GEOLOGICAL IMPORTANCE OF SEDIMENTATION 565 



land- deposited beds followed by 50 feet of irregular massive beds 

 of yellowish-brown sandstone and 25 feet of light gray shaly sand- 

 stone. These by their contrast in color presumably represent the 

 off-shore deposits of a transgressing sea and are followed by 50 to 

 150 feet of massive, gray to reddish magnesian limestone, closing 

 the deposits of the Unkar terrane. 



The detailed section of the following Chuar terrane is not quoted 

 in full as the beds are not described by Walcott as mud-cracked, 

 nor do most of them by their other characters strongly suggest sub- 

 aerial deposition. As previously noted, they embrace 5,120 feet 

 of strata of which 285 feet are limestones. The greater portion 

 of the balance consists of brown to black, gray, or variegated shales, 

 with some reddish-brown sandstones, often shaly. Deposition in 

 off-shore waters beyond the reach of beach action is suggested as 

 the mode of origin of much of the formation by the prevailing differ- 

 ence in color, the more shaly character, and the beds of limestone 

 scattered at intervals through the entire terrane. Occasional transi- 

 tions to shore deposits of an arid climate are similarly suggested 

 by the variegated shales, and especially masses of white and pink 

 gypsum found in a few localities in one horizon which consists of 

 black argillaceous shale with chocolate and greenish, sandy and 

 argillaceous shales beneath, carrying hard layers of sandstones. 



A classification of the Chuar section into appreciably calcareous 

 and arenaceous portions suggests three movements of subsidence 

 with invasions of the sea and four periods of halting or possibly 

 elevation with approach of the httoral to this region, the series ending 

 with 125 feet of massive, reddish-brown sandstone, with irregular 

 layers of similar color and containing numerous fragments of sand- 

 stone-shale of lighter color. 



The thorough and accurate observations of Walcott have made 

 it possible to give this discussion of continental as opposed to marine 

 origin both for the Belt and Grand Canyon series, although the 

 problem of a possible continental origin is not discussed in the 

 original papers and possibly was not seriously in mind, since such 

 a question had never been raised in regard to them. Since the 

 facts have been freely quoted, however, it is also desirable to give 

 the observer's interpretation, which is throughout that of a marine 



