National Geographic : 1893 Feb 08
Deflation and Desert Varnish. 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 depression, 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 was 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 crawl 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 whether the columns had at one time been washed by de scending rain rills or whether that had been the case with the holes between them. By the recent observations it is placed beyond doubt that only the holes can have been formed by water. Professor Sickenberger of Cairo has been engaged for a year on investigations on the chemical processes involved in the forma tion of desert varnish, and important results are to be expected from him, confirming the views here set forth. I attach great importance to a letter I received frcm Professor von Streeruwitz, who, in his laboratory at Austin, " Kept a pro tective coat, absolutely free from manganese, in an ozonized atmosphere for two weeks. It rapidly grew darker, and in the course of a few months assumed the color which those rocks had before the cloud-burst." This fact shows how correct is G. Rohlfs in calling attention, as he did again recently, to the significance of electricity in deserts, and how important would be the insti tution of exact determinations of electricity in some desert. 24-NAT. GEOG.MAG., VOL.IV, 1892.
1893 Feb 20
1892 May 15