UPPER RED BEDS OF THE BLACK HILLS 39 1 



not borne out by the presence of associated, parti}- decomposed 

 minerals. 



Excluding these adventitious constituents, the analyses show 

 the red beds of the Black hills to be composed essentially of 

 silica, alumina, ferric oxide, potash, and water. These are char- 

 acteristic components of residual soils, the corresponding min- 

 eralogical composition being the stable species, quartz, muscovite, 

 kaolin, and the red pigment with occasional bits of decomposed 

 feldspar. 



The result of microscopic examination also shows the similar- 

 ity of these red beds with residual red clay. Some of the quartz 

 grains have intricate contours, as if etched by alkalies derived 

 from decomposing feldspar. The disposition of the pigment as a 

 coating to the individual rock particles is like that in residual red 

 soils. And the inclusion of the red pigment in secondary quartz 

 and kaolin is significant. 



This inclusion implies the formation of the including miner- 

 als in the presence of the red pigment. It is not probable that 

 these minerals were formed in the area of deposition when the 

 sediments were accumulating so as to receive inclusions of iron 

 precipitated from solution ; nor is it a likely assumption that 

 much decomposition took place subsequent to the formation of 

 the red beds, and that the secondary minerals received inclusions 

 of iron from percolating solutions. There is but little unSecom- 

 posed material in the red beds, whereas considerable remains 

 would be expected did alteration take place after sedimentation. 

 The composition of the red beds is essentially of stable minerals, 

 but some of the quartz and all the kaolin are products derived 

 from decomposition. On the land area where the parent red 

 soil of the red beds was accumulating, it is to be expected that 

 in the intimate association of the constituents, iron oxide became 

 included in the secondary minerals that were formed as decom- 

 position products. Such inclusions are common in residual red 

 clay in the District of Columbia. 



It seems probable therefore that the dominant factor in the 

 production of the color of the red beds of the Black hills was a 

 residual red soil on the land mass which supplied the sediments. 



