May 26, 1905.] 



SCIENCE. 



827 



the rocks resulted in the production of fine 

 dusty waste; and that, inasmuch as the winds 

 know no baselevel, there would be no definitely 

 assignable limit to the unevenness of the sur- 

 face thus produced. This might be true in ab- 

 solutely rainless regions; but such regions are 

 not known. The most desert regions of the 

 world have occasional rainfall, and are from 

 time to time visited by showers heavy enough 

 to cause floods ; and the intermittent action of 

 such floods will put an effectual stop to the 

 development of deep basins by wind action. 

 As soon as the winds succeed in sweeping out 

 a shallow depression, that part of the in- 

 tegrated drainage slopes which leads toward 

 the depression will, when rain falls and floods 

 are formed, provide a supply of waste with 

 which the depression will be aggraded. Fur- 

 ther deepening of the depression below its 

 surroundings is thus effectually hindered. 

 The wind may then begin the excavation of 

 another depression elsewhere, only again to be 

 defeated by the local inwash of a waste cover. 

 Not an uneven surface of many hills and hol- 

 lows, but a remarkably even plain must result 

 from the long continuance of these antago- 

 nistic processes. 



During the development of such a plain, a 

 series of systematically irregular changes will 

 run their course. As the exportation of desert 

 waste by the winds continues, the area of the 

 central aggraded basin floor must diminish, 

 while that of the surrounding degraded rock 

 plains must increase. At the same time, the 

 integrated drainage system of maturity will 

 be more and more completely disintegrated and 

 replaced by many local and variable systems 

 of extremely indefinite separation. Eventually 

 all the central accumulation of waste will have 

 been exported by the winds ; the rock-floored 

 plain will have been worn down lower than 

 the bottom of the deepest initial depression, 

 so that it will then extend throughout the 

 region, except for residual mountains of rocks 

 most resistant to dry weathering. Thin 

 veneers of gravelly waste will remain, swept 

 hither and yon by the intermittent fluctuating 

 disintegrated drainage ; shallow ' saltpans ' may 

 occur from place to place and from time to 

 time; but large areas of rock plains carrying 



only scattered stony waste, will abound; this 

 is the condition of true old age in such a 

 region. Once attained, it persists, slowly worn 

 lower and lower, possibly sinking below sea- 

 level, until disturbed by crustal movements or 

 climatic change. It is old rock-floored desert 

 plains of this character and apparently of this 

 origin that Passarge describes as occupying 

 thousands of square miles in South Africa. 



Two interesting consequences of this scheme 

 should be pointed out. 



Every truncated upland that has been de- 

 scribed as an uplifted and more or less dis- 

 sected peneplain should now be reexamined 

 with the object of learning whether it may 

 not have originated as a desert plain at its 

 present altitude above sealevel, and afterwards 

 suffered dissection as a result of climatic 

 change. True, we are to-day more accus- 

 tomed to movements of the earth crust, in the 

 way of elevations and depressions, than to cli- 

 matic changes, in the way of transforming 

 arid regions to humid regions and vice versa; 

 but perhaps this habit of thought is only a 

 fashion of our time. A century ago, move- 

 ments of the earth's crust indicated by the dis- 

 covery of marine fossils on the higher peaks 

 of the Alps were regarded with astonishment, 

 not to say incredulity. A century hence, 

 variations of climate may be accepted as freely 

 as changes of level are now. The way 

 towards such an opinion is opened by the dis- 

 covery of glacial periods in various geological 

 ages, and it is not hindered so much as it was 

 once by supposed evidence of the correspond- 

 ence of earlier climatic zones with those of 

 to-day. We should, therefore, open our minds 

 widely to the possibility of explaining trun- 

 cated uplands as ancient desert plains not 

 changed in elevation, but only in climate; and 

 this possibility should not be set aside because 

 it seems improbable, but only because it may 

 be shown on good and sufficient grounds to be 

 inappropriate to the case under consideration. 

 It may be added that, as far as I have under- 

 taken a revision of the origin of truncated 

 txplands, as is suggested above, nearly all the 

 familiar cases seem to possess characteristics 

 that accord with their origin as uplifted pene- 

 plains and not as desert plains; and that there 



