22 BULLETIN 16 9, UNITED STATES NATIONAL MUSEUM 



rable throughout the field, and their distinction makes discussion 

 and records more exact. Both are said to be in large part andesitic, 

 and both are characterized by their somber color, the shales gen- 

 erally greenish and the sandstones dark brown, gray, or gray-green. 



The No. 1 beds, or lower Lebo (shown in pi. 2), are characterized 

 by numerous lenses and beds of hard and resistant dark sandstone, so 

 that this unit is generally topographically positive and forms a ridge 

 or series of ridges. This characteristic is visible throughout the 

 field; for instance, typically in the gentle anticline in the northern 

 part of T. 5 N., Ks. 15-16 E,, where the No. 1 forms an elevated hilly 

 area surrounded by a horseshoe valley developed on the No. 2 beds. 

 The upper limit of the No. 1 is taken at the top of the highest and 

 most persistent of its hard sandstones. The thickness, as measured 

 in sees. 15-22, T. 6 N., K. 15 E., is 496 feet (Stanton and Silberling). 

 It has not elsewhere been accurately measured. It may be somewhat 

 thicker in the western and thinner in the eastern parts of the field 

 but apparently does not vary greatly. 



The No. 2 beds are topographically negative (see pi. 3). They 

 form valleys between the No. 1 and No. 3 sandstones, or slopes 

 beneath the latter. The characteristic material is greenish shale, 

 often rather coarse and sandy, with lenses and beds of gray sandstone. 

 When unweathered, these sandstones may be hard, for instance in 

 the overburden of the Gidley Quarrj^, but they weather rapidly and 

 are not resistant to erosion. It is this nonresistant nature of its 

 sandstones, and generally their somewhat lighter color, that dis- 

 tinguish this member most sharply from the No. 1. The thickness in 

 the measured section mentioned above is 840 feet and probably 

 averages 800 to 900 feet throughout the field. Stone and Calvert 

 (1910, p. 753) give a total thickness of 2,080 feet for the Lebo on Lebo 

 Creek. I did not measure the beds here (where they are not very 

 well exposed and have a variable and uncertain dip) but estimated 

 the thickness at not much over 1,500 feet, with about 600 feet in the 

 No. 1 and 900 in the No. 2. They give a total thickness of the Lebo 

 in T. 6 N., R. 16 E. (that is, near the north end of Bear Butte) of 

 only 463 feet, which I think is surely much too small. It is highly 

 unlikely that this persistent formation thins out from 1,334 to 463 

 feet in less than 8 miles. The dip in this region changes rapidly, as 

 the beds are around the Bear Butte syncline, and exposures are not 

 continuous, so that exact measurement is not possible, but it is unlikely 

 that the thickness is much if any less than 1,200 feet here, with some 

 700 or 800 in the No. 2 and 500 or 400 in the No. 1. 



The Lebo as a whole forms a narrow band along the northern edge 

 of the field, from the northeastern end of the Crazy Mountains almost 

 due east, but with some sinuosity, to sec. 23, T. 6 N., R. 15 E. Here 

 they turn abruptly southward, and their exposure widens greatly 



