THE ORIGIN OF RED BEDS 



165 



from all sources. The following figures from Clarke 1 are to 

 the point : 



See also analyses K and K 2 , on p. 160, supra. 



Microscopic evidence. — -Russell 2 has presented evidence to show- 

 that the red coloring matter in sandstones of the Newark series in 

 Virginia occurs as a coating on the sand grains, and that it existed 

 in that form even before the transportation of the sand from the 

 residual soils from which it was derived. Since much of his argu- 

 ment would apply equally well to the red sandstones of western 

 Red Beds, it is worth investigation in this connection. In a few 

 thin sections of Newark sediments from Virginia which have been 

 available for study by the writer, it is shown clearly that ferruginous 

 matter exists there, both in the form described above and as a 

 later interstitial filling between grains. Many of the sand grains, 

 both of quartz and of feldspar, are well rounded. Most of them 

 are surrounded by a thin coat of hematitic material, whose outer 

 surface is smooth and even, but whose inner border may show slight 

 irregularities penetrating into the body of the grain. This red 

 film may not be entirely continuous around the grain; and this fact, 

 together with the smoothness of its outer surface, suggests, as 

 Russell maintained, that the coating was acquired by the sand grain 

 before its final deposition, and that the coating suffered wear during 

 transportation, without being completely removed. 



In a number of instances it was found that part of the cementa- 

 tion of the rock had been accomplished by enlargement of the 

 original grains of quartz and feldspar in optical continuity, outside 

 of the red coating; and that outside of the enlargements there 

 existed other bodies of hematitic matter, irregularly scattered 



1 F. W. Clarke, "The Data of Geochemistry," 2d ed., U.S. Geol. Survey Bull. No. 

 4Qi, 1911, chaps, iii, v, vi. 



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



