242 C. W. TOMLINSON 



Deposits of desert lakes or playas. — For an example of the sixth 

 type of present-day red sediments, deposited under water in desert 

 lakes or playas, we turn to Chinese Turkestan. The following 

 description, by Huntington, relates to the northern extremity of 

 the bed of Lop Nor, near the southern base of the Kuruk-Tagh 

 or Dry Mountains: "Beyond the fatiguing plain of salt [dry bed 

 of the dwindled lake Lop Nor] we found easy traveling for a time. 

 A fantastic red plain, the soft, dry bed of an older expansion of the 

 lake, glittered with innumerable gypsum crystals." 1 



Here we have a recent deposit of gypsum (or, more properly, 

 selenite), in which the crystals presumably are imbedded in a red 

 clay or mud. The relations here described could be duplicated by 

 many minor deposits of gypsum in the Red Beds of the western 

 United States. Farther out toward the center of the lake floor 

 occur the purer non-clastic sediments, which in the case of Lop Nor 

 are described as salt beds. The coloring matter of the clays 

 probably was derived, as in the two preceding types of modern 

 red sediments, from the decay of rocks on the neighboring uplands. 



Red dune sands. — The seventh and last type differs from all 

 the others in being an eolian deposit. Red dune sands are excep- 

 tional rather than the rule in the desert regions of today, but they 

 occur in sufficient abundance to warrant attention. Their most 

 striking occurrence is in the Nefood or Red Desert of North- 

 western Arabia. The following quotation is from Palgrave's 

 narrative of a journey taken in 1862: "We were now traversing 

 an immense ocean of loose reddish sand, unlimited to the eye, and 

 heaped up in enormous ridges, .... undulation after undulation, 

 each swell two or three hundred feet in average height." 2 



The extreme breadth of the Nefood is about 150 miles, its 

 greatest length about 400 miles. 3 



Huntington mentions "an almost absolutely barren area of 

 reddish or yellowish sand dunes, from ten to a hundred or more feet 



'Ellsworth Huntington, The Pulse of Asia (Boston and New York: Houghton 

 Mifflin Co.), p. 254. 



2 W. G. Palgrave, Central and Eastern Arabia (London: Macmillan, 1908), pp. 

 62-63. 



3 See J. A. Phillips, "The Red Sands of the Arabian Desert," Qnar. Jour. Geol. Soc. 

 London, XXXVIII (1881), 1 10-13. 



