170 Dr Johannes Walther — The North American Deserts. 



Thus rocks are destroyed by dry heat at a tmie when denuda- 

 tion by water sinks to a minimum. On the 300 dry days of the 

 year the process of rock destruction continues uninterruptedly. 

 On rainy days, of course, the loosened rock material is carried 

 off by running water. That such a desert rain-storm, falling 

 like a cloud-burst, may carry off immense masses of debris needs 

 no proof. The question is merely this : Is the transportation of 

 the products of deiiudation at a standstill in the desert during 

 the 300 dry days ? To this question also we must reply in the 

 negative. Almost daily I saw columns of dust traveling slowly 

 over the plain. These raise great quantities of loose material 

 high in the air ; there this material is caught by horizontal air 

 currents and carried farther away. I also saw in Colorado how 

 within one-quarter of an hour the desert plain was wrapped in 

 clouds of dust so dense that one could hardly see two kilometers 

 away, while previously the eye might discern everything Avithin 

 a radius of many miles. 



Thus we see that even when it does not rain there exists in 

 the desert a transporting force, and that on the 300 dry days 

 neither the destruction of rock nor the transportation of the 

 products of destruction is at rest. We also recognize that this 

 " dry denudation," as it has been called b}'' an English reviewer, 

 is of intense power and may well be compared qualitatively, 

 though not quantitatively, with the denuding effect of water. 

 It will be exceedingly difficult, however, to find a scale by which 

 erosion, that is, denudation by water, can be compared with 

 deflation, that is, denudation by wind; and so long as such a 

 scale is wanting, all conclusions regarding their relative activity 

 must rest on subjective estimates. 



Many rocks and rocky surfaces of the African deserts are cov- 

 ered with a peculiar coating, which may be designated as "brown 

 protective coat " or " desert varnish." This coating is also found 

 widely distributed in the deserts of North America, and if I did 

 not succeed in forming a definitive judgment concerning the 

 origin of this product in Egypt, I have now in America made 

 observations which promise to bring the problem nearer to its 

 solution. Mr von StreeruAvitz in the beginning of September 

 had made excursions to several parts of the Sierra del Diablo, 

 observing instances of the j)rotective coat, Avhich he intended 

 afterAvard to shoAv me. ToAvard the end of September cloud- 



