GRAND CANYON TERRANES 215 



explain the absence of lower Cambrian rocks and fossils and to warrant 

 our placing the Belt terrane in the pre-Cambrian Al^onkian system of 

 formations. 



GRAND CANYON TERRANES 



Composition and character. — The Algonkian formations composing the 

 Grand Canyon series of Arizona differ materially, in the order of their 

 succession, from the Belt series of formations ; but the sedimentation is 

 strikingly similar in many of the beds, especially those of the Chuar 

 terrane. These embrace limestones, shales, and interbedded sandstones, 

 which are of the same type lithologically as those of the Belt terrane in 

 the Little Belt and Big Belt mountains. 



The plane of unconformity between the Cambrian and the Algonkian 

 formations is more marked than that between the Cambrian and the Belt 

 in Montana. The plane of pre-Cambrian erosion cut across the entire 

 Grand Canyon series and extended a long distance beyond, across the 

 subjacent schists and granites. The extent of the unconformity is illus- 

 trated by figure 5. 



Chuar terrane. — This section is arranged from the summit of the forma- 

 tions downward.* The limestone in the upper division of the section is 

 138 feet, while the limestone of the lower division is 147 feet, making a 

 total thickness of limestone of 285 feet. 



Upper Division : Feet 



1. Massive reddish brown sandstone, passing into shales below, with 



irregular layers of similar color, and containing numerous frag- 

 ments of sandstone shale of lighter color 425 



2. Alternating shales and limestone ; traces of fossils occur in the lower 



portion of the shale ; Chuaria circularis is the only species that 



has been defined 337 



3. Gray Stromatopora (?) f limestone 8 



4. Black argillaceous shale, with variegated shales below ; on the 



slopes light drab, pea-green, vermilion, chocolate, maroon, and 

 buff-colored shales of various shades alternate 740 



5. Massive stratum of concretionary limestone, with sandstones and 



shales below ; reddish brown sandy shale 190 



Total thickness of upper division 1,700 



Lower Division : 



1. Brown sandy shales, passing below into chocolate and dark argil- 

 laceous shales that pass below into alternating sandy and argilla- 

 ceous shales, with thin belts of limestone from 6 inches to 4 feet 

 in thickness ; Stromatopora (?) limestone of 4 feet near base 625 



*This section is given in detail in tlie Fourteenth Ann. Report U. S. Geol. Survey, 1895, pp. 

 508-512. 

 f Probably a species of Cryptozoon. 



XXXII— Bull. Geol. Soc. Am., Vol. 10, 1898 



