UPPER RED BEDS OF THE BLACK HILLS 379 



by 0.006"'" wide. Sericite occurs sparingly in irregular patches 

 averaging 0.06 or 0.07™™ in cross-section and consisting of 

 minute fibrous plates. 



Carbonates occur scattered throughout the slides in rhombs 

 that average about 0.03™™ in cross-section and in small irregular 

 particles. 



The presence of kaolin is suggested under the microscope by 

 dull white flakes of irregular outline and of low refraction and 

 birefringence. Bits of red pigment are frequently included. 

 And the presence of kaolin is probable from chemical tests. The 

 color was discharged from a piece of red clay by digestion in 

 hydrochloric acid. The resulting gray powder was examined 

 under the microscope, and white flakes of low double refraction 

 were isolated, excluding quartz as far as possible. These flakes 

 reacted for aluminum and water before the blowpipe. 



Feldspars are very sparingly present. Some clouded frag- 

 ments seem to be decomposed orthoclase. A few irregular areas 

 of microperthite of fibrous appearance are also present. And 

 very rarely bits of decomposed plagioclase occur. 



Gypsum and dolomite, or magnesite, were not determined 

 under the microscope, but their presence is shown by chemical 

 analysis. 



Amorphous red pigment is prominent in the slides. It irreg- 

 ularly coats and spots the minerals, and is included in some. 

 The inclusions seem to be restricted to quartz and kaolin. This 

 pigment constitutes the chief interstitial substance. It does not 

 occur in continuous vein-like impregnations, nor does it form 

 bands or local accumulations. 



Analysis shows that the red pigment is iron oxide and that 

 probably it is completely anhydrous. Sample No. 54, in which 

 the complete analysis shows 2.84 per cent, of combined water, 

 was treated with hot hydrochloric acid until all of the iron was 

 dissolved. The residue was found to contain 1.90 per cent, of 

 combined water, leaving 0.94 per cent, available for the hydrous 

 minerals which hydrochloric acid would attack. Gypsum, and 

 possibly an iron hydrate, are the only ones that the acid would 

 decampose. Calculating the amount of gypsum from the 



