UPPER RED BEDS OF THE BLA CK HILLS 



joo 



origin of the color of the red beds of the Black hills. In this 

 way C. H. Smyth ^ accounts for the origin of some of the Clin- 

 ton ores. And W. Spring^ in his recent paper on the color of 

 red beds accepts this theory and devotes his attention to details 

 of how dehydration may take place after the iron has been pre- 

 cipitated as hydrate. 



Another theory is that the color of red rocks may be caused 

 by the sedimentation of a residual red soil. A. W. McKay ^ 

 thus explained the color of the red sandstone of Nova Scotia. 

 And I. C. RusselH has elaborated this theory and has applied it 

 to the explanation of the color of the rocks of the Newark 

 system. 



la. A variation in this process may occur when the iron in 

 the soil, which furnishes the sediments of red beds, is not com- 

 pletely changed to the red ferric hydrate or to the anhydride 

 previous to sedimentation. In such a case the soil would 

 have a mottled color due to different stages of hydration of the 

 disseminated iron compounds. Such mottled material may 

 become uniformly red by dehydration of the disseminated iron 

 subsequent to or during sedimentation. 



J. D. Dana appealed to such a possibility in criticising 

 Russell's widespread application of the theory of original depo- 

 sition of red beds as such. Dana regarded conditions attending 

 the consolidation of the rocks sufficient to cause the dehydra- 

 tion necessary to produce the uniform red. In the case of the 

 rocks of the Newark system he considered the influence of the 

 associated trap dykes, by virtue of their raising the temperature 

 of interstitial water, potent to change to the anhydrous red 

 oxide any limonite present about the individual rock particles. 5 



Dehydration may occur also during the process of sedimenta- 



' C. H. Smyth, American Journal oJ Science, Vol. XLIII (1892), p. 487. 



*W. Spring, Recueil des travaux chimiques des Pays-Bas et de la Belgique, Vol. 

 XVII (i898),No. 2, p. 202. 



^Report British Assoc. Adv. Sci., Thirty-fifth Meeting (Birmingham, 1S65), 

 Part II, p. 67. 



^ Bulletin §2, U. S. Geological Survey, 1 888. 



^American Journal of Science, Vol. XXXIX (1890), p. 319. 



