COLORS OF CONTINENTAL CLASTICS 621 



gling of the vegetation with the soil. In such cases oxidation may 

 proceed without hindrance, as in the case of the eastern slopes of 

 Nicaragua, where red soil from 3 to 10 meters deep underlies the 

 dense vegetation. 



In seasons of dryness, when the amount of vegetation is small, 

 the iron of the sediments of deltas and alluvial fans may become 

 thoroughly oxidized. Where dryness prevails for most of the 

 year, and where vegetation is as a result scanty, such oxidation 

 may be especially favored. Thus semiarid or even desert regions 

 would furnish the best conditions for such oxidation. On river 

 flood plains there is always sufficient moisture to result in the 

 formation of hydroxides of iron, and hence the colors of such 

 deposits will range from yellows to ocher and brown. It is only 

 under conditions of intense heat that dehydration will result with 

 a consequent change in color toward the reds. Such change of 

 color may, however, take place as the result of aging of the deposit, 

 as pointed out by Crosby. In such a case dehydration is virtually 

 spontaneous, and "... the color of the deposit, so far as it 

 is due to ferric oxide, is, other things being equal, a function of its 

 geological age." "In other words,'' says Crosby further, "the color 

 naturally tends with the lapse of time to change from yellow to 

 red ; and, although this tendency exists independently of the tem- 

 perature, it is undoubtedly greatly favored by a warm climate." 



(15.) 



Barrell (4:288) holds that "a still more potent cause exists 

 . . . in the dehydration effected by the great increase in pres- 

 sure and moderate rise in temperature which takes place upon 

 the burial of the material to some thousands of feet beneath later 

 accumulations." Under such pressure the tendency would be for 

 the oxide to give up its water with corresponding reduction in 

 volume (see ante, p. 177), just as shales are formed by the giving 

 off of about one-half the combined water by the silicate of alumina, 

 and this at temperatures probably often far below boiling point. 



It is, of course, necessary that organic matter should be absent 

 from such a deposit, for its presence would prevent oxidation in 

 the first place. It is not necessary that the absolute amount of iron 

 should be very large to affect the color of the deposit. The bril- 

 liant red Vernon shales of the eastern New York Siluric carry 

 only 2.25 per cent, of ferric iron and 0.75 per cent, of ferrous iron 

 (Miller-39). The chief desideratum is that the iron should be in 

 an extremely fine state of subdivision and intimately disseminated 

 or diffused through- the mud or dust deposits. This fine division 

 and diffusion of the iron have been noted by Dawson for the red 



