THE GEOGRAPHICAL CYCLE IN AN ARID CLIMATE 383 



the less arid highlands to the more arid depressions. Each basin 

 system will therefore consist of many separate streams, which may 

 occasionally, in time of flood or in the cooler season of diminished 

 evaporation, unite in an intermittent trunk river, and even form a 

 shallow lake in the basin bed, but which will ordinarily exist inde- 

 pendently as disconnected headwater branches. 



Youthful stage — In the early stage of a normal cycle the relief is 

 ordinarily and rapidly increased by the incision of consequent valleys 

 by the trunk rivers that flow to the sea. In the early stage of the arid 

 cycle the relief is slowly diminished by the removal of waste from the 

 highlands, and its deposition on the lower gentler slopes and on the 

 basin beds of all the separate centripetal drainage systems. Thus all 

 the local baselevels rise. The areas of removal are in time dissected 

 by valleys of normal origin : if the climate is very arid, the uplands 

 and slopes of these areas are either swept bare, or left thinly veneered 

 with angular stony waste from which the finer particles are carried 

 away almost as soon as they are weathered; if a less arid climate 

 prevails on the uplands and highlands, the plants that they support 

 will cause the retention of a larger proportion of finer waste on the 

 slopes. The areas of deposition are, on the other hand, given a nearly 

 level central floor of fine waste, with the varied phenomena of shallow 

 lakes, playas, and salinas, surrounded with graded slopes of coarser 

 waste. The deposits thus accumulated will be of variable composi- 

 tion and, toward the margin, of irregular structure. The coarser 

 deposits will exhibit a variety of materials, mechanically comminuted, 

 but not chemically disintegrated, and hence in this respect unlike the 

 less heterogeneous deposits of humid climates from which the more 

 easily soluble or decomposable minerals have been largely removed. 

 The finer deposits will vary from sand and clay to salt and gypsum. 

 The even strata that are supposed to characterize lake deposits may 

 follow or precede irregular or cross-bedded strata, as the lake invades 

 or is invaded by the deposits of streams or winds. While many 

 desert deposits may be altogether devoid of organic remains, others 

 may contain the fossils of land, stream, or lake organisms. 



The Basin Range province of the western United States gives 

 examples of dissected mountains from which descend many wither- 

 ing streams that belong to separate drainage systems of the kind 



