UPPER RED BEDS OF THE BLACK HILLS 385 



explanation accounts for the red coating of the individual 

 particles, and that it is difficult to disprove, are in its favor. 

 Nevertheless, while it cannot be maintained that no ferric hydrate 

 was precipitated from solution among the accumulating sedi- 

 ments under consideration, yet the evidence is that such was not 

 the principal source of the pigment. 



It will be shown presently that the climate during the sedi- 

 mentation of the red beds probably was arid. Under such con- 

 ditions vegetation would be scant, and surface waters would 

 not be heavily charged with solvents. Conditions then were 

 not specially favorable for rock decay, nor for the transportation 

 of iron in solution to the area of deposition during red-bed 

 time. Besides, if such were the origin of the pigment, local 

 accumulations of it would be expected as in the case of those 

 Clinton ores for which such an origin is accepted, and in the 

 case of bog ores now forming. On the contrary, in none of the 

 rocks examined microscopically does the thickness of the 

 pigment amount to half a millimeter, and usually it is much 

 thinner. The exact equilibrium required for the chemical pre- 

 cipitation of ferric hydrate to be just sufficient to coat each 

 sedimentary particle, no more nor less, is extremely improbable. 



Then there is the theory, emphasized by Russell, that red 

 beds may be formed by the sedimentation of a residual red soil, 

 and the evidence seems to be in favor of such an origin for the 

 red beds of the Black hills. 



It is a familiar fact that under suitable conditions rocks 

 which contain iron-bearing minerals weather to a red clay. In 

 the process of rock disintegration and decomposition the iron- 

 bearing minerals alter easily. Ferrous iron — in biotite, horn- 

 blende, and pyroxene, for example — becomes oxidized and 

 hydrated to limonite, which dehydrates and passes through 

 stages corresponding to gothite and turgite, to the stable red 

 hematite.' A late stage of residual soil, from a variety of 

 parent rocks, consists of the stable minerals quartz, kaolin, and 

 muscovite, traces of original rock constituents in various stages 



'W. O. Crosby, American Geologist, Vol. VIII (1891), p. 72; G. P. Merrill, 

 Rocks, Rock Weathering and Soils, 1897, p. 299. 



