172 Dr JohcmnesWalther — The North American Deserts. 



From what has thus far been stated, it is evident that the 

 North American deserts agree with the Egyptian deserts in all 

 essential and characteristic points. There are striking differ- 

 ences in plant geography, it is true, in that the American deserts 

 on the whole are much richer in vegetation, probably by reason 

 of greater precipitation. It is probably for this reason that ero- 

 sion plays a larger and more important part than in Egypt. But 

 the topographic character is the same : the- prevalence of hori- 

 zontal plains with island-like mountains rising from them; the 

 frequency of isolated " Zeugenberge,'' or island mountains and 

 of amphitheaters in the valleys ; . the intense energy of insolation, 

 cracking bowlders and pebbles and causing varicolored granite 

 to crumble into loose gravel ; the appearance of mushroom rocks 

 and columned galleries ; and the wide distribution of the desert 

 varnish, a phenomenon which must be regarded as a specific 

 effect of dry climate and scarcity of rain. The denuding effect 

 of wind is apparent not only in the characters of the surface 

 forms just named, which are essentially different from the forms 

 of erosion, but it may also be observed directly when dust storms 

 career over the desert. As in northern Africa, so in North 

 America there are found four types of denudation products or 

 sediments : gravel deposits, sand dunes, clay tracts and salt 

 deposits. 



In view of such agreement in the primary and secondary geo- 

 logic phenomena of the deserts, geographically so far apart, of 

 northern Africa and North America, despite the different condi- 

 tions of vegetation, one is justified in i-egarding the phenomenon 

 of desert formation as one of the great telluric processes, a process 

 having its own laws just as much as the glacial j^henomena of 

 the polar zone or the cumulative weathering of the tropics. 

 Transferring this principle to the domain of earth history, there 

 arises the problem of searching for the remains of fossil deserts 

 in the strata of the earth's crust with the same care with which 

 in recent time fossil glacial periods are reconstructed. 



But the study of deserts has another imj^ortant consequence. 

 The desert is an extreme of climatologic conditions. Dry air 

 and dry heat, which in our temperate climate make their ap- 

 pearance on a few days only in the year, are active in the desert 

 for the larger j^art of the year. Their effect in the desert is 

 prominent ; in our regions it occupies a very modest place ; but 



