376 GEORGE B. RICHARDSON 



Specimen a fine-textured arenaceous red shale, with occa- 

 sional minute glistening particles of quartz and muscovite. The 

 rock crumbles between the fingers to a fine powder, which with 

 water can be readily molded; breathed upon, it gives the charac- 

 teristic clay odor. The rock has no pronounced structure. It 

 is coherent, yet easily friable, and breaks unevenly, with a 

 tendency to a hackly fracture. Bedding planes are feebly 

 developed and usually cannot be distinguished. Occasionally, 

 though, when sand admixture becomes so prominent as to 

 produce a clayey sandstone, thin flaky bedding planes become 

 distinct. 



Streaks and spots of green in the midst of the red shales form 

 local variations. The green streaks seldom are continuous, but 

 occur irregularly, often with uneven and wavy surfaces of con- 

 tact with the red rocks. In places the green streaks follow small 

 joint planes. The size of the streaks varies from a small fraction 

 of an inch to three or four inches in thickness. The green spots 

 are irregularly distributed and roughly spheroidal in shape ; 

 usually of about the diameter of a pin-head, they sometimes 

 reach half an inch in diameter. In composition the green differs 

 from the red shale by being poorer in iron and having a higher 

 ratio of ferrous oxide. An analysis of adjacent green and red 

 shale gave : 



Green Shale. Red Shale. 



FcjOs - - 1.85 per cent. FegOs - - 4.61 per cent, 



FeO - - - 1.04 " FeO - - - 1.24 



Beds of gypsum occur at different horizons throughout the 

 extent of the Spearfish formation, the greatest development 

 being toward the middle of the series. Longitudinally no indi- 

 vidual bed can be traced far. The thickness of the gypsum 

 varies from a fraction of an inch to a maximum of about forty feet. 

 Generally the gypsum is remarkably pure and the color a clear 

 white. Occasionally admixtures of red clay produce a mottled 

 appearance. 



The following is an analysis of a sample of pure white gypsum 

 collected near Cascade Springs: ^ 



^By Mr. George Steiger. 



