386 GEORGE B. RICHARDSON 



of alteration, and red pigment indiscriminately distributed 

 among the individual soil particles. Instances of residual red 

 soils are numerous. Notable occurrences are : the soils of the 

 Piedmont plateau of the southern Appalachians, the terra rossa 

 of Europe, the laterite of India and the red soil of the valley of 

 the Amazon. 



Streams coursing over lands mantled with residual red soil 

 transport it to areas of deposition, and thus tend to cause the 

 sedimentation of red rocks. In many instances, however, in 

 regions where red soils are abundant the red material washed 

 from the land turns brown, and often greenish or bluish, before 

 final deposition among accumulating sediments. For instance, 

 in the Piedmont plateau region of the southern Appalachians red 

 detritus in the streams generally becomes decolorized to the 

 more somber tints of ferrous compounds, because of the deoxi- 

 dizing influence of abundant decomposing organic matter in the 

 water. But such destruction of the red color of detrital material 

 so as to prevent the actual deposition and accumulation of red 

 sediment is not universal. Thus enough of the red material 

 brought down by the Amazon escapes deoxidation, so that vast 

 deposits of red rocks are now accumulating along the coast of 

 Brazil.^ And it is probable that under more favorable conditions 

 red rocks would accumulate more generally than now occurs. 

 Such conditions would be the greater prevalence of areas cov- 

 ered with residual red soil and the absence of much organic 

 matter in the waters concerned with the transportation and 

 deposition of the red material. 



In the case of the red beds of the Black hills it seems probable 

 that unusually favorable conditions did exist both for the forma- 

 tion of a parent residual soil and for its accumulation as red 

 sediment. 



The geography of the Rocky mountain and adjacent regions 

 in red-bed time^ remains to be worked out. Still it is generally 



^JOHN Murray, Challenger, Reports Deep Sea Deposits, 1891, p. 234. 



^The age of the red beds of the Black hills, considered as an entire series, is not 

 satisfactorily known, because no fossils have been found in the upper and lower 

 formations. The intermediate limestone, however, carries fossils which indicate it to 

 be Permian, but whether the lower red beds are in part Carboniferous or whether the 

 upper are partly Triassic there is no direct evidence. 



