THE TERTIARY PERIOD. 239 



scope magnifying 90,000 times, or 300 diameters, the space of about 

 one-eighth of an inch, and of incalculable thinness. These speci- 

 mens, under such high powers, are translucent. Many other curi- 

 ous microscopic forms occur. The chemical analysis of this earth, 

 however, is very different from tripoli. It is proved to be a silicate 

 of the alkaline earths, and most generally of soda, potash, magnesia 

 or lime. Sometimes only one, and sometimes several of these alka- 

 lies are present. It ranges in color from light gray to snow white, 

 green and yellowish. All these colors are sometimes found in the same 

 bed, and the chemical composition varies even more than the color. 

 To the touch it feels very much like flour. The best specimens 

 have no grit, and when used as a polishing powder no scratches can 

 be detected, even with the microscope. It is most abundant along 

 the Republican, where it is found in almost every county. The 

 following is a characteristic section, taken at a bed in Furnas 

 County, south of the Republican, and about eight miles southeast 

 of Arapahoe. It is exposed on the East half of Northeast of 8, 

 and on West half of Northwest of 9, Township 3 North, and 21 

 West, of 6th Principal Meridian. One of these exposures here is 

 near a quarter of a mile long. The measurements are from the top 



down : 



Section. 



1. Loess, from three to 6 



2. Drift 3 feet. 



3. Compact silicate of lime and limestone 3 " 



4. Flour-like earth 12 " 



This bed is made up of layers one-fourth of an inch in thickness, 

 of snowy whiteness, and other layers, from nine inches to a foot 

 thick, of a grayish white color. Nine feet from the top there is a 

 layer two inches thick, of a greenish color, which contains potash 

 and iron. 



As already intimated, it polishes as successfully and as finely as 

 the best tripoli. 



Origin of this Flour-like Earth. — Near or in many of these beds 

 all over the Pliocene region of the plains are found many extinct 

 geyser tubes, and sometimes old geyser basins. Of these I ob- 

 served at least thirty between Arapahoe and the west line of the 

 State. I have also found them in the Loup region and on the Nio- 

 brara. As some of these geyser tubes had their exit in the Fort 

 Pierre Group, on the upper Republican, it is probable that they 



