3Iesozoic and Ccenozoic Geology and Baloeontoloyg. 175 



peculiar significauce, and is applied to the rugged and desolate country 

 formed where the soft ciaye}^ formations are undergoing rapid waste. 

 Steep irregular hills of cla}^ on which scared}^ a trace of vegetation 

 exists, are found, separated by deep, nearly perpendicular-sided, and 

 often well nigh impassable valleys; or when denudation has advanced to 

 a further stage — and especialh^vhen some more resisting stratum forms 

 a natural base to the claj'C}' beds — an arid flat, paved with the washed- 

 down claj^s, almost as hard as stone when dr}", is produced, and sup- 

 ports irregular cones and buttes of clay, the remnants of a former 

 high-level plateau. Denudation, in these regions, proceeds with ex- 

 treme rapiditj^ during the short period of each year, in which the soil 

 is saturated with water. T\ie term, first and typically applied to the 

 newer White river Tertiaries of Nebraska, has been extended to cover 

 countr}' of similar nature in the lignite regions of the Upper Mis. 

 souri and other areas of the West, In the Bad Lands, south of Wood 

 Mountain, the hills assume the form of broken plateaus; degenerating 

 graduall}^ into conical peaks, when a . harder la3^er of sandstone, or 

 material indurated b}^ the combustion of lignite beds, forms a resist- 

 ant capping. Where no such protection is afibrded, rounded mud- 

 lumps are produced from the homogeneous, arenaceous cla^^s. Waste 

 proceeds entirel}' by the power of falling rain, and the sliding down of 

 the half-liquid clays, in the period of the melting snow in spring. The 

 cla}^ hills are consequently^ furrowed, from top to base, by innumerable 

 runnels, converging into larger furrows below. The small streams, 

 rapidly cutting back among these hills, have formed many narrow, 

 steep-walled gullies, while the larger brooks have produced wide, flat- 

 bottomed valleys at a lower level, in which the streams pursue a ver}^ 

 serpentine course. Denudation is even here, however, still going on 

 as from the frequent change in the channel of the stream, it is con- 

 stantl}' encroaching on the banks of the main valley, undercutting 

 them and causing landslips. 



The general section at this place, in descending order, is as follows: 



1. Yellowish sand and arenaceous cla}-, sometimes indurated in cer- 

 tain layers and forming a soft sandstone. It forms the flat plateau — 

 like tops of the highest hills seen. About 50 feet. 



2. Clays and arenaceous claj^s, with a general purplish-gray color 

 when viewed from a distance. It contains a lignite-bearing zone and 

 beds, rich in the remains of plants, and in the lower part, the remains 

 of vertebrate animals. About 150 feet. 



3. Yellowish and rust}^ sands, in some places approaching arenaceous 

 clavs, often nodular. About 80 feet. 



