566 PRINCIPLES OF STRATIGRAPHY 



the winds from the Desert of Gobi in central Mongolia. Evidence 

 of fluvial action is, however, not wanting, many gravel beds of great 

 extent and great distance from their source pointing to river activi- 

 ties. It has been thought (Wright-54) that the distribution indi- 

 cates a vast body of standing water which temporarily occupied the 

 loess-covered area in late Tertiary or Quaternary time, but this in- 

 terpretation does not sufficiently account for the absence of fossils 

 in the loess, nor does it do justice to the ability of rivers and wind 

 to distribute fine sediments in broad level plains. For the whole 

 of Asia the loess-covered area comprises about 1.324,000 square 

 kilometers (511,150 square miles) (v. Tillo), which, with an aver- 

 age estimated thickness of 30 meters, gives a total of nearly 40,000 

 cubic kilometers of loess material. All this material has of course 

 been removed from the rock surfaces of the country where it origi- 

 nated. (Walther-51 :jO.) The loess is characteristically pene- 

 trated by vertical tubes, the calcite-filled hollows left by the decay 

 of grasses and roots. These help to produce a vertical splitting of 

 the mass, which otherwise shows no planes of separation, since 

 stratification is for the most part absent. As a result, the walls 

 of loess left on the dissection of the loess area by the Yellow River 

 and its tributaries form vertical bluffs of great height. "Millions 

 of Chinese live on the valley floors of dissected basins of this kind, 

 for the loess is extremely fertile where well watered. Great num- 

 bers of the people inhabit cave-like dwellings excavated in loess 

 bluffs ; in a thickly populated district not a house may be seen. 

 The yellowish color of loess prevails everywhere. It gives color 

 and name to the great river of the region and to the sea into which 

 the river flows." (Davis-13 : j//, 318.) The loess of the Mis- 

 sissippi \'alley region and of the plains generally, is also commonly 

 interpreted as a deposit of wind-blown material, though indications 

 of aqueous activities are not wanting. The principal evidence for the 

 eolian origin of this "Prairie loess" (Shimek-46) is found (i) in 

 the absence in the deposit of shells belonging to distinctively water 

 species; (2) the presence of land shells of species that live on the 

 shores of ponds; (3) the difficulty of imagining a submergence of 

 the loess-covered area of such character as to account for its pe- 

 culiarities ; for in a permanently standing body of water the uni- 

 formity of distribution of material of the given degree of fineness 

 characteristic of the loess could not be secured ; (4) the reasonable 

 influence of vegetation in arresting wind-blown dust. Professor 

 Wright (55:^05) gives some facts which seem to oppose the eolian 

 origin of the Mississippi valley loess: (i) The distribution of the 

 loess in about equal proportions upon both sides of the INIissouri 



