380 



GEORGE B. RICHARDSON 



amount of SO3, it is found that 4.8 per cent, of the rock is 

 gypsum. This requires i per cent, of water, which practically 

 is the amount available. No water, therefore, is left for combi 

 nation with the iron. It is true this calculation is but an approx- 

 imation, but it appears evident that the red pigment is either 

 anhydrous or nearly so. 



Chemical analysis. — Two specimens of the Spearfish formation 

 were analyzed by. Mr. George Steiger, of the U. S. Geological 

 Survey, with the following results : 



Red Sandy Shale from East 

 of Newcastle. No. 54. 



Red Sandy Shale from East 

 of Spearfish. No. 55. 



100.19 99-69 



The elements are so distributed that an exact determination 

 of the relative abundance of the minerals in the rocks analyzed 

 is impossible. Estimates based on the analyses and on the 

 appearance under the microscope give the following approximate 

 mineral composition of average red shale: 



