392 GEORGE B. RICHARDSON 



But inasmuch as favorable conditions for dehydration existed in 

 the area of deposition, it must not be asserted that the color of 

 the red beds of the Black hills was entirely formed in the parent 

 soil, and that none of the color was formed during sedimentation. 



W. O. Crosby has called attention to the effect of exposure 

 of iron-bearing sediments in shallow-water areas of deposition 

 upon the production of red pigment.^ This action depends on 

 the dehydration of hydrated iron compounds and is a further 

 operation of the influences which have been emphasized as the 

 effective cause in the production of a residual red soil. 



Another factor in the production of red pigment from hydrated 

 iron compounds is the dehydrating effect of salt water discovered 

 by W. Spring.^ Because the red beds of the Black hills were 

 deposited in concentrated waters, this influence operated and 

 may have been important. 



The presumption is, however, under the favorable climatic 

 conditions and from analogy with the homogeneous red tint of 

 many present residual red soils, that before the soil particles were 

 actually deposited they had become completely red. But the 

 possibility of these two causes having acted must not be forgot- 

 ten. And, too, the fact must be borne in mind that the dehy- 

 dration of ferric hydrates tends to go on under ordinary con- 

 ditions without any unusual cause. ^ So that it is unnecessary to 

 assume the action of further dehydrating agencies than those 

 operating on the land which supplied the residual soil. 



Whether a change in the pigment occurred subsequent to 

 deposition, as suggested by Dana,'* should be considered, 

 inasmuch as the Black hills have been subjected to igneous 

 intrusions. There are, however, in the central west province, 

 red beds — similar to those in the Black hills and apparently 

 genetically connected with them — which are not associated with 



'W. O. Crosby, Proc Boston Soc. Nat. Hist., Vol. XXIII (1888), p. 509. 



=0/. cit. 



3 This has been repeatedly demonstrated by experiment: Wittstein, Viertel- 

 jahresschrift fiir PhartJiacie, Vol. I (1852), p. 275; Davies, Jour. Chem. Soc. of 

 London, Vol. XIX (1866), p. 69; Van Bemmeln, Recueil des travaux chimiques des 

 Pays-Bas et de la Belgique, Vol. VII (1888), p. 106. 



" Op. cit. 



