THE ORIGIN OF RED BEDS 169 



This variegation itself constitutes one of the most unanswerable 

 arguments against weathering in the present cycle as the cause of 

 the red color. The distribution of gray and green beds among the 

 red does not appear to have any definite relation to coarseness of 

 grain, as in the case of intensity of the red color, nor to any other 

 single stratigraphic feature; and it certainly bears no constant 

 relation to the topographic surface. 



Origin of mottling. — The green spots present in some otherwise 

 red strata have been explained, on the hypothesis that the red 

 color is due to recent oxidation in weathering, as remnants of the 

 original color of the beds; but, as will be shown presently, it is 

 much better in accordance with fact to explain them as spots in 

 originally red sediments, deoxidized by the agency of some particle 

 of organic matter which was present in the original sediment. . 



Dale 1 discusses the origin of the green spots in red and purple 

 slates in part as follows : 



The difference in color from the green to purple to red is manifestly due to 

 the differences in the amount of hematite. [See analyses, p. 160.] The green 

 fossil impressions in purple slate may throw some light on the origin of these 

 spots. In this case the effect of organic matter, whether the carbonaceous 

 matter of the lining of an annelid boring or from a marine alga, has been to 

 diminish the quantity of Fe 2 3 in the slate. The increase of the carbonates 

 may be directly connected with the production of C0 2 by decaying organisms 

 and the consequent decrease of the Fe 2 3 . In view of all these facts and indica- 

 tions, the spots may be safely regarded as probably produced by chemical 

 changes consequent upon the decay of organisms. 



The same conclusion is reached by Miller with respect to green 

 spots in the red Vernon shale (Silurian, central New York) . Miller 2 

 finds dark organic centers in many of the spots, and attributes the 

 color of green shales associated with the red strata to more abundant 

 dissemination of organic matter. 



It is probable that a very small quantity of organic matter may 

 reduce or prevent the oxidation of a considerable amount of iron. 

 The ferruginous matter necessary to stain a sediment is so small in 

 amount that the quantity of organic matter necessary to accom- 

 plish the reduction of a patch less than an inch in diameter, like 



1 T. N. Dale, The Slate Belt of Eastern New York and Western Vermont, Ann. 

 Rept. U.S. Geol. Survey No. 19, 1899, Part 3. 



2 Op. oil. 



