THE ORIGIN OF RED BEDS 173 



occurrence of traces of organic matter in red strata may be explained 

 by an unusually high content of ferric oxide in those strata, or by 

 later reoxidation of iron at one time in the ferrous state. The fact 

 that much the greater part of the occurrences of colors characteristic 

 of ferrous compounds are in distinct beds with definite boundaries, 

 indistinguishable in most other characteristics from other beds 

 which are not so colored, is good evidence that this distribution of 

 ferrous and ferric compounds, or of the substances responsible for 

 these compounds, was accomplished for the most part at the time of 

 sedimentation. It is well to remember that the gray and greenish 

 strata are very subordinate in the Red Beds of the western United 

 States. 



The general conclusion to be drawn from the preceding dis- 

 cussion is that there has been in the western Red Beds no extensive 

 change of ferrous to ferric iron, or vice versa, since the time of 

 sedimentation; and also that the original distribution of these 

 compounds in the series was influenced most largely by the dis- 

 tribution of organic matter. 



Variations in hydration of ferric oxide.— That various degrees of 

 hydration exist in the ferric oxide of the Red Beds today is clear 

 from the variety of red, brown, and yellow hues which appear 

 in some members of the group. The major part of the ferric oxide 

 in the Red Beds is no doubt but poorly hydrated. 1 The bright- 

 and deep-red and red-brown colors (which are most common in the 

 western series) may be attributed to hematite (anhydrous Fe 2 3 ) 

 or to turgite ( 2 Fe 2 3 -H 2 0). The lighter browns, yellow-browns, 

 and yellow tints are referable to gothite (Fe 2 3 -H 2 0) or limonite 

 (2Fe 2 3 -3H 2 0), or possibly in some cases even to xanthosiderite 

 (Fe 2 3 - 2 H 2 0). 



The freer passage of water through the sandstones as com- 

 pared with the shales makes the constituents of the former, after 

 consolidation, more susceptible of hydration than those of the 

 latter. It is entirely probable that in some cases at least this factor 

 of porosity heightens the contrast in color between coarse and fine 

 sediments ; but it apparently has not affected the greater part of the 

 Red Beds, in which the ferric oxide is relatively anhydrous. Van 



1 Cf. Richardson's investigation, discussed on p. 166. 



