THE ORIGIN OF RED BEDS 239 



the pre-Cambrian series, and that much of the material of the Red 

 Beds is known to have been derived from other rocks. 



Arkosic stream deposits. — These are illustrated by deposits of 

 limited extent in and downstream from the region occupied by 

 the Sherman granite of Wyoming, which weathers to a coarse 

 pink gravel, owing its color to a high content of undecomposed 

 pink orthoclase. It is obvious that much the greater part of the 

 color of the Red Beds is not due to pink feldspar; but in some of 

 the very arkosic sediments of the Cutler and Dolores formations, 

 and probably elsewhere, this is an element not to be ignored. 1 



Stream deposits deriving their coloring matter from ferruginous 

 residual soils.— -The fourth type of modern red sediments is exempli- 

 fied by the continental portion of the deposits of the lower Amazon, 

 and by smaller deposits in some of the rivers of the United States. 

 Russell says: 



Each grain [of sand in residual soils left by the decomposition of crystalline 

 rocks in the southern Appalachian Piedmont] is coated with a thin shell having 

 a brownish or red color. Prolonged washing fails to remove this superficial 

 coating, a fact which is well illustrated by the color of the sands deposited by 

 the streams of Virginia and the Carolinas in the regions underlaid by crystalline 

 rocks. 2 



Russell appears to assume that all of the ferric oxide produced 

 by the decay of the crystalline rocks of this area is attached to 

 grains of other minerals in this way. That which fills interstices 

 between grains of sand in the final deposit, 3 as distinct from that 

 which occurs in coatings on the grains, probably persisted inde- 

 pendently, however, and was transported as a fine sediment like 

 clay. 3 



The relation between surface weathering of the Piedmont 

 crystalline rocks and the color of the Newark elastics in the neigh- 

 boring areas, as developed by Russell, is very much the same as a 

 relation recently advocated by Beede 4 between weathering of lime- 



1 Cf. Whitman Cross, Telluride Folio (No. 57), Geol. Atlas of the U.S., U.S. Geo!. 

 Survey, 1899, p. 2. 



2 1. C. Russell, "Subaerial Decay of Rocks," U.S. Geol. Survey Bull. No. 52, 1889, 

 p. 14. 



3 See p. 164, this volume. 



< J. W. Beede, "Origin of the Sediments and Coloring Matter of the Eastern 

 Oklahoma Red Beds," abstract in Bull. Geol. Soc. America, XXIII (1912), 723-24. 



