THE ORIGIN OF RED BEDS 159 



others. The occurrence of coarse-grained massive buff sand- 

 stones in a series of maroon or chocolate shales has been noted by 

 many writers. This association holds true in many other Red Beds 

 besides the group here under consideration. Thwaites 1 reports it 

 as an almost unfailing relation in the Lake Superior sandstone 

 series of northern Wisconsin, and Geikie 2 mentions its existence in 

 the Triassic New Red Sandstone of Great Britain. The respective 

 tints are understood to be uniform throughout the strata in which 

 they are noted, and not to be merely the surface staining of the 

 beds, which might have a very different origin. 



Distribution of greenish colors. — Greenish tints occur in the Red 

 Beds as a fairly even color through continuous beds interstratified 

 more or less closely with red strata, in streaks and blotches in red 

 strata themselves, and occasionally in strips following joint planes. 

 The completely greenish strata, and likewise the mottled beds, 

 include both shales and sandstones, and are described from many 

 districts. Green spots in red strata are as a rule irregularly 

 ellipsoidal in shape, somewhat flattened parallel to the bedding, 

 and indefinite in outline. They may vary in diameter from a few 

 millimeters to several inches. 



Variations in color along the strike are as common and as much 

 to be expected as similar variations in texture, cross-bedding, or 

 any other stratigraphic feature. The Red Beds of the Southwest 

 are noted for their inconstancy and frequency of change along the 

 strike; those of Central Wyoming and the Black Hills are fairly 

 continuous in lithologic characters for considerable distances. 

 Where other features are variable, the color is variable also; and 

 where other features are constant, color likewise is constant. 



Nature of the coloring matter. — Available data as to the chemical 

 composition of the green bands and spots and other variations in 

 color in the western Red Beds are very meager; but many of the 

 same phenomena occur in roofing slates, whose commercial value 

 has been the cause of painstaking investigation of their occurrence 

 and character. The close dependence of color upon chemical 



1 F. T. Thwaites, "Sandstones of the Wisconsin Coast of Lake Superior," Bull. 

 Wis. Geol. and Nat. Hist. Survey No. 25, 1912, p. 31. 



2 Archibald Geikie, Text-Book of Geology (London: Macmillan, 1903), p. 1064. 



