24 THE ORIGIN OF ALKALI 



have been some shifting and readjusting of the land as the 

 Dakota beds arc found to be quite thick in the northern 

 section where the Mancos are thin; while in the southern 

 section the Mancos are found to be exceedingly thick in 

 places where the Dakota is comparatively thin. 



"Where they are not capped with the sandstone the 

 beds do not form abrupt ledges, but weather off into rather 

 rounded symmetrical clay hills at least they appear 





Fig. i. Salt-bearing Shale Formation. This Type of Soil- 

 forming Material is a Common Source of Alkali. 



to be clay hills. This disintegration of the shales gives 

 rise to a very sticky, plastic clay which forms numerous 

 cracks when dry, but becomes a continuous coat of plastic 

 clay when wet. The material is so close grained that 

 when rain falls upon it, it seals up all the pores and cracks 

 so that water does not seem to penetrate it. These hills 

 are very sparsely covered with vegetation and it is not an 

 unusual thing to see an area of more than an acre which 

 does not contain a single plant. 



"On these rounded clay hills one seldom has to dig more 

 than a foot before the shale is found in place. However, 

 the material covered is not uniform, especially on top of the 

 clay knolls. The usual condition is that on the surface 

 is from one to two inches of earthy clay, under which is 



