* Mesozoic and Coinozoic Geology and Paloeontoloyg. 119 



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

 formed where the soft clayey formations are undergoing rapid waste. 

 Steep irregular hills of clay, on which scarcely 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 especially when some more resisting stratum forms 

 a natural base to the clayey beds — an arid flat, paved with the washed- 

 down clays, almost as hard as stone when dry, 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 rapidity during the short period of each year, in which the soil 

 is saturated with water. The term, first and typically applied to the 

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

 country 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 

 gradually into conical peaks, when a harder layer of sandstone, or 

 material indurated by the combustion ot lignite beds, forms a resist- 

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

 lumps are produced from the homogeneous, arenaceous clays. Waste 

 proceeds entirely 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 

 clay 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 very 

 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 vallej^, undercutting 

 them and causing landslips. 



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



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

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

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



2. Clays and arenaceous clays, 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 

 clays, often nodular. About 80 feet. 



