260 ANNUAL REPORT. SMITHSONIAN INSTITUTION, 1916. 



Though well known, it may not be amiss to recall some features of 

 these three periods of widely extended desert conditions. 



ARID ZONES OF THE PLEISTOCENE AND PRESENT. 



The map of the world shows two zones which are largely desert, 

 one in each hemisphere, with a broad zone of heavy equatorial rain- 

 fall between. To the north of the northern desert belt there are 

 moister conditions, and the same is true to the south of the southern 

 one. There is reason to believe that Antarctica is arid, evaporation 

 exceeding precipitation, and the same may be true of some Arctic 

 lands. The precipitation on Spitzbergen is stated to be only 6 inches 

 per annum. 



The two belts of deserts do not run quite parallel to the Equator. 

 The northern one, beginning with the Sahara and Nubian Deserts, in 

 Africa, runs northeastward through the Arabian and Indian Deserts 

 to those of central Asia, wdiere the desert of Gobi reaches nearly 50'^ 

 of north latitude. In North America desert conditions are less ex- 

 tensive and do not extend beyond latitude 40° or 45°. 



In the Southern Hemisphere the bodies of land are much smaller, 

 and the deserts of South Africa, Australia, and South America are 

 correspondingly small as compared w-ith those north of the Equator. 

 Their southern limits are, roughly, 30°, 40°, and 45° south latitude. 



Penck has shown, I think satisfactorily, that these desert belts 

 migrate toward the Equator in cold periods, narrowing the zone of 

 tropic rains, and move respectively north and south in warmer 

 periods. In the mildest geological periods it would almost seem as if 

 the equatorial belt of warmth and moisture expanded to cover the 

 whole earth, abolishing both deserts and ice-sheets, and these appear 

 to be the normal conditions when peneplanation has advanced far 

 and shallow seas transgress widely over the continents.^ 



ARID PERIOD OF THE PERMIAN AND TRIASSIO. 



Going back to Permian and Triassic timas, much of the evidence 

 has been buried or destroyed ; jet it is certain that deserts extended 

 widely in many lands. Red sa^ndstones, arkoses, and shales with mud 

 cracks and footprints, beds of salt and gjq^sum, are reported from 

 England, Germany, Austria, and Russia in regions now well watered. 

 In North America there were the widespread red beds of the Rocky 

 Mountain region and the band of desert sandstones extending from 

 Prince Edward Island southwest to Virginia ; so that arid conditions 

 covered far more of Europe and North America than now. In India 



1 Die Formen der Landoberfliiche u. Verschiebungen der Klimagiirtel, Koenlgliche, Preus. 

 Ak., V«l. 4, 1913, 



