'vVANLESS— LITHOLOGY OF WHITE RIVER SEDIMENTS. 199 



weathered feldspars, and mica foils can be detected in all samples, as 

 well as occasional fragments of pumice and volcanic glass. The latter 

 fact shows that showers of volcanic dust were being deposited 

 throughout White River time, but first became the dominant source 

 of sediment at the beginning of the Leptauchenia beds stage. The 

 clay beds represent the normal sheet-flood deposits of a level country 

 and probably are mainly formed of kaolinized feldspars from the 

 granites and pegmatites of the Harney Peak mass, as well as from 

 decomposition products of other minerals less stable in the zone of 

 weathering. 



As mentioned above, the profiles developed on the Oreodon clays, 

 as a rule, have steeper slopes and a more angular appearance than 

 those developed on the Titanotherium beds which weather with gentler 

 slopes and more rounded surfaces. The chemical tests suggest that 

 this is due to a difference in the amount of calcareous cement in the 

 two series. The samples of the Oreodon clays averaged lo to 12 per 

 cent, soluble cement, while the Titanotherium clays averaged only 

 about 4 per cent. 



V. Evidence from Shapes of Sand Grains. 



Inasmuch as the method of transportation and deposition of sands 

 can often be definitely established by the shapes and average sizes of 

 sand grains, a study of this feature of the White River sands was 

 attempted. 



The channel sands, as a rule, have large rounded grains, but the 

 smaller grains are almost all angular and with sharp conchoidal frac- 

 ture. This is because grains with a diameter less than o.i mm. are 

 coated, while submerged, with a film of water which prevents one 

 grain coming into physical contact with another, and thus prevents 

 abrasion. On the other hand, the air forms no such cushion for the 

 smaller grains, and thus wind-rounded sands may show rounded 

 grains down to the finest sands present. The only sample of sand 

 found in the White River which showed thorough rounding even to 

 the finest particles is a fine-grained sandy silt occurring 1 1 feet above 

 the base of the Leptauchenia beds in the Spring Draw section, Sheep 

 Mountain. This sand has well-rounded grains even in the smallest 

 sizes and contains a notable amount of magnetite and green horn- 



PROC. AMER. PHIL. SOC , VOL. LXI., O, NOV. I7, I922. 



