Deflation and Desert Varnish. 171 



bursts descended there, and when in the beginning of October 

 we entered the valleys of the Sierra, we found to our astonish- 

 ment that the protective coat had everywhere been torn away, 

 and only a few shreds of it hung against some of the walls. Even 

 where there had merely been a pool of water in a deiDression, 

 and where therefore the chemical rather than the mechanical 

 force of the water had been active, we found the desert varnish 

 removed. From this appears with certainty what previously I 

 could express only as conjecture: 



(1) That the brown protective coat is not formed by the aid 

 of water, and — 



(2) That it is torn off and removed wherever rainwater has 

 access. 



Now, the latter fact also throws some light on a phenome- 

 non which Avas previously a perfect enigma. In the African 

 deserts, sandstones or limestones, more rarely granite, are found 

 weathered in such manner that the face of a rock wall is broken 

 by niches or crannies 10 to 100 centimeters high, 5 to 50 centi- 

 meters broad, separated by columns reaching a meter in height. 

 Behind these columns — that is to say, in the interior of the rock 

 wall — runs a passageway, at times large enough to allow a man 

 to craAvl along it. Both Professor Schweinfurth and myself were 

 convinced that in the formation of these columned passages rain- 

 water had played a part ; our views diverged only on the ques- 

 tion Avhether the columns had at one time been washed by de- 

 scending rain rills or whether that had been the case Avith the 

 holes betAveen them. By the recent obserA^ations it is placed 

 beyond doubt that only the holes can have been formed by Avater. 



Professor Sickenberger of Cairo has been engaged for a year on 

 investigations on the chemical processes iuA^olved in the forma- 

 tion of desert A^arnish, and important results are to be expected 

 from him, confirming the vieAvs here set forth. 



I attach great importance to a letter I received frcm Professor 

 von StreeruAvitz, Avho, in his laboratory at Austin, " Kept a pro- 

 tective coat, absolutely free from manganese, in an ozonized 

 atmosphere for tAVO Aveeks. It rapidly grcAV darker, and in the 

 course of a few months assumed the color which those rocks had 

 before the cloud-burst." This fact shoAVS how correct is G. Rohlfs 

 in calling attention, as he did again recently, to the significance 

 of electricity in deserts, and how important Avould be the insti- 

 tution of exact determinations of electricity in some desert. 



24— Nat. Geos. BIag., vol. IV, 1892. 



