THE GEOGRAPHICAL CYCLE IN AN ARID CLIMATE 



403 



On the other hand, great area, moderate altitude, inclosure by 

 mountain barriers, and small exterior rainfall are favorable to the 

 leveling of interior desert plains; and to these favoring conditions 

 should be added a long geological period of quiet. The greater 

 the area and the less the altitude, the less the opportunity that exterior 

 streams will have to establish relations with the interior streams. 

 The higher the inclosing mountains, the longer the interior region 

 will be left to itself, but the more dust it will have to export before 

 a general rock-floor can be developed; the desert of Gobi offers 

 an example of this kind, for its surface must long continue to suffer 

 aggradation before the lofty ranges around its depressed surface 

 are worn down to its level. South Africa would seem to offer, accord- 

 ing to the descriptions by Passarge, excellent opportunity for the 

 successful advance of the arid cycle far into old age, because of the 

 large extent of the land area, its sufficient height and inclosure, its 

 long-undisturbed history, and its persistently arid climate. 



It thus seems evident that the conditions necessary for desert- 

 leveling are actually present in greater or less degree in different 

 parts of the world. 



The scheme of the arid cycle as an aid to observation. — The normal 

 cycle has now been practically used by so many observers, and with 

 so many advantageous results, that it is not unfair to expect similar 

 advantage from the use of the arid cycle as an aid to observation in 

 regions where it may be appropriately applied. Certain it is that 

 many observations now on record with regard to arid regions do not 

 suffice to indicate clearly the stage of erosion in the arid cycle there 

 reached; and this, not because the observers had either reason or 

 wish to dissent from the principles of the scheme, but because it was 

 not consciously present in their minds when the observations were 

 made. The same is often true of the scheme of the normal cycle. 

 In both cases the failure of the observant explorer to refer the facts 

 that he finds to some comprehensive scheme for their systematic 

 treatment not only results in the accidental overlooking of certain 

 significant facts and in the insufficient description of others, but it 

 leaves the reader in great difficulty when he t ries to visualize what 

 the observer has seen. It is as if the writer and the reader had no 

 common language in which the observations and thoughts of the one 



