sedimentary origin. Their relations to the 

 Algonkian rocks are those of unconformity. The Algonkian rocks 

 are found in the mountain tracts of the Little Belt range, in Castle 

 Mountain, and in the low range crossed by Sixteenmile Creek in the 

 southwest corner of the Little Belt Mountain quadrangle. They are 

 divided into the Neihart quartzite and the Belt formation,"" both of 

 which are parts of what Mr. Walcott has called the Belt Terrane. 



The Neihart quartzite is a hard pink and gray quartzite forming 

 the base of the Belt Terrane for this area. It is found in the vicinity 

 of Neihart in the Little Belt Mountains. Its thickness is about six 

 hundred feet. The Belt formation consists mainly of slaty, siliceous 

 shales, but also contains interbedded limestone and quartzite. Fossils 

 found in this series (in the shales above the formation which Mr. 

 Walcott has named the Newland limestone member of the Belt Ter- 

 rane), represent the earliest forms of life yet known. Near Neihart 

 the Algonkian period is represented by 4000 feet of beds, while further 

 south and west the thickness is much greater. 



Overlying the Algonkian rocks conformably are rocks containing 

 Middle Cambrian fossils. North of Neihart they rest directly on the 

 Archean. 



Reconnaissance geological surveys in Alaska and adjacent portions 

 of British Columbia, by United States and Canadian government 

 parties, have shown the basal rock over considerable areas to be a 

 granite, which is provisionally assigned to the Archean. ^ Such granite 



'Fort Benton and Little Belt Mountains Folios, by Walter Harvey Weed: 

 Geol. Atlas of the U. S., Nos. 55 and 56, 1899. 



See also Geology of the Little Belt Mountains, Montana : Twentieth Ann. Rept. 

 U. S. Geol Surv., 1898-9, Pt. Ill, 1900, pp. 278-284. 



' The Belt formation includes the various lithological members of the Belt Terrane 

 which Mr. Walcott has named the Chamberlin shale, the Newland limestone, the 

 Greyson shale, the Spokane shale, and the Empire shale. 



3 Usually in the sense of pre-Cambrian. 



