180 A. P. COLEMAN DRY LAND IN GEOLOGY 



Ordovician seem to have escaped the drought, and it may be that a more 

 careful search through the literature would disclose deserts there also. 



A number of the suggestions noted are only tentative, however, and 

 wide-spread and unmistakable desert formations seem confined to the 

 Pleistocene, Triassic, Permian, Devonian, and late Precambrian. Of 

 these the Pleistocene deserts may be looked on as continuing to the pres- 

 ent, the Triassic deserts form an aftermath of the arid conditions of the 

 Permian, and the Devonian deserts seem less extensive than the others. 

 The three times of greatest aridity appear to be, 1, the Pleistocene con- 

 tinuing to the present; 2, the Permian-Triassic ; 3, the late Precambrian. 



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

 these three periods of widely extended desert conditions. 



Arid Zones oe 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 rainfall 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 Spitz- 

 bergen 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, where the desert of Gobi reaches nearly 50° of north lati- 

 tude. In North America desert conditions are less extensive 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 correspond- 

 ingly small as compared with 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 mild- 

 est 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. 2 



2 Die Formen der Landoberflache u. Verscbiebungen der Klimagiirtel, Koenigliche, 

 Preus. Ak., vol. Iv, 1913. 



