HKliSHEY.J 



The River Terraces of the Orleans Basin. 



intimation of climatic change is the color of the various flood- 

 plain deposits. We have in the basin a sudden change from 

 red to dark brown. I believe that the red indicates a warm but 

 not arid or semi-arid climate. In a paper entitled "Subaerial 

 Decay of Rocks and Origin of the Red Color of Certain Forma- 

 tions,"* I. C. Russell shows that red residuary soils are charac- 

 teristic of countries having- a warm, moist climate, and not an 

 arid climate. The same red soil material deposited in bodies of 

 water in the absence of much organic matter will yield red for- 

 mations. J. S. Newberry says: 1"The fact that it is generally 

 in this condition, and therefore the rock is red, proves that it 

 contained little or no organic matter when deposited, for when- 

 ever decaying organic matter is present in any considerable 

 quantity it reduces he peroxide of iron to protoxide, and makes 

 the color, so far as influenced by the salts of iron, gray, green 

 or blue." However, on the Isthmus of Panama, I found a deep 

 red soil prevailing in the dense jungles, even where the under- 

 lying rock was black, gray or yellow, while on the grassy plains 

 that are dry half the year, unless the underlying rock was red, 

 the soil was never of that color. Red and other bright hues 

 occur on deserts, but it is not the brick red tint of the moist 

 tropics. The prevailing colors of desert soils are ashen gray and 

 light brown, unless the underlying rocks are red. 



It is a question whether the red color of the upper terraces 

 was original or has subsequently been produced by oxidation. 

 One fact leads me to think that the flood-plain silts were depos- 

 ited red. The color is uniformly reddish brown (except in the 

 few places where it is dark blue because of deoxidation on 

 account of the presence of much drift-wood), and these silts 

 preserve this color where they pass under thick masses of gray 

 slate debris or green serpentine landslides. If originally red, 

 it implies that the river drained a country having a prevailingly 

 red soil. Whether the red color of the silts was original or 

 subsequently produced, it indicates a comparatively warm and 

 not arid climate for the earlier portion of the terrace period. 

 In fact, these red silts occur down to the seventy-foot terrace. 



* Bulletin No. 52, IT. S. Geo!. Survey. 



f Monograph No.' 14, U. S. Geol. Survey, pp. 7, 8. 



