826 



SCIENCE. 



[X. S. Vol. XXI. Xo. 543. 



conclusion is announced, is well worthy of at- 

 tention from American physiographers. 



The principle of leveling without baselevel- 

 ing, or Passarge's law, as it may he called, in 

 contrast to Powell's law of leveling by base- 

 leveling, suggests that the scheme of the nor- 

 mal cycle of erosion, so generally applicable 

 in regions of ordinary or normal climate, 

 should be systematically modified in such ways 

 as will adapt it to the conditions of an ab- 

 normally dry or arid climate. This modifica- 

 tion I have lately attempted in an article that 

 will soon be published in the Journal of Geol- 

 ogy; it is here presented in outline. 



An extensive region of any structure up- 

 lifted in an arid zone to any altitude and with 

 any form will, in the youthful stage of its 

 cycle of erosion, be characterized by as many 

 independent and incomplete centripetal drain- 

 age systems as there are depressed areas or 

 basins within its limits : independent systems, 

 because in an arid climate the basins can not 

 be filled with overflowing lakes; incomplete 

 systems, because many of the intermittent 

 centripetal streams will wither away on the 

 slopes and fail to join forces in trunk streams 

 on the basin floors. The early stage of a 

 normal cycle, where all basins are filled to 

 overflowing and where all streams are con- 

 tinued until they unite in trunk rivers which 

 reach the sea, is characterized by a rapid in- 

 crease of relief, due to the incision of valleys. 

 The early stage of the arid cycle is, on the 

 other hand, characterized by a decrease of re- 

 lief, due to the aggradation of the basins with 

 the waste washed down from the enclosing 

 highlands. As youth advances towards ma- 

 turity, the initially independent basins will 

 become more and more completely confluent, 

 either by headward erosion on the slopes of the 

 lower basins, or by the overflow of waste across 

 depressions in the borders of the higher basins ; 

 thus from original independence will be de- 

 veloped a maturely integrated and interde- 

 pendent system of drainage slopes, although 

 trunk rivers will still be wanting. Maturity 

 may be said to be fully established when large 

 areas are thus brought into systematic correla- 

 tion. At this stage, there may still be some 

 unreduced uplands, but there will also be in- 



creasing piedmont areas of degraded, rock- 

 floored plains, inclined gently towards the 

 greatly enlarged central aggraded basin floor; 

 and the composite jAain thus produced will 

 have no definite relation to normal baselevel. 

 In so far as the erosion of arid regions has 

 previously been discussed, it would appear that 

 this stage, here called maturity, has been re- 

 garded as the old age of a desert, and that it 

 has been taken to mark the end of the changes 

 to which an interior basin is subject, unless it 

 is attacked by the headward extension of ex- 

 terior streams and thus dissected and reduced 

 to normal baselevel; but as Passarge clearly 

 shows, the old age is yet to come, and with a 

 systematic sequence and grouping of features 

 essentially unlike those just described. The 

 action of the wind is yet to be considered. 



In the earlier stages of the cycle, while the 

 slopes are still varied and strong, transporta- 

 tion and trituration by the wind is probably 

 of small value in proportion to that of the 

 occasional streams and floods. But as the 

 barren surface becomes more and more even, 

 the relative importance of wind action in- 

 creases; for unlike running water, the wind 

 does not depend on local slopes for its activity ; 

 it is about equally strong everywhere on a 

 surface of moderate relief, and has no sub- 

 division into subordinate parts that correspond 

 to small headwater streams, whose slopes must 

 be steeper than that of their trunk river. 

 The wind may sweep sand along a level floor, 

 or even up a moderate slope ; and whirlwinds 

 may raise dust high into the air, and there 

 give it to the upper currents ; both of these 

 processes may carry desert waste outside of 

 the desert region under consideration, and 

 thus the mean level of the desert may be very 

 slowly reduced. The surface may, indeed, in 

 this wa.y eventually be worn below sealevel, as 

 several writers have suggested; but the form 

 that the surface will exhibit during its slow 

 reduction has not, to my knowledge, been espe- 

 cially considered until in the recent statement 

 of tliis aspect of the question by Passarge. 



It might at first sight appear that when the 

 winds gain the upper hand in the processes 

 of transportation, they would tend to excavate 

 extensive basins wherever the weathering of 



